Draft:Female masculinization hypothesis

The female masculinization hypothesis emerged from decades of research on mammalian development, social behavior, and endocrinology, with its origins from studies of the spotted hyena. Early naturalists and behavioral ecologists noted unusual characteristics of female spotted hyenas, particularly male-like dominance, aggressive behavior, and the presence of a pseudopenis—an enlarged, erectile clitoris that closely resembles the male penis. These traits challenged traditional understandings of sexual dimorphism within the zoological community and raised questions about the developmental processes underlying them.{{Cite journal |last1=Young |first1=William C. |last2=Goy |first2=Robert W. |last3=Phoenix |first3=Charles H. |date=1964-01-17 |title=Hormones and Sexual Behavior |url=https://www.science.org/doi/10.1126/science.143.3603.212 |journal=Science |volume=143 |issue=3603 |pages=212–218 |doi=10.1126/science.143.3603.212|pmid=14077548 |bibcode=1964Sci...143..212Y }}

Initial studies in the mid-20th century documented the social and reproductive traits of spotted hyenas. Fieldwork by Dutch zoologist {{Ill|Hans Kruuk|ca}} in the 1970s provided detailed accounts of their matriarchal social structure, where females dominated males in both size and behavior. The physiological mechanisms driving these traits remained poorly understood until advances in endocrinology provided new tools for investigation.{{Cite book |last=Kruuk |first=H. (Hans) |url=https://archive.org/details/hyaena0000kruu |title=Hyaena |date=1975 |publisher=London; New York : Oxford University Press |others=Internet Archive |isbn=978-0-19-857395-1}}{{Cite book |last=Kruuk |first=H. (Hans) |url=https://archive.org/details/spottedhyenastud0000kruu_z0n8 |title=The spotted hyena : a study of predation and social behavior |date=1974 |publisher=Chicago : University of Chicago Press |others=Internet Archive |isbn=978-0-226-45507-5}}

Glickman, Frank, and their team at the University of California, Berkeley were among the first to systematically investigate the role of prenatal androgens in the development of masculinized traits in female spotted hyenas.{{Cite journal |last1=Frank |first1=L. G. |last2=Glickman |first2=S. E. |last3=Powch |first3=Irene |date=1990 |title=Sexual dimorphism in the spotted hyaena (Crocuta crocuta) |url=https://zslpublications.onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-7998.1990.tb04001.x |journal=Journal of Zoology |language=en |volume=221 |issue=2 |pages=308–313 |doi=10.1111/j.1469-7998.1990.tb04001.x |issn=1469-7998}} Their research in the 1980s and 1990s demonstrated that high levels of androgens produced by the mother during pregnancy were linked to the development of both the pseudopenis and aggressive behaviors in female offspring.{{Cite journal |last1=Glickman |first1=S. E. |last2=Frank |first2=L. G. |last3=Pavgi |first3=S. |last4=Licht |first4=P. |date=1992-07-01 |title=Hormonal correlates of 'masculinization' in female spotted hyaenas (Crocuta crocuta). 1. Infancy to sexual maturity |url=https://rep.bioscientifica.com/view/journals/rep/95/2/jrf_95_2_013.xml |journal=Reproduction |language=en-US |volume=95 |issue=2 |pages=451–462 |doi=10.1530/jrf.0.0950451 |pmid=1518001 |issn=0022-4251}}Baker, M.G. (1990) Effects of ovariectomy on dyadic aggression and submission in a colony of peripubertal spotted hyaenas (Crocuta crocuta). MA thesis, University of California, Berkeley.

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Studies by Glickman revealed that androgen exposure in utero was critical for the formation of masculinized genitalia, while additional research explored the organizational effects of these hormones on behavior.{{Cn|date=February 2025}} Their{{Whose|date=February 2025}} experiments showed that manipulating androgen levels during gestation could alter the degree of masculinization in both morphology and social interactions. This work provided the foundation for the formal articulation of the female masculinization hypothesis.{{Cite journal |last1=Glickman |first1=Stephen E. |last2=Frank |first2=Laurence G. |last3=Licht |first3=Paul |last4=Yalcinkaya |first4=Tamer |last5=Siiteri |first5=Pentti K. |last6=Davidson |first6=Julian |date=1992 |title=Sexual Differentiation of the Female Spotted Hyena |url=https://nyaspubs.onlinelibrary.wiley.com/doi/abs/10.1111/j.1749-6632.1992.tb22858.x |journal=Annals of the New York Academy of Sciences |language=en |volume=662 |issue=1 |pages=135–159 |doi=10.1111/j.1749-6632.1992.tb22858.x |pmid=1456636 |bibcode=1992NYASA.662..135G |issn=1749-6632}}{{Cite journal |last1=Licht |first1=P. |last2=Frank |first2=L. G. |last3=Pavgi |first3=S. |last4=Yalcinkaya |first4=T. M. |last5=Siiteri |first5=P. K. |last6=Glickman |first6=S. E. |date=1992-07-01 |title=Hormonal correlates of 'masculinization' in female spotted hyaenas (Crocuta crocuta). 2. Maternal and fetal steroids |url=https://rep.bioscientifica.com:443/view/journals/rep/95/2/jrf_95_2_014.xml |journal=Reproduction |language=en-US |volume=95 |issue=2 |pages=463–474 |doi=10.1530/jrf.0.0950463 |pmid=1518002 |issn=0022-4251}}

Following its development in the context of spotted hyenas, the hypothesis was expanded to explain similar traits observed in other mammals. Research by Tim Clutton-Brock and colleagues on meerkats highlighted how dominant females in cooperative breeding systems exhibited elevated aggression and androgen-linked traits, such as increased body size and reproductive control.{{Cite journal |last1=Davies |first1=Charli S. |last2=Smyth |first2=Kendra N. |last3=Greene |first3=Lydia K. |last4=Walsh |first4=Debbie A. |last5=Mitchell |first5=Jessica |last6=Clutton-Brock |first6=Tim |last7=Drea |first7=Christine M. |date=2016-10-18 |title=Exceptional endocrine profiles characterise the meerkat: sex, status, and reproductive patterns |journal=Scientific Reports |language=en |volume=6 |issue=1 |pages=35492 |doi=10.1038/srep35492 |issn=2045-2322 |pmc=5067592 |pmid=27752129|bibcode=2016NatSR...635492D }} Parallel studies on banded mongooses (Mungos mungo) and other species demonstrated that masculinized traits in females often correlated with intense intrasexual competition or high social dominance, suggesting a broader applicability of the hypothesis.{{Cite web |title=Sexual Selection Not Just for Males Anymore |url=https://phys.org/news/2007-01-sexual-males-anymore.html |access-date=2025-01-02 |website=phys.org |language=en}}{{Cite journal |last1=Johnstone |first1=Rufus A. |last2=Cant |first2=Michael A. |last3=Cram |first3=Dominic |last4=Thompson |first4=Faye J. |date=2020-11-24 |title=Exploitative leaders incite intergroup warfare in a social mammal |journal=Proceedings of the National Academy of Sciences |volume=117 |issue=47 |pages=29759–29766 |doi=10.1073/pnas.2003745117 |doi-access=free |pmc=7703641 |pmid=33168743|bibcode=2020PNAS..11729759J }}

The female masculinization hypothesis gained traction as it aligned with emerging frameworks in behavioral endocrinology, particularly the organizational-activational hypothesis.{{Cite journal |last1=Glickman |first1=Stephen E. |last2=Cunha |first2=Gerald R. |last3=Drea |first3=Christine M. |last4=Conley |first4=Alan J. |last5=Place |first5=Ned J. |date=2006-11-01 |title=Mammalian sexual differentiation: lessons from the spotted hyena |url=https://linkinghub.elsevier.com/retrieve/pii/S1043276006001767 |journal=Trends in Endocrinology & Metabolism |language=English |volume=17 |issue=9 |pages=349–356 |doi=10.1016/j.tem.2006.09.005 |issn=1043-2760 |pmid=17010637}} This model describes how hormones shape behavior and physiology during critical developmental windows. It provided a theoretical backbone for understanding the lasting effects of prenatal androgen exposure. Advances in molecular biology techniques, such as hormone assays and gene expression analyses, further validated and refined the hypothesis.{{Cite journal |last1=Yalcinkaya |first1=Tamer M. |last2=Siiteri |first2=Pentti K. |last3=Vigne |first3=Jean-Louis |last4=Licht |first4=Paul |last5=Pavgi |first5=Sushama |last6=Frank |first6=Laurence G. |last7=Glickman |first7=Stephen E. |date=1993-06-25 |title=A Mechanism for Virilization of Female Spotted Hyenas in Utero |url=https://www.science.org/doi/10.1126/science.8391165 |journal=Science |volume=260 |issue=5116 |pages=1929–1931 |doi=10.1126/science.8391165|pmid=8391165 |bibcode=1993Sci...260.1929Y }}{{Cite journal |last=Drea |first=Christine M. |date=2009-08-01 |title=Endocrine Mediators of Masculinization in Female Mammals |url=https://journals.sagepub.com/doi/10.1111/j.1467-8721.2009.01640.x |journal=Current Directions in Psychological Science |language=en |volume=18 |issue=4 |pages=221–226 |doi=10.1111/j.1467-8721.2009.01640.x |issn=0963-7214}}

These mechanisms of prenatal androgens, which include hormones such as testosterone, influence the development of male-associated traits in females. These involve hormonal, genetic, and epigenetic factors, shaping both morphology and behavior through distinct developmental processes.

Prenatal androgens play a central role in the masculinization of tissues and behaviors. During critical periods of fetal development, elevated androgen levels influence the differentiation of reproductive organs, neural structures, and behavioral tendencies. In spotted hyenas, high androgen concentrations in pregnant females contribute to the formation of a pseudopenis and male-like social dominance in female offspring.

The effects of androgens are commonly categorized as:

• Organizational effects: Permanent changes established during early development, such as the masculinization of genitalia and brain structures.{{Cite journal |last=Arnold |first=Arthur P. |date=2009-05-01 |title=The organizational–activational hypothesis as the foundation for a unified theory of sexual differentiation of all mammalian tissues |journal=Hormones and Behavior |series=50th Anniversary of the Publication of Phoenix, Goy, Gerall & Young 1959: Organizational Effects of Hormones |volume=55 |issue=5 |pages=570–578 |doi=10.1016/j.yhbeh.2009.03.011 |issn=0018-506X |pmc=3671905 |pmid=19446073}}
• Activational effects: Temporary changes triggered later in life, often during puberty, that amplify pre-established traits, such as aggression and dominance behaviors.{{Cite journal |last1=Schulz |first1=Kalynn M. |last2=Molenda-Figueira |first2=Heather A. |last3=Sisk |first3=Cheryl L. |date=2009-05-01 |title=Back to the future: The organizational-activational hypothesis adapted to puberty and adolescence |journal=Hormones and Behavior |volume=55 |issue=5 |pages=597–604 |doi=10.1016/j.yhbeh.2009.03.010 |issn=1095-6867 |pmc=2720102 |pmid=19446076 }}

Prenatal androgen exposure can result in the development of male-typical anatomical features in females:

• In spotted hyenas, the fusion of the labia and elongation of the clitoris to form a pseudopenis are direct consequences of androgenic effects on the urogenital sinus.
• Similar masculinized genital traits have been documented in other mammals, such as certain rodent and carnivore species, though the extent and visibility of these traits vary across taxa.{{Cite journal |last1=Cunha |first1=Gerald R. |last2=Risbridger |first2=Gail |last3=Wang |first3=Hong |last4=Place |first4=Ned J. |last5=Grumbach |first5=Mel |last6=Cunha |first6=Tristan J. |last7=Weldele |first7=Mary |last8=Conley |first8=Al J. |last9=Barcellos |first9=Dale |last10=Agarwal |first10=Sanjana |last11=Bhargava |first11=Argun |last12=Drea |first12=Christine |last13=Hammond |first13=Geoffrey L. |last14=Siiteri |first14=Penti |last15=Coscia |first15=Elizabeth M. |date=2014 |title=Development of the external genitalia: perspectives from the spotted hyena (Crocuta crocuta) |journal=Differentiation; Research in Biological Diversity |volume=87 |issue=1–2 |pages=4–22 |doi=10.1016/j.diff.2013.12.003 |issn=1432-0436 |pmc=4069199 |pmid=24582573}}

Masculinized behaviors, including increased aggression, dominance, and territoriality, are also linked to prenatal androgen exposure. Neural pathways that regulate these behaviors are shaped by androgenic effects during development:

• In meerkats, dominant females with elevated androgen levels exhibit aggressive behaviors typically associated with males, aiding in maintaining their social hierarchy.{{Cite journal |last1=Carlson |first1=Anne A |last2=Young |first2=Andrew J |last3=Russell |first3=Andrew F |last4=Bennett |first4=Nigel C |last5=McNeilly |first5=Alan S |last6=Clutton-Brock |first6=Tim |date=2004-08-01 |title=Hormonal correlates of dominance in meerkats (Suricata suricatta) |url=https://linkinghub.elsevier.com/retrieve/pii/S0018506X04000686 |journal=Hormones and Behavior |volume=46 |issue=2 |pages=141–150 |doi=10.1016/j.yhbeh.2004.01.009 |pmid=15256303 |issn=0018-506X}}
• Similar effects have been observed in banded mongooses and other cooperative breeders where reproductive competition is intense.{{Cite journal |last1=de Luca |first1=D. W. |last2=Ginsberg |first2=J. R. |date=2001-01-01 |title=Dominance, reproduction and survival in banded mongooses: towards an egalitarian social system? |url=https://linkinghub.elsevier.com/retrieve/pii/S0003347200915598 |journal=Animal Behaviour |volume=61 |issue=1 |pages=17–30 |doi=10.1006/anbe.2000.1559 |pmid=11170693 |issn=0003-3472}}

• Genetic pathways: Androgen receptors mediate the effects of hormones on target tissues. Variations in receptor density and sensitivity influence how masculinized traits manifest.{{Cite journal |last1=Traish |first1=Abdulmaged M |last2=Kim |first2=Noel |last3=Min |first3=Kweonsik |last4=Munarriz |first4=Ricardo |last5=Goldstein |first5=Irwin |date=2002-04-06 |title=Role of androgens in female genital sexual arousal: receptor expression, structure, and function |url=https://linkinghub.elsevier.com/retrieve/pii/S0015028202029783 |journal=Fertility and Sterility |volume=77 |pages=11–18 |doi=10.1016/s0015-0282(02)02978-3 |pmid=12007897 |issn=0015-0282 |via=Elsevier}}
• Epigenetic modifications: Environmental and maternal conditions can modulate the hormonal milieu during development, influencing gene expression and shaping phenotypic outcomes.{{Cite journal |last=Crews |first=David |date=2008-06-01 |title=Epigenetics and its implications for behavioral neuroendocrinology |journal=Frontiers in Neuroendocrinology |volume=29 |issue=3 |pages=344–357 |doi=10.1016/j.yfrne.2008.01.003 |issn=1095-6808 |pmc=2394853 |pmid=18358518 }}

Although the specific effects of androgens vary across species, the underlying principles of prenatal hormonal influence remain consistent. Differences in the timing, intensity, and duration of androgen exposure account for the diversity of masculinized traits observed among mammals.

The female masculinization hypothesis has been explored across various species, providing evidence for how such masculinization can manifest in different ways.{{Cite journal |last=Drea |first=Christine M. |date=2009-08-01 |title=Endocrine Mediators of Masculinization in Female Mammals |url=https://journals.sagepub.com/doi/10.1111/j.1467-8721.2009.01640.x |journal=Current Directions in Psychological Science |language=en |volume=18 |issue=4 |pages=221–226 |doi=10.1111/j.1467-8721.2009.01640.x |issn=0963-7214}}

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In rodent species, particularly laboratory mice and rats, androgen exposure during prenatal and postnatal development has been shown to alter sexual differentiation. Females exposed to elevated levels of testosterone during gestation or immediately after birth often display masculinized behaviors, including increased aggression, exploration, and mounting behavior typically associated with males.{{Cite journal |last1=Quadagno |first1=D. M. |last2=Shryne |first2=J. |last3=Anderson |first3=C. |last4=Gorski |first4=R. A. |date=1972-11-01 |title=Influence of gonadal hormones on social, sexual, emergence, and open field behaviour in the rat (Rattus norvegicus) |url=https://linkinghub.elsevier.com/retrieve/pii/S0003347272801453 |journal=Animal Behaviour |volume=20 |issue=4 |pages=732–740 |doi=10.1016/S0003-3472(72)80145-3 |pmid=4661317 |issn=0003-3472}}{{Cite journal |last1=Chu |first1=Xi |last2=Snoeren |first2=Eelke |last3=Södersten |first3=Per |last4=Ågmo |first4=Anders |date=2021-08-01 |title=Sexual incentive motivation and male and female copulatory behavior in female rats given androgen from postnatal day 20 |url=https://linkinghub.elsevier.com/retrieve/pii/S0031938421001529 |journal=Physiology & Behavior |volume=237 |pages=113460 |doi=10.1016/j.physbeh.2021.113460 |pmid=33991538 |issn=0031-9384}} These behaviors are commonly studied in rodent models that have been manipulated genetically or hormonally to study sex differences. For instance, studies on house mice (Mus musculus) and Norway rats (Rattus norvegicus) have demonstrated that females with higher prenatal testosterone levels may show reduced maternal care and increased territoriality, behaviors typically more common in males.{{Cite journal |last1=Zielinski |first1=William J. |last2=Vandenbergh |first2=John G. |last3=Montano |first3=Monica M. |date=1991-01-01 |title=Effects of social stress and intrauterine position on sexual phenotype in wild-type house mice (Mus musculus) |url=https://linkinghub.elsevier.com/retrieve/pii/003193849190241F |journal=Physiology & Behavior |volume=49 |issue=1 |pages=117–123 |doi=10.1016/0031-9384(91)90241-F |pmid=2017464 |issn=0031-9384}}{{Cite journal |last1=Pellis |first1=Sergio M. |last2=Pellis |first2=Vivien C. |last3=McKenna |first3=Mario M. |date=1994 |title=Feminine dimension in the play fighting of rats (Rattus norvegicus) and its defeminization neonatally by androgens. |url=https://doi.apa.org/doi/10.1037/0735-7036.108.1.68 |journal=Journal of Comparative Psychology |language=en |volume=108 |issue=1 |pages=68–73 |doi=10.1037/0735-7036.108.1.68 |pmid=8174346 |issn=1939-2087}}{{Cite journal |last1=Saunders |first1=Paul A. |last2=Franco |first2=Thomas |last3=Sottas |first3=Camille |last4=Maurice |first4=Tangui |last5=Ganem |first5=Guila |last6=Veyrunes |first6=Frédéric |date=2016-03-11 |title=Masculinised Behaviour of XY Females in a Mammal with Naturally Occurring Sex Reversal |journal=Scientific Reports |language=en |volume=6 |issue=1 |pages=22881 |doi=10.1038/srep22881 |pmid=26964761 |pmc=4786791 |bibcode=2016NatSR...622881S |issn=2045-2322}}{{Cite journal |last1=Biancardi |first1=Manoel F. |last2=Perez |first2=Ana P. S. |last3=Caires |first3=Cássia Regina Suzuki |last4=Góes |first4=Rejane M. |last5=Vilamaior |first5=Patrícia S. L. |last6=Santos |first6=Fernanda C. A. |last7=Taboga |first7=Sebastião R. |date=2015-09-14 |title=Prenatal exposure to testosterone masculinises the female gerbil and promotes the development of lesions in the prostate (Skene's gland) |url=https://www.publish.csiro.au/rd/RD13387 |journal=Reproduction, Fertility and Development |language=en |volume=27 |issue=7 |pages=1000–1011 |doi=10.1071/RD13387 |pmid=25483231 |issn=1448-5990}}

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In primates, particularly non-human primates such as rhesus macaques (Macaca mulatta) and vervet monkeys (Chlorocebus pygerythrus), evidence for the female masculinization hypothesis comes from observations of behaviors and morphological traits that resemble male counterparts in females with elevated androgen exposure.{{Cite journal |last1=Tomaszycki |first1=Michelle L. |last2=Gouzoules |first2=Harold |last3=Wallen |first3=Kim |date=2005 |title=Sex differences in juvenile rhesus macaque (Macaca mulatta) agonistic screams: Life history differences and effects of prenatal androgens |url=https://onlinelibrary.wiley.com/doi/10.1002/dev.20102 |journal=Developmental Psychobiology |language=en |volume=47 |issue=4 |pages=318–327 |doi=10.1002/dev.20102 |pmid=16284962 |issn=1098-2302}} Female rhesus macaques with higher levels of testosterone during the prenatal period exhibit more dominant behaviors, such as higher social ranking and increased aggression compared to their female counterparts with normal testosterone levels.{{Cite journal |last1=Wallen |first1=K. |last2=Hassett |first2=J. M. |date=2009 |title=Sexual Differentiation of Behaviour in Monkeys: Role of Prenatal Hormones |journal=Journal of Neuroendocrinology |language=en |volume=21 |issue=4 |pages=421–426 |doi=10.1111/j.1365-2826.2009.01832.x |issn=1365-2826 |pmc=2704567 |pmid=19207815}}{{Cite journal |last1=Hemelrijk |first1=Charlotte K. |last2=Wantia |first2=Jan |last3=Isler |first3=Karin |date=2008-07-16 |title=Female Dominance over Males in Primates: Self-Organisation and Sexual Dimorphism |journal=PLOS ONE |language=en |volume=3 |issue=7 |pages=e2678 |doi=10.1371/journal.pone.0002678 |doi-access=free |issn=1932-6203 |pmc=2441829 |pmid=18628830|bibcode=2008PLoSO...3.2678H }} In some primate species, females may develop more male-like physical traits, including increased size or altered secondary sexual characteristics, which can be linked to hormonal imbalances during development.{{Cite journal |last1=Zehr |first1=Julia L. |last2=Van Meter |first2=Page E. |last3=Wallen |first3=Kim |date=2005-05-01 |title=Factors Regulating the Timing of Puberty Onset in Female Rhesus Monkeys (Macaca mulatta): Role of Prenatal Androgens, Social Rank, and Adolescent Body Weight1 |url=https://academic.oup.com/biolreprod/article-abstract/72/5/1087/2667047?redirectedFrom=fulltext |journal=Biology of Reproduction |volume=72 |issue=5 |pages=1087–1094 |doi=10.1095/biolreprod.104.027755 |pmid=15625235 |issn=0006-3363}}{{Cite journal |last=Wrangham |first=R. W. |date=1981-08-01 |title=Drinking competition in vervet monkeys |url=https://linkinghub.elsevier.com/retrieve/pii/S0003347281800279 |journal=Animal Behaviour |volume=29 |issue=3 |pages=904–910 |doi=10.1016/S0003-3472(81)80027-9 |issn=0003-3472}}

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Among ungulate species such as sheep and cattle, female masculinization has also been observed under the influence of hormones. Female sheep (Ovis aries) that were exposed to high levels of testosterone during fetal development often exhibit masculine behaviors, including mounting other ewes and engaging in male-like sexual behaviors.{{Cite journal |last1=Roberts |first1=Eila K. |last2=Padmanabhan |first2=Vasantha |last3=Lee |first3=Theresa M. |date=2008-07-01 |title=Differential Effects of Prenatal Testosterone Timing and Duration on Phenotypic and Behavioral Masculinization and Defeminization of Female Sheep1 |url=https://academic.oup.com/biolreprod/article-abstract/79/1/43/2557500 |journal=Biology of Reproduction |volume=79 |issue=1 |pages=43–50 |doi=10.1095/biolreprod.107.067074 |pmid=18385446 |issn=0006-3363}} This phenomenon is thought to be due to maternal stress or hormonal fluctuations that alter the hormonal environment of the developing fetus.{{Cite journal |last1=McFadden |first1=Dennis |last2=Pasanen |first2=Edward G. |last3=Valero |first3=Michelle D. |last4=Roberts |first4=Eila K. |last5=Lee |first5=Theresa M. |date=2009-01-01 |title=Effect of prenatal androgens on click-evoked otoacoustic emissions in male and female sheep (Ovis aries) |journal=Hormones and Behavior |volume=55 |issue=1 |pages=98–105 |doi=10.1016/j.yhbeh.2008.08.013 |issn=0018-506X |pmc=2649662 |pmid=18834887}}{{Cite journal |last1=Vázquez |first1=Reyes |last2=Orihuela |first2=Agustín |last3=Aguirre |first3=Virginio |date=2014-05-01 |title=A note on the effect of number (single or twin) and sex of contemporary siblings on male-like play behavior of lambs (Ovis aries) |url=https://linkinghub.elsevier.com/retrieve/pii/S1558787814000094 |journal=Journal of Veterinary Behavior |volume=9 |issue=3 |pages=132–135 |doi=10.1016/j.jveb.2014.01.003 |issn=1558-7878}} Similarly, in cattle (Bos taurus), females with abnormal androgen levels often show signs of masculinization in terms of both behavior and morphology, such as increased body size and altered reproductive anatomy.{{Citation |last=Popescu |first=PAUL C. |title=Chromosomes of the Cow and Bull |date=1990-01-01 |work=Advances in Veterinary Science and Comparative Medicine |volume=34 |pages=41–71 |editor-last=McFeely |editor-first=Richard A. |url=https://linkinghub.elsevier.com/retrieve/pii/B9780120392346500070 |access-date=2025-01-02 |series=Domestic Animal Cytogenetics |publisher=Academic Press |doi=10.1016/b978-0-12-039234-6.50007-0|isbn=978-0-12-039234-6 }}{{Cite journal |last1=Graïc |first1=Jean-Marie |last2=Corain |first2=Livio |last3=Peruffo |first3=Antonella |last4=Cozzi |first4=Bruno |last5=Swaab |first5=Dick F. |date=2018 |title=The bovine anterior hypothalamus: Characterization of the vasopressin–oxytocin containing nucleus and changes in relation to sexual differentiation |url=https://onlinelibrary.wiley.com/doi/10.1002/cne.24542 |journal=Journal of Comparative Neurology |language=en |volume=526 |issue=17 |pages=2898–2917 |doi=10.1002/cne.24542 |pmid=30255945 |issn=1096-9861}}

File:Alpha female of the Canyon pack in the Lower Geyser Basin (67cf7f56-f9df-4c77-a389-c7050e836f87).jpg

In carnivorous mammals, such as domestic dogs (Canis lupus familiaris) and wild species like wolves (Canis lupus), female masculinization has been documented in certain breeds or populations that exhibit elevated androgen exposure, which can result in increased male-typical behaviors such as territorial marking and aggression.{{Cite journal |last1=Pavlicev |first1=Mihaela |last2=Herdina |first2=Anna Nele |last3=Wagner |first3=Günter |date=2022-09-01 |title=Female Genital Variation Far Exceeds That of Male Genitalia: A Review of Comparative Anatomy of Clitoris and the Female Lower Reproductive Tract in Theria |url=https://academic.oup.com/icb/article/62/3/581/6582219 |journal=Integrative and Comparative Biology |volume=62 |issue=3 |pages=581–601 |doi=10.1093/icb/icac026 |issn=1540-7063 |pmc=9494530 |pmid=35524696}} Studies in these species have shown that such masculinization is not solely a result of genetic differences but can also be influenced by environmental factors, such as exposure to endocrine-disrupting chemicals.{{Cite book |url=https://archive.org/details/isbn_9780791419120 |title=Conservation of endangered species in captivity : an interdisciplinary approach |date=1995 |publisher=Albany, N.Y. : State University of New York Press |others=Internet Archive |isbn=978-0-7914-1911-3}}

File:Little brown bat hanging in cave - DPLA - bcf12feccba7527205fd16b7a26b0a21.jpg

Some studies on bats, particularly in species like the little brown bat (Myotis lucifugus), have also contributed to the understanding of female masculinization. In certain populations, females exposed to high androgen levels exhibit male-like vocalizations and mating behaviors. These behaviors can influence their social roles within the group and, in some cases, may lead to alterations in reproductive success, as masculinized females may compete more aggressively for mates or territories.{{Citation |last1=Kunz |first1=Thomas H. |title=10 - Parental Care and Postnatal Growth in the Chiroptera |date=2000-01-01 |work=Reproductive Biology of Bats |pages=415–468 |editor-last=Crichton |editor-first=Elizabeth G. |url=https://linkinghub.elsevier.com/retrieve/pii/B9780121956707500114 |access-date=2025-01-02 |place=London |publisher=Academic Press |doi=10.1016/b978-012195670-7/50011-4 |isbn=978-0-12-195670-7 |last2=Hood |first2=Wendy R. |editor2-last=Krutzsch |editor2-first=Philip H.}}{{Cite journal |last1=Drake |first1=Gabrielle Jeanne-Clare |last2=Cowl |first2=Veronica |last3=Ashpole |first3=Ian |last4=Rowland |first4=Hannah |last5=White |first5=David |last6=McDonald |first6=Monica |last7=Chantrey |first7=Julian |last8=Lopez |first8=Javier |last9=Feltrer-Rambaud |first9=Yedra |date=2021-10-31 |title=The use of etonogestrel (Nexplanon®) and aglepristone (Alizin®) for population management of a colony of Rodrigues fruit bats (Pteropus rodricensis) |url=https://www.jzar.org/jzar/article/view/627 |journal=Journal of Zoo and Aquarium Research |language=en |volume=9 |issue=4 |pages=239–246 |doi=10.19227/jzar.v9i4.627 |issn=2214-7594}}

In domesticated species, female masculinization has been studied extensively due to its implications for animal husbandry and breeding. Female pigs (Sus scrofa domesticus) and horses (Equus ferus caballus) have been subjects of study in which exposure to androgens has been linked to increased male-typical behaviors.{{Citation |last=Signoret |first=J. P. |title=Sexualisation of Behavior During Development in the Pig |date=1993 |work=The Development of Sex Differences and Similarities in Behavior |pages=279–289 |editor-last=Haug |editor-first=Marc |url=https://link.springer.com/chapter/10.1007/978-94-011-1709-8_16 |access-date=2025-01-02 |place=Dordrecht |publisher=Springer Netherlands |language=en |doi=10.1007/978-94-011-1709-8_16 |isbn=978-94-011-1709-8 |editor2-last=Whalen |editor2-first=Richard E. |editor3-last=Aron |editor3-first=Claude |editor4-last=Olsen |editor4-first=Kathie L.}} In some cases, masculinized females are more aggressive, less likely to display maternal behaviors, and show altered reproductive cycles. This has led to investigations into how androgen manipulation during development can be used to improve productivity or manage behavior in domestic species.{{Cite thesis |last=Geijer-Simpson |first=Annika Vera |title=The sex biased litter in utero and its effect on post-natal health, development, and reproductive capacity of the commercial pig |date=2022-04-22 |degree=phd |publisher=University of Leeds |url=https://etheses.whiterose.ac.uk/31672/ |language=en}}{{Citation |last1=Park |first1=Jin Ho |title=Chapter 8 - Behavioral Neuroendocrinology of Reproduction in Mammals |date=2011-01-01 |work=Hormones and Reproduction of Vertebrates |pages=139–173 |editor-last=Norris |editor-first=David O. |url=https://linkinghub.elsevier.com/retrieve/pii/B9780123749284100082 |access-date=2025-01-02 |place=London |publisher=Academic Press |doi=10.1016/b978-0-12-374928-4.10008-2 |isbn=978-0-12-374928-4 |last2=Rissman |first2=Emilie F. |editor2-last=Lopez |editor2-first=Kristin H.}}

The evidence across various mammalian species supports the female masculinization hypothesis, demonstrating that androgen exposure during critical developmental windows can lead to the expression of male-typical traits. These findings have implications for understanding sexual differentiation, reproductive behavior, and social roles in mammals, as well as for applied fields like animal breeding and conservation.

Spotted hyenas are one of the most well-known examples of female masculinization. Females exhibit male-like external genitalia, including a pseudo-penis. These traits are attributed to elevated androgen levels during prenatal development. The masculinization may be linked to dominance advantages in the matriarchal social system.{{Citation |last1=Holekamp |first1=Kay E. |title=Chapter 6 - Intraspecific Variation in the Behavioral Ecology of a Tropical Carnivore, the Spotted Hyena |date=2010-01-01 |work=Advances in the Study of Behavior |volume=42 |pages=189–229 |editor-last=Macedo |editor-first=Regina |url=https://linkinghub.elsevier.com/retrieve/pii/S0065345410420069 |access-date=2025-01-09 |series=Behavioral ecology of tropical animals |publisher=Academic Press |doi=10.1016/s0065-3454(10)42006-9 |last2=Dloniak |first2=Stephanie M.|isbn=978-0-12-380894-3 }}

Female ring-tailed lemurs are behaviorally and socially dominant over males, which is hypothesized to involve elevated androgens, leading to masculinized behaviors.

In cooperative breeders like marmosets and tamarins, dominant females suppress the reproduction of subordinate females. While not strictly masculinized in appearance, their hormonal profiles and behaviors reflect dominance-associated androgenic effects.{{Cite journal |last1=Ross |first1=Corinna N. |last2=French |first2=Jeffrey A. |date=2018-06-14 |title=Female marmosets' behavioral and hormonal responses to unfamiliar intruders |journal=American Journal of Primatology |volume=73 |issue=10 |pages=1072–1081 |doi=10.1002/ajp.20975 |issn=1098-2345 |pmc=6000820 |pmid=21748772}}{{Cite journal |last1=Smith |first1=Adam S. |last2=Birnie |first2=Andrew K. |last3=French |first3=Jeffrey A. |date=2010-04-01 |title=Maternal androgen levels during pregnancy are associated with early-life growth in Geoffroy's marmosets, Callithrix geoffroyi |journal=General and Comparative Endocrinology |volume=166 |issue=2 |pages=307–313 |doi=10.1016/j.ygcen.2009.10.008 |issn=0016-6480 |pmc=2834851 |pmid=19854190}}{{Cite journal |last1=French |first1=Jeffrey A. |last2=Mustoe |first2=Aaryn C. |last3=Cavanaugh |first3=Jon |last4=Birnie |first4=Andrew K. |date=2013 |title=The influence of androgenic steroid hormones on female aggression in 'atypical' mammals |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=368 |issue=1631 |pages=20130084 |doi=10.1098/rstb.2013.0084 |issn=1471-2970 |pmc=3826213 |pmid=24167314}}

Female Asian elephants occasionally display musth-like behaviors, a condition typically associated with heightened androgens in males. This has been attributed to androgen fluctuations.{{Cite journal |last1=Ghosal |first1=Ratna |last2=Ganswindt |first2=André |last3=Seshagiri |first3=Polani B. |last4=Sukumar |first4=Raman |date=2013 |title=Endocrine correlates of musth in free-ranging Asian elephants (Elephas maximus) determined by non-invasive faecal steroid hormone metabolite measurements |journal=PLOS ONE |volume=8 |issue=12 |pages=e84787 |doi=10.1371/journal.pone.0084787 |doi-access=free |issn=1932-6203 |pmc=3866162 |pmid=24358371|bibcode=2013PLoSO...884787G }}{{Cite journal |last1=Swain |first1=Debabrata |last2=Singh |first2=L.A.K. |date=2003-08-21 |title=Musth in female Asian Elephant |url=https://zoosprint.org/index.php/zpj/article/view/6188/ |journal=Zoos' Print Journal |volume=18 |issue=9 |pages=1202 |doi=10.11609/JoTT.ZPJ.18.9.1202}}

Banded mongooses live in cooperative breeding groups where reproductive opportunities are limited, and only a subset of females breed successfully. Female mongooses exhibit elevated androgen levels, particularly during pregnancy, which influences both their behavior and physiology.{{Cite journal |last1=Kern |first1=Julie M. |last2=Morris-Drake |first2=Amy |last3=Radford |first3=Andrew N. |date=2023-08-16 |title=Behavioural, demographic and fitness consequences of social instability in cooperatively breeding dwarf mongoose groups |journal=Proceedings of the Royal Society B: Biological Sciences |volume=290 |issue=2005 |pages=20230901 |doi=10.1098/rspb.2023.0901 |pmc=10427820 |pmid=37583317}} The heightened androgens increase aggression and dominance, allowing these females to compete more effectively for access to mates and resources. In addition, masculinized traits may help dominant females suppress the reproductive efforts of subordinate group members, ensuring that their offspring receive the majority of care and resources.{{Cite journal |last1=Creel |first1=Scott |last2=Creel |first2=Nancy |last3=Wildt |first3=David E. |last4=Monfort |first4=Steven L. |date=1992-02-01 |title=Behavioural and endocrine mechanisms of reproductive suppression in Serenge dwarf mongooses |url=https://linkinghub.elsevier.com/retrieve/pii/S0003347205802192 |journal=Animal Behaviour |volume=43 |issue=2 |pages=231–245 |doi=10.1016/S0003-3472(05)80219-2 |issn=0003-3472}}{{Cite journal |last1=Müller |first1=Corsin A |last2=Manser |first2=Marta B |date=2007-01-16 |title='Nasty neighbours' rather than 'dear enemies' in a social carnivore |journal=Proceedings of the Royal Society B: Biological Sciences |volume=274 |issue=1612 |pages=959–965 |doi=10.1098/rspb.2006.0222 |pmc=2141673 |pmid=17251103}}

In naked mole rat colonies, the queen is the sole breeding female, and she dominates the social hierarchy. The queen's dominance is maintained through aggression and hormonal control, including elevated androgen levels that suppress reproduction in subordinates. While external masculinization is not prominent, behavioral masculinization through heightened aggression is clear.{{Cite journal |last1=Clarke |first1=F. M. |last2=Faulkes |first2=C. G. |date=1997-07-22 |title=Dominance and queen succession in captive colonies of the eusocial naked mole–rat, Heterocephalus glaber |journal=Proceedings of the Royal Society of London. Series B: Biological Sciences |volume=264 |issue=1384 |pages=993–1000 |doi=10.1098/rspb.1997.0137 |pmc=1688532 |pmid=9263466}}{{Cite journal |last1=Faulkes |first1=C. G. |last2=Abbott |first2=D. H. |last3=Jarvis |first3=J. U. M. |date=1991-03-01 |title=Social suppression of reproduction in male naked mole-rats, Heterocephalus glaber |url=https://rep.bioscientifica.com:443/view/journals/rep/91/2/jrf_91_2_021.xml |journal=Reproduction |language=en-US |volume=91 |issue=2 |pages=593–604 |doi=10.1530/jrf.0.0910593 |pmid=2013881 |issn=0022-4251}}

Dominant female meerkats often show elevated androgen levels, especially during pregnancy. This hormonal shift enhances aggression, allowing them to suppress subordinate females and maintain exclusive breeding rights.{{Cite journal |last1=Drea |first1=Christine M. |last2=Davies |first2=Charli S. |date=2022-09-01 |title=Meerkat manners: Endocrine mediation of female dominance and reproductive control in a cooperative breeder |url=https://linkinghub.elsevier.com/retrieve/pii/S0018506X22001398 |journal=Hormones and Behavior |volume=145 |pages=105245 |doi=10.1016/j.yhbeh.2022.105245 |pmid=35988450 |issn=0018-506X}}

Female moles possess ovotestes—gonads with both ovarian and testicular tissue—resulting in significant androgen production. This androgenization leads to traits like increased aggression, aiding in territorial defense.{{Cite journal |last1=M. Real |first1=Francisca |last2=Haas |first2=Stefan A. |last3=Franchini |first3=Paolo |last4=Xiong |first4=Peiwen |last5=Simakov |first5=Oleg |last6=Kuhl |first6=Heiner |last7=Schöpflin |first7=Robert |last8=Heller |first8=David |last9=Moeinzadeh |first9=M-Hossein |last10=Heinrich |first10=Verena |last11=Krannich |first11=Thomas |last12=Bressin |first12=Annkatrin |last13=Hartmann |first13=Michaela F. |last14=Wudy |first14=Stefan A. |last15=Dechmann |first15=Dina K. N. |date=2020-10-09 |title=The mole genome reveals regulatory rearrangements associated with adaptive intersexuality |journal=Science |volume=370 |issue=6513 |pages=208–214 |doi=10.1126/science.aaz2582 |pmc=8243244 |pmid=33033216|bibcode=2020Sci...370..208M }}{{Cite book |last1=Place |first1=Ned J. |last2=Glickman |first2=Stephen E. |chapter=Masculinization of Female Mammals: Lessons from Nature |series=Advances in Experimental Medicine and Biology |date=2004 |volume=545 |editor-last=Baskin |editor-first=Laurence S. |title=Hypospadias and Genital Development |chapter-url=https://link.springer.com/chapter/10.1007/978-1-4419-8995-6_15 |language=en |location=Boston, MA |publisher=Springer US |pages=243–253 |doi=10.1007/978-1-4419-8995-6_15 |pmid=15086031 |isbn=978-1-4419-8995-6}}{{Cite journal |last1=Whitworth |first1=Deanne J. |last2=Licht |first2=Paul |last3=Racey |first3=Paul A. |last4=Glickman |first4=Stephen E. |date=1999-02-01 |title=Testis-Like Steroidogenesis in the Ovotestis of the European Mole, Talpa europaea1 |url=https://academic.oup.com/biolreprod/article-abstract/60/2/413/2741126?redirectedFrom=fulltext |journal=Biology of Reproduction |volume=60 |issue=2 |pages=413–418 |doi=10.1095/biolreprod60.2.413 |pmid=9916009 |issn=0006-3363}}

The female masculinization hypothesis has significant evolutionary implications, providing context for the role of hormones in shaping sexual dimorphism, reproductive strategies, and social dynamics across species. Understanding how and why females may develop male-typical traits under certain conditions provides valuable insights into evolutionary pressures, mechanisms of sexual selection, and the flexibility of sexual differentiation processes in mammals.

One of the evolutionary implications of female masculinization is its potential impact on sexual selection. In species where masculinized females exhibit behaviors or physical traits typically associated with males, such as increased aggression, territoriality, or dominance, there may be changes in mating dynamics.{{Cite journal |last1=Emlen |first1=Stephen T. |last2=Oring |first2=Lewis W. |date=1977 |title=Ecology, Sexual Selection, and the Evolution of Mating Systems |url=https://www.jstor.org/stable/1744497 |journal=Science |volume=197 |issue=4300 |pages=215–223 |doi=10.1126/science.327542 |jstor=1744497 |pmid=327542 |bibcode=1977Sci...197..215E |issn=0036-8075}} Females with masculine traits could gain an advantage in securing mates or competing for resources. In species where males typically compete for dominance, masculinized females may have the opportunity to access mates more effectively or even engage in mate guarding. This shift can alter traditional mating hierarchies and reproductive strategies, especially in species where female choice plays a critical role in mate selection.{{Citation |last1=Kauffman |first1=Alexander S. |title=Female Sexual Behavior and Hormones in Mammals☆ |date=2019-01-01 |encyclopedia=Encyclopedia of Animal Behavior (Second Edition) |pages=403–419 |editor-last=Choe |editor-first=Jae Chun |url=https://linkinghub.elsevier.com/retrieve/pii/B9780128096338206870 |access-date=2025-01-02 |place=Oxford |publisher=Academic Press |doi=10.1016/b978-0-12-809633-8.20687-0 |isbn=978-0-12-813252-4 |last2=Micevych |first2=Paul E.}}{{Cite journal |last1=Sharma |first1=Usha R. |last2=Rissman |first2=Emilie F. |date=1994 |title=Testosterone Implants in Specific Neural Sites Activate Female Sexual Behaviour |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1365-2826.1994.tb00603.x |journal=Journal of Neuroendocrinology |language=en |volume=6 |issue=4 |pages=423–432 |doi=10.1111/j.1365-2826.1994.tb00603.x |pmid=7987373 |issn=1365-2826}}

In some species, feminized males—males with characteristics typically associated with females, such as nurturing behaviors or higher parental investment—have also been observed, creating a broader context for understanding sexual selection.{{Citation |last1=Sciences (US) |first1=National Academy of |title=Sexual Selection and Mating Systems |date=2009 |work=In the Light of Evolution: Volume III: Two Centuries of Darwin |url=https://www.ncbi.nlm.nih.gov/books/NBK219741/#:~:text=Darwin%20was%20first%20to%20recognize%20the%20power%20of,%E2%80%9Cspecial%20circumstances%E2%80%9D%20in%20which%20reproduction%20occurs%20within%20species. |access-date=2025-01-02 |publisher=National Academies Press (US) |language=en |last2=Avise |first2=John C. |last3=Ayala |first3=Francisco J.}} This duality between masculinized females and feminized males could reflect adaptive strategies that optimize reproductive success under varying environmental conditions.{{Cite journal |last1=Wong |first1=Bob B. M. |last2=Candolin |first2=Ulrika |date=2005 |title=How is female mate choice affected by male competition? |url=https://onlinelibrary.wiley.com/doi/10.1017/S1464793105006809 |journal=Biological Reviews |language=en |volume=80 |issue=4 |pages=559–571 |doi=10.1017/S1464793105006809 |pmid=16221329 |hdl=1912/256 |issn=1469-185X}}{{Cite journal |last=Amundsen |first=Trond |date=2018-06-01 |title=Sex roles and sexual selection: lessons from a dynamic model system |url=https://academic.oup.com/cz/article/64/3/363/4986908 |journal=Current Zoology |volume=64 |issue=3 |pages=363–392 |doi=10.1093/cz/zoy036 |issn=2396-9814 |pmc=6007278 |pmid=30402079}}

The female masculinization hypothesis also considers the evolutionary flexibility in sexual dimorphism, the condition where males and females of a species exhibit distinct physical or behavioral differences.{{Cite web |date=1994-06-16 |title=Sexual Selection {{!}} Princeton University Press |url=https://press.princeton.edu/books/paperback/9780691000572/sexual-selection |access-date=2025-01-02 |website=press.princeton.edu |language=en}} While sexual dimorphism is common in many species, it is typically seen as the result of selective pressures on males to exhibit traits that increase their chances of reproduction, such as larger size, stronger features, or more elaborate courtship displays.{{Cite book |url=https://archive.org/details/reproductivesucc0000unse |title=Reproductive success : studies of individual variation in contrasting breeding systems |date=1988 |publisher=Chicago : University of Chicago Press |others=Internet Archive |isbn=978-0-226-11058-5}} The female masculinization hypothesis challenges the rigid distinction between male and female traits, suggesting that under certain conditions—such as hormonal imbalances or environmental stress—females can also exhibit male-typical traits.{{Cite journal |last1=Berglund |first1=Anders |last2=Bisazza |first2=Angelo |last3=Pilastro |first3=Andrea |date=1996-08-01 |title=Armaments and ornaments: an evolutionary explanation of traits of dual utility |url=https://academic.oup.com/biolinnean/article-abstract/58/4/385/2662833?redirectedFrom=fulltext |journal=Biological Journal of the Linnean Society |volume=58 |issue=4 |pages=385–399 |doi=10.1111/j.1095-8312.1996.tb01442.x |issn=0024-4066}}

This flexibility could have evolutionary advantages, allowing populations to adapt to fluctuating environmental pressures. For example, in species with skewed sex ratios or where males are scarce, females with masculinized traits may be able to fill reproductive niches traditionally occupied by males, ensuring the survival and continuation of the population.{{Cite journal |last1=Hangartner |first1=Sandra |last2=Sgrò |first2=Carla M. |last3=Connallon |first3=Tim |last4=Booksmythe |first4=Isobel |date=2022 |title=Sexual dimorphism in phenotypic plasticity and persistence under environmental change: An extension of theory and meta-analysis of current data |journal=Ecology Letters |language=en |volume=25 |issue=6 |pages=1550–1565 |doi=10.1111/ele.14005 |issn=1461-0248 |pmc=9311083 |pmid=35334155|bibcode=2022EcolL..25.1550H }} In these cases, the development of masculine traits in females could be a form of sexual plasticity, helping species navigate changing ecological conditions.{{Cite journal |last1=Fox |first1=Rebecca J. |last2=Fromhage |first2=Lutz |last3=Jennions |first3=Michael D. |date=2019-01-28 |title=Sexual selection, phenotypic plasticity and female reproductive output |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=374 |issue=1768 |pages=20180184 |doi=10.1098/rstb.2018.0184 |pmc=6365872 |pmid=30966965}}{{Cite journal |last=Vilas |first=Julia Sanchez |date=2018-01-01 |title=Sexual dimorphism in response to stress |url=https://www.academia.edu/116420747 |journal=Environmental and Experimental Botany|volume=146 |page=1 |doi=10.1016/j.envexpbot.2017.12.006 |bibcode=2018EnvEB.146....1R |hdl=10347/16235 }}

Hormonal influences on sexual differentiation also have broader evolutionary implications. The female masculinization hypothesis suggests that variations in androgen exposure during critical developmental periods can lead to lasting effects on behavior and physiology, potentially influencing natural selection over time.{{Cite journal |last1=Hines |first1=Melissa |last2=Constantinescu |first2=Mihaela |last3=Spencer |first3=Debra |date=2015 |title=Early androgen exposure and human gender development |journal=Biology of Sex Differences |volume=6 |pages=3 |doi=10.1186/s13293-015-0022-1 |doi-access=free |issn=2042-6410 |pmc=4350266 |pmid=25745554}} If masculinized females exhibit enhanced survival traits—such as increased aggression that helps protect offspring or territory—these traits may be favored by natural selection, even in females. This hypothesis highlights the potential for hormones to drive evolutionary change in ways that go beyond the typical male-female distinctions.{{Cite journal |last1=Tenugu |first1=Swathi |last2=Senthilkumaran |first2=Balasubramanian |date=2022-09-01 |title=Sexual plasticity in bony fishes: Analyzing morphological to molecular changes of sex reversal |url=https://linkinghub.elsevier.com/retrieve/pii/S2468550X22000521 |journal=Aquaculture and Fisheries |series=Reproduction and Gonadal Development in Bony Fishes |volume=7 |issue=5 |pages=525–539 |doi=10.1016/j.aaf.2022.02.007 |bibcode=2022AqFis...7..525T |issn=2468-550X}}

In species with environmental stressors, such as pollution or changes in habitat, exposure to endocrine-disrupting chemicals can mimic the effects of increased androgen levels, leading to unexpected shifts in sexual behavior and traits.{{Cite journal |last1=Toppari |first1=Jorma |last2=Skakkeb˦k |first2=Niels E. |date=1998-04-01 |title=Sexual differentiation and environmental endocrine disrupters |url=https://linkinghub.elsevier.com/retrieve/pii/S0950351X98805296 |journal=Baillière's Clinical Endocrinology and Metabolism |volume=12 |issue=1 |pages=143–156 |doi=10.1016/S0950-351X(98)80529-6 |pmid=9890066 |issn=0950-351X}} These changes could have long-term evolutionary consequences, possibly influencing how species adapt to changing environments or how they respond to anthropogenic pressures.{{Cite journal |last1=Patisaul |first1=Heather B. |last2=Adewale |first2=Heather B. |date=2009-06-29 |title=Long-term effects of environmental endocrine disruptors on reproductive physiology and behavior |journal=Frontiers in Behavioral Neuroscience |language=English |volume=3 |page=10 |doi=10.3389/neuro.08.010.2009 |doi-access=free |issn=1662-5153 |pmc=2706654 |pmid=19587848}}{{Cite journal |last1=McWhinnie |first1=Kirsty |last2=Negi |first2=Deepti |last3=Tanner |first3=K. Elizabeth |last4=Parsons |first4=Kevin J. |date=2023-11-28 |title=Functional trait plasticity diverges between sexes in African cichlids: A contribution toward ecological sexual dimorphism? |journal=Ecology and Evolution |volume=13 |issue=11 |pages=e10702 |doi=10.1002/ece3.10702 |issn=2045-7758 |pmid=38034329 |pmc=10682861 |bibcode=2023EcoEv..1310702M }}

The expression of male-typical traits in females can also affect social structures and roles within species. In social mammals, particularly those with complex social hierarchies (such as primates, wolves, or elephants), masculinized females may challenge traditional gender roles, leading to shifts in leadership, dominance, and cooperative behaviors.{{Cite web |last=Novak |first=Sara |title=Females Dominate Males in Many Primate Species |url=https://www.scientificamerican.com/article/females-dominate-males-in-many-primate-species/ |access-date=2025-01-02 |website=Scientific American |language=en}} These females may form alternative mating strategies or leadership structures, which could alter the overall dynamics of group living.{{Cite journal |last1=Hemelrijk |first1=Charlotte K. |last2=Wantia |first2=Jan |last3=Isler |first3=Karin |date=2008-07-16 |title=Female dominance over males in primates: self-organisation and sexual dimorphism |journal=PLOS ONE |volume=3 |issue=7 |pages=e2678 |doi=10.1371/journal.pone.0002678 |doi-access=free |issn=1932-6203 |pmc=2441829 |pmid=18628830|bibcode=2008PLoSO...3.2678H }}{{Cite journal |last=Lewis |first=Rebecca J. |date=2018 |title=Female Power in Primates and the Phenomenon of Female Dominance |url=https://www.jstor.org/stable/48550921 |journal=Annual Review of Anthropology |volume=47 |pages=533–551 |doi=10.1146/annurev-anthro-102317-045958 |jstor=48550921 |issn=0084-6570}}

The evolutionary impact of such shifts is particularly relevant in species where cooperation between males and females is key to survival.{{Cite web |last=Waal |first=Frans de |date=2014-09-01 |title=Why Humans and Other Primates Cooperate |url=https://www.scientificamerican.com/article/why-humans-and-other-primates-cooperate/ |access-date=2025-01-02 |website=Scientific American |language=en}}{{Cite journal |last=Bertram |first=Brian C. R. |date=1975 |title=The Social System of Lions |url=https://www.jstor.org/stable/24949799 |journal=Scientific American |volume=232 |issue=5 |pages=54–65 |doi=10.1038/scientificamerican0575-54 |jstor=24949799 |bibcode=1975SciAm.232e..54B |issn=0036-8733}} If masculinized females are able to contribute to or even dominate certain aspects of the social structure (such as by engaging in male-typical behaviors like aggression or territory defense), this could drive changes in the evolution of cooperation, leadership, and resource distribution within groups.{{Cite journal |last1=Robinson |first1=Elva J. H. |last2=Barker |first2=Jessica L. |date=2017-03-01 |title=Inter-group cooperation in humans and other animals |journal=Biology Letters |volume=13 |issue=3 |pages=20160793 |doi=10.1098/rsbl.2016.0793 |pmc=5377026 |pmid=28250206 }}

The evolution of parental investment is another critical area affected by female masculinization. In species where males typically invest heavily in parental care, the appearance of masculinized females who take on more active roles in offspring care or territorial defense could alter the reproductive strategies of both sexes.{{Cite web |title=Parental Investment and Sexual Selection |url=https://www.researchgate.net/publication/288653750 |archive-url=http://web.archive.org/web/20231211190144/https://www.researchgate.net/publication/288653750_Parental_Investment_and_Sexual_Selection |archive-date=2023-12-11 |access-date=2025-01-02 |website=ResearchGate |language=en}}{{Cite journal |last1=Klug |first1=Hope |last2=Bonsall |first2=Michael B. |date=2010-03-01 |title=Life History and the Evolution of Parental Care |url=https://academic.oup.com/evolut/article-abstract/64/3/823/6853592?redirectedFrom=fulltext |journal=Evolution |volume=64 |issue=3 |pages=823–835 |doi=10.1111/j.1558-5646.2009.00854.x |pmid=19796144 |issn=0014-3820}} In certain rodent species, masculinized females may be more aggressive in defending nests or resources, potentially improving offspring survival. This shift in parental roles may influence the evolution of parenting behaviors across generations, reshaping the traditional division of labor between sexes.{{Cite journal |last1=Rymer |first1=T. L. |last2=Pillay |first2=N. |date=2018 |title=An integrated understanding of paternal care in mammals: lessons from the rodents |url=https://zslpublications.onlinelibrary.wiley.com/doi/10.1111/jzo.12575 |journal=Journal of Zoology |language=en |volume=306 |issue=2 |pages=69–76 |doi=10.1111/jzo.12575 |issn=1469-7998}}{{Cite journal |last=Wynne-Edwards |first=Katherine E. |date=2001-09-01 |title=Hormonal Changes in Mammalian Fathers |url=https://linkinghub.elsevier.com/retrieve/pii/S0018506X0191699X |journal=Hormones and Behavior |volume=40 |issue=2 |pages=139–145 |doi=10.1006/hbeh.2001.1699 |pmid=11534974 |issn=0018-506X}}

While the female masculinization hypothesis has gained significant interest in the fields of endocrinology, evolutionary biology, and animal behavior, it also has critiques and controversies. These include concerns regarding the generalizability of findings, the complexity of hormonal interactions, and the ethical implications of research in this area.

One major critique of the female masculinization hypothesis is the potential over-simplification of the relationship between hormonal exposure and the development of masculine traits. While androgens like testosterone play a central role in sexual differentiation, critics argue that the hormonal environment is complex and influenced by a multitude of factors.{{Cite journal |last1=McEwen |first1=Bruce S. |last2=Milner |first2=Teresa A. |date=2017-01-02 |title=Understanding the broad influence of sex hormones and sex differences in the brain |journal=Journal of Neuroscience Research |volume=95 |issue=1–2 |pages=24–39 |doi=10.1002/jnr.23809 |issn=1097-4547 |pmc=5120618 |pmid=27870427}}{{Cite journal |last1=Negri-Cesi |first1=P. |last2=Colciago |first2=A. |last3=Celotti |first3=F. |last4=Motta |first4=M. |date=2004 |title=Sexual differentiation of the brain: role of testosterone and its active metabolites |url=https://pubmed.ncbi.nlm.nih.gov/15481811/ |journal=Journal of Endocrinological Investigation |volume=27 |issue=6 Suppl |pages=120–127 |issn=0391-4097 |pmid=15481811}} These include genetic variation, environmental influences, and interactions between different hormones, which can complicate the interpretation of findings. Elevated androgen levels may not consistently lead to masculinized behaviors or characteristics, and individuals may exhibit a range of responses depending on their genetic and developmental context. Attributing masculine traits solely to hormonal imbalances may not fully capture the complexity of sexual differentiation.{{Cite journal |last=Balthazart |first=Jacques |date=2019-10-01 |title=New concepts in the study of the sexual differentiation and activation of reproductive behavior, a personal view |journal=Frontiers in Neuroendocrinology |volume=55 |pages=100785 |doi=10.1016/j.yfrne.2019.100785 |issn=1095-6808 |pmc=6858558 |pmid=31430485 }}{{Cite journal |last1=Matsushita |first1=Shoko |last2=Suzuki |first2=Kentaro |last3=Murashima |first3=Aki |last4=Kajioka |first4=Daiki |last5=Acebedo |first5=Alvin Resultay |last6=Miyagawa |first6=Shinichi |last7=Haraguchi |first7=Ryuma |last8=Ogino |first8=Yukiko |last9=Yamada |first9=Gen |date=2018-06-18 |title=Regulation of masculinization: androgen signalling for external genitalia development |url=https://www.nature.com/articles/s41585-018-0008-y |journal=Nature Reviews Urology |language=en |volume=15 |issue=6 |pages=358–368 |doi=10.1038/s41585-018-0008-y |pmid=29670181 |issn=1759-4820}}

Another area of concern is the potential bias in the research models used to study female masculinization. Much of the research in this area has been conducted in species with well-established laboratory populations or domesticated animals, which may not accurately represent natural variation in wild populations.{{Cite journal |last=Sjoberg |first=Espen A. |date=2017-02-15 |title=Logical fallacies in animal model research |journal=Behavioral and Brain Functions |volume=13 |issue=1 |pages=3 |doi=10.1186/s12993-017-0121-8 |issn=1744-9081 |pmc=5312558 |pmid=28202023 |doi-access=free}}{{Cite journal |last1=Barroca |first1=Nayara Cobra Barreiro |last2=Della Santa |first2=Giovanna |last3=Suchecki |first3=Deborah |last4=García-Cairasco |first4=Norberto |last5=Umeoka |first5=Eduardo Henrique de Lima |date=2022-09-01 |title=Challenges in the use of animal models and perspectives for a translational view of stress and psychopathologies |url=https://linkinghub.elsevier.com/retrieve/pii/S0149763422002603 |journal=Neuroscience & Biobehavioral Reviews |volume=140 |pages=104771 |doi=10.1016/j.neubiorev.2022.104771 |issn=0149-7634 |pmid=35817171}} These species may be subject to artificial selection pressures or other human interventions that could influence the expression of masculinized traits. In addition, the focus on a small number of model species may limit the generalizability of findings to other mammals, especially those with different ecological niches, social structures, or reproductive strategies.{{Cite book |url=https://academic.oup.com/book/51178 |title=Phenotypic Plasticity |date=2004-01-15 |publisher=Oxford University PressNew York, NY |isbn=978-0-19-513896-2 |editor-last=DeWitt |editor-first=Thomas J |language=en |doi=10.1093/oso/9780195138962.001.0001 |editor-last2=Scheiner |editor-first2=Samuel M}}{{Cite journal |last1=Kaiser |first1=Sylvia |last2=Hennessy |first2=Michael B. |last3=Sachser |first3=Norbert |date=2015-08-24 |title=Domestication affects the structure, development and stability of biobehavioural profiles |journal=Frontiers in Zoology |volume=12 |issue=1 |pages=S19 |doi=10.1186/1742-9994-12-S1-S19 |issn=1742-9994 |pmc=5385816 |pmid=28400853 |doi-access=free}}

The manipulation of hormones in animal models to study female masculinization also raises ethical concerns, particularly when it comes to animal welfare. Hormonal interventions, such as administering synthetic androgens or altering prenatal hormone exposure, can have unintended consequences on the health and well-being of the animals involved.{{Citation |last1=Stokes |first1=William S. |title=Chapter 9 - Animal Welfare Considerations in Biomedical Research and Testing |date=2014-01-01 |work=Laboratory Animal Welfare |pages=115–140 |editor-last=Bayne |editor-first=Kathryn |url=https://linkinghub.elsevier.com/retrieve/pii/B9780123851031000099 |access-date=2025-01-02 |series=American College of Laboratory Animal Medicine |place=Boston |publisher=Academic Press |doi=10.1016/b978-0-12-385103-1.00009-9 |isbn=978-0-12-385103-1 |last2=Marsman |first2=Daniel S. |editor2-last=Turner |editor2-first=Patricia V.}} For example, exposing female animals to elevated testosterone may result in physical deformities, altered reproductive function, or behavioral changes that are not typical for their species. These concerns highlight the need for careful consideration of the ethical implications of such studies, especially in terms of balancing scientific advancement with the responsibility to minimize harm to research subjects.{{Cite journal |last1=Patisaul |first1=Heather B. |last2=Fenton |first2=Suzanne E. |last3=Aylor |first3=David |date=2018-06-01 |title=Animal models of endocrine disruption |journal=Best Practice & Research. Clinical Endocrinology & Metabolism |volume=32 |issue=3 |pages=283–297 |doi=10.1016/j.beem.2018.03.011 |issn=1878-1594 |pmc=6029710 |pmid=29779582}}{{Cite journal |last1=Qaid |first1=Mohammed M. |last2=Abdoun |first2=Khalid A. |date=2022-12-31 |title=Safety and concerns of hormonal application in farm animal production: a review |url=https://www.tandfonline.com/doi/full/10.1080/09712119.2022.2089149 |journal=Journal of Applied Animal Research |volume=50 |issue=1 |pages=426–439 |doi=10.1080/09712119.2022.2089149 |issn=0971-2119 |via=Tandfonline}}

The interpretation of behavioral changes in masculinized females is also a point of contention. Critics argue that behaviors associated with masculinization, such as aggression, increased territoriality, or dominance, may be more nuanced than simply being male-typical traits. Many behaviors attributed to masculinization may reflect adaptive responses to environmental pressures, social structures, or reproductive strategies rather than a direct expression of male-like traits.{{Cite journal |title=Hormones and neuroplasticity: A lifetime of... : Neuroscience and Biobehavioral Reviews |url=https://www.ovid.com/journals/nebior/abstract/10.1016/j.neubiorev.2021.11.029 |access-date=2025-01-02 |journal=Neuroscience and Biobehavioral Reviews |date=2022 |language=en |doi=10.1016/j.neubiorev.2021.11.029 |pmid=34808191 |last1=Been |first1=L. E. |author2=Sheppard PAS |author3=Galea LAM |last4=Glasper |first4=E. R. |volume=132 |pages=679–690 }} This raises questions about whether behaviors like aggression or dominance truly represent a form of masculinization or whether they are part of broader strategies to cope with ecological or social challenges. As such, some researchers caution against over-interpreting behavioral differences as a clear sign of masculinization without considering the full ecological and social context.{{Cite journal |last1=Hau |first1=Michaela |last2=Goymann |first2=Wolfgang |date=2015-08-24 |title=Endocrine mechanisms, behavioral phenotypes and plasticity: known relationships and open questions |journal=Frontiers in Zoology |volume=12 |issue=1 |pages=S7 |doi=10.1186/1742-9994-12-S1-S7 |doi-access=free |issn=1742-9994 |pmc=4722346 |pmid=26816524}}{{Cite journal |last1=Holekamp |first1=Kay E |last2=Strauss |first2=Eli D |date=2016-12-01 |title=Aggression and dominance: an interdisciplinary overview |url=https://linkinghub.elsevier.com/retrieve/pii/S2352154616301553 |journal=Current Opinion in Behavioral Sciences |series=Behavioral ecology |volume=12 |pages=44–51 |doi=10.1016/j.cobeha.2016.08.005 |issn=2352-1546}}

The concept of female masculinization has also faced criticism for being potentially misinterpreted in a sociocultural context. Some critics argue that framing female behaviors or traits as "masculine" could reinforce gender stereotypes and perpetuate harmful biases about what is considered "appropriate" behavior for women, both in humans and in animals.{{Citation |last1=Differences |first1=Institute of Medicine (US) Committee on Understanding the Biology of Sex and Gender |title=The Future of Research on Biological Sex Differences: Challenges and Opportunities |date=2001 |work=Exploring the Biological Contributions to Human Health: Does Sex Matter? |url=https://www.ncbi.nlm.nih.gov/books/NBK222296/ |access-date=2025-01-02 |publisher=National Academies Press (US) |language=en |last2=Wizemann |first2=Theresa M. |last3=Pardue |first3=Mary-Lou}} These concerns highlight the importance of using neutral language when discussing masculinization in the animal kingdom, as there is the risk that such discussions could be misapplied to human social dynamics in ways that misrepresent biological phenomena.{{Cite journal |last=Roselli |first=C. E. |date=2018 |title=Neurobiology of gender identity and sexual orientation |journal=Journal of Neuroendocrinology |language=en |volume=30 |issue=7 |pages=e12562 |doi=10.1111/jne.12562 |issn=1365-2826 |pmc=6677266 |pmid=29211317}}{{Citation |last1=Wood |first1=Wendy |title=Chapter two - Biosocial Construction of Sex Differences and Similarities in Behavior |date=2012-01-01 |work=Advances in Experimental Social Psychology |volume=46 |pages=55–123 |editor-last=Olson |editor-first=James M. |url=https://linkinghub.elsevier.com/retrieve/pii/B9780123942814000027 |access-date=2025-01-02 |publisher=Academic Press |doi=10.1016/b978-0-12-394281-4.00002-7 |last2=Eagly |first2=Alice H. |editor2-last=Zanna |editor2-first=Mark P.}}

Some researchers have raised questions about whether the concept of female masculinization is universally applicable across species. While the hypothesis has been supported in several mammalian species, there are notable exceptions where females do not exhibit significant masculinized traits, even when exposed to elevated androgen levels.{{Cite journal |last1=Davies |first1=William |last2=Wilkinson |first2=Lawrence S. |date=2006-12-18 |title=It is not all hormones: Alternative explanations for sexual differentiation of the brain |url=https://linkinghub.elsevier.com/retrieve/pii/S000689930602854X |journal=Brain Research |series=Sex, Genes and Steroids |volume=1126 |issue=1 |pages=36–45 |doi=10.1016/j.brainres.2006.09.105 |pmid=17101121 |issn=0006-8993}} Additionally, in species with highly rigid sex roles or clear-cut sexual dimorphism, the potential for female masculinization may be limited or absent altogether. This has led some to argue that the hypothesis may not be as broadly relevant as initially thought and that its application may be constrained by factors such as evolutionary history, ecological context, and species-specific reproductive strategies.{{Cite journal |last=Shine |first=Richard |date=1989 |title=Ecological Causes for the Evolution of Sexual Dimorphism: A Review of the Evidence |url=https://www.jstor.org/stable/2830103 |journal=The Quarterly Review of Biology |volume=64 |issue=4 |pages=419–461 |doi=10.1086/416458 |jstor=2830103 |pmid=2697022 |issn=0033-5770}}{{Cite journal |last1=Kappeler |first1=Peter M. |last2=Benhaiem |first2=Sarah |last3=Fichtel |first3=Claudia |last4=Fromhage |first4=Lutz |last5=Höner |first5=Oliver P. |last6=Jennions |first6=Michael D. |last7=Kaiser |first7=Sylvia |last8=Krüger |first8=Oliver |last9=Schneider |first9=Jutta M. |last10=Tuni |first10=Cristina |last11=van Schaik |first11=Jaap |last12=Goymann |first12=Wolfgang |date=2023 |title=Sex roles and sex ratios in animals |url=https://onlinelibrary.wiley.com/doi/10.1111/brv.12915 |journal=Biological Reviews |language=en |volume=98 |issue=2 |pages=462–480 |doi=10.1111/brv.12915 |pmid=36307924 |issn=1469-185X}}

While much of the research on the female masculinization hypothesis has focused on mammals, this concept has also been explored in other vertebrate and invertebrate species, where hormonal exposure during development can result in masculinized traits in females. Studies in birds, reptiles, amphibians, fish, and even some invertebrates have provided valuable insights into the broader applicability of the hypothesis and the role of androgens in shaping sexual differentiation across species.

File:Alectoris chukar 3 - Israel.jpg

In birds, particularly in species like domestic chickens (Gallus gallus domesticus) and chukars (Alectoris chukar), hormonal manipulation during early development has been shown to induce masculinized behaviors in females.{{Cite report |url=https://peerj.com/preprints/26994/ |title=Hens (Gallus gallus domesticus) with larger spurs have higher faecal testosterone levels; evidence of female to male - like phenotype |last1=Kent |first1=John P. |last2=Hynes |first2=Niamh M. |last3=Hayden |first3=Thomas J. |last4=Murphy |first4=Kenneth J. |last5=O'Dywer |first5=Laurence |date=2018-06-20 |publisher=PeerJ Preprints |issue=e26994v1 |language=en}} Elevated levels of androgens, such as testosterone, can lead to changes in aggression, territoriality, and courtship behaviors. Female chickens exposed to high levels of testosterone during embryonic development may display male-typical behaviors, such as increased aggression toward other females and more active participation in territorial defense.{{Cite journal |last1=Jessl |first1=Luzie |last2=Lenz |first2=Rebecca |last3=Massing |first3=Fabian G. |last4=Scheider |first4=Jessica |last5=Oehlmann |first5=Jörg |date=2018-07-03 |title=Effects of estrogens and antiestrogens on gonadal sex differentiation and embryonic development in the domestic fowl (Gallus gallus domesticus) |journal=PeerJ |language=en |volume=6 |pages=e5094 |doi=10.7717/peerj.5094 |doi-access=free |pmid=30002959 |pmc=6034593 |issn=2167-8359}} Similarly, in some species of quail, females exposed to elevated androgens show more dominant behaviors in social hierarchies and engage in competitive interactions usually associated with males.{{Cite journal |last=West |first=J. L. |date=1974 |title=Case Report: Arrhenoblastoma in a Chukar Partridge |url=https://www.jstor.org/stable/1589136 |journal=Avian Diseases |volume=18 |issue=2 |pages=258–261 |doi=10.2307/1589136 |jstor=1589136 |issn=0005-2086}}{{Cite journal |last=Adkins-Regan |first=Elizabeth |date=1985-08-01 |title=Exposure of embryos to an aromatization inhibitor increases copulatory behaviour of male quail |url=https://linkinghub.elsevier.com/retrieve/pii/0376635785900567 |journal=Behavioural Processes |volume=11 |issue=2 |pages=153–158 |doi=10.1016/0376-6357(85)90056-7 |pmid=24895921 |issn=0376-6357}}

Research in birds has been instrumental in understanding the neuroendocrine mechanisms underlying sexual differentiation, particularly how early exposure to sex hormones can permanently alter the development of brain regions associated with behavior, including aggression and mating competition.{{Cite journal |last=Paster |first=Michael B. |date=1991-11-01 |title=Avian Reproductive Endocrinology |url=https://linkinghub.elsevier.com/retrieve/pii/S0195561691501431 |journal=Veterinary Clinics of North America: Small Animal Practice |volume=21 |issue=6 |pages=1343–1359 |doi=10.1016/S0195-5616(91)50143-1 |pmid=1767479 |issn=0195-5616}} These studies also demonstrate the flexibility of sexual behavior and the potential for females to express traits traditionally associated with males under certain hormonal conditions.{{Cite journal |last1=Major |first1=Andrew T. |last2=Smith |first2=Craig A. |date=2016-08-17 |title=Sex Reversal in Birds |url=https://karger.com/sxd/article-abstract/10/5-6/288/296439/Sex-Reversal-in-Birds?redirectedFrom=fulltext |journal=Sexual Development |volume=10 |issue=5–6 |pages=288–300 |doi=10.1159/000448365 |pmid=27529790 |issn=1661-5425}}{{Cite journal |last1=Arnold |first1=Arthur P. |last2=Itoh |first2=Yuichiro |date=2011-07-01 |title=Factors Causing Sex differences in Birds |journal=Avian Biology Research |language=en |volume=4 |issue=2 |pages=44–51 |doi=10.3184/175815511X13070045977959 |issn=1758-1559 |pmc=3864897 |pmid=24353746}}

File:Female Gambusia affinis UMFS 2015.jpg

Fish species provide another diverse group where the female masculinization hypothesis has been applied. In species like the zebrafish (Danio rerio) and the mosquito fish (Gambusia affinis), researchers have manipulated hormonal exposure to examine its effects on female behavior and physiology.{{Cite journal |last1=Lee |first1=Stephanie L J |last2=Horsfield |first2=Julia A |last3=Black |first3=Michael A |last4=Rutherford |first4=Kim |last5=Gemmell |first5=Neil J |date=2018-08-01 |title=Identification of sex differences in zebrafish (Danio rerio) brains during early sexual differentiation and masculinization using 17α-methyltestoterone† |url=https://academic.oup.com/biolreprod/article-abstract/99/2/446/4767998?redirectedFrom=fulltext |journal=Biology of Reproduction |volume=99 |issue=2 |pages=446–460 |doi=10.1093/biolre/iox175 |pmid=29272338 |issn=0006-3363}}{{Cite journal |last1=Hou |first1=Li-Ping |last2=Yang |first2=Yang |last3=Shu |first3=Hu |last4=Ying |first4=Guang-Guo |last5=Zhao |first5=Jian-Liang |last6=Fang |first6=Gui-Zhen |last7=Xin |first7=Li |last8=Shi |first8=Wen-Jun |last9=Yao |first9=Li |last10=Cheng |first10=Xue-Mei |date=2018-01-01 |title=Masculinization and reproductive effects in western mosquitofish (Gambusia affinis) after long-term exposure to androstenedione |url=https://linkinghub.elsevier.com/retrieve/pii/S0147651317304992 |journal=Ecotoxicology and Environmental Safety |volume=147 |pages=509–515 |doi=10.1016/j.ecoenv.2017.08.004 |pmid=28915398 |bibcode=2018EcoES.147..509H |issn=0147-6513}} In other fish species, females exposed to elevated levels of androgens during development exhibit male-like traits, including increased aggression, territorial behavior, and changes in mate choice preferences. In the case of the mosquito fish, masculinized females may compete with males for mates or display more dominant behaviors in social groups.{{Cite journal |last1=Chen |first1=Lu |last2=Wang |first2=Li |last3=Cheng |first3=Qiwei |last4=Tu |first4=Yi-Xuan |last5=Yang |first5=Zhuang |last6=Li |first6=Run-Ze |last7=Luo |first7=Zhi-Hui |last8=Chen |first8=Zhen-Xia |date=2020-01-07 |title=Anti-masculinization induced by aromatase inhibitors in adult female zebrafish |journal=BMC Genomics |language=en |volume=21 |issue=1 |pages=22 |doi=10.1186/s12864-019-6437-z |doi-access=free |issn=1471-2164 |pmc=6947999 |pmid=31910818}}{{Cite journal |last1=Deaton |first1=Raelynn |last2=Cureton |first2=James C. |date=2011-12-01 |title=Female masculinization and reproductive life history in the western mosquitofish (Gambusia affinis) |url=https://link.springer.com/article/10.1007/s10641-011-9878-z |journal=Environmental Biology of Fishes |language=en |volume=92 |issue=4 |pages=551–558 |doi=10.1007/s10641-011-9878-z |bibcode=2011EnvBF..92..551D |issn=1573-5133}}

Fish are often used in studies on endocrine-disrupting chemicals (EDCs), which can lead to masculinized or feminized traits depending on the timing and level of exposure to environmental contaminants. These studies highlight the potential for EDCs to affect sexual differentiation in fish and the broader ecological consequences of widespread chemical pollutants on reproductive and social behaviors in aquatic environments.{{Cite journal |last1=Baumann |first1=Lisa |last2=Knörr |first2=Susanne |last3=Keiter |first3=Susanne |last4=Nagel |first4=Tina |last5=Segner |first5=Helmut |last6=Braunbeck |first6=Thomas |date=2015-11-01 |title=Prochloraz causes irreversible masculinization of zebrafish (Danio rerio) |url=https://link.springer.com/article/10.1007/s11356-014-3486-3 |journal=Environmental Science and Pollution Research |language=en |volume=22 |issue=21 |pages=16417–16422 |doi=10.1007/s11356-014-3486-3 |pmid=25163568 |bibcode=2015ESPR...2216417B |issn=1614-7499}}{{Cite journal |last1=Hou |first1=Liping |last2=Chen |first2=Shangduo |last3=Chen |first3=Hongxing |last4=Ying |first4=Guangguo |last5=Chen |first5=Diyun |last6=Liu |first6=Juan |last7=Liang |first7=Ye |last8=Wu |first8=Rongrong |last9=Fang |first9=Xuwen |last10=Zhang |first10=Cuiping |last11=Xie |first11=Lingtian |date=2019-02-01 |title=Rapid masculinization and effects on the liver of female western mosquitofish (Gambusia affinis) by norethindrone |url=https://linkinghub.elsevier.com/retrieve/pii/S004565351831991X |journal=Chemosphere |volume=216 |pages=94–102 |doi=10.1016/j.chemosphere.2018.10.130 |pmid=30359922 |bibcode=2019Chmsp.216...94H |issn=0045-6535}}

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The female masculinization hypothesis has also been explored in amphibians, particularly in species with environmental sex determination. In amphibians such as frogs and toads, elevated levels of androgens during critical periods of development can result in masculinized physical traits and altered mating behaviors in females. In some species of frogs, females exposed to increased levels of testosterone can exhibit male-typical behaviors, such as increased aggression or competition for mates, and may even develop male-like reproductive organs.{{Cite journal |last=Eggert |first=Christophe |date=2004-11-01 |title=Sex determination: the amphibian models |url=https://rnd.edpsciences.org/articles/rnd/abs/2004/07/r4608/r4608.html |journal=Reproduction Nutrition Development |language=en |volume=44 |issue=6 |pages=539–549 |doi=10.1051/rnd:2004062 |pmid=15762298 |issn=0926-5287}}{{Cite journal |last1=Wallace |first1=H. |last2=Badawy |first2=G. M. I. |last3=Wallace |first3=B. M. N. |date=1999-06-01 |title=Amphibian sex determination and sex reversal |journal=Cellular and Molecular Life Sciences CMLS |language=en |volume=55 |issue=6 |pages=901–909 |doi=10.1007/s000180050343 |issn=1420-9071 |pmc=11147005 |pmid=10412371}}

These studies contribute to our understanding of how environmental factors, such as temperature and hormonal exposure, interact to influence sexual differentiation in amphibians. They also highlight the potential for environmental stressors to disrupt the typical development of sexual traits in both males and females.{{Cite journal |last1=Orton |first1=Frances |last2=Tyler |first2=Charles R. |date=2015 |title=Do hormone-modulating chemicals impact on reproduction and development of wild amphibians? |url=https://onlinelibrary.wiley.com/doi/10.1111/brv.12147 |journal=Biological Reviews |language=en |volume=90 |issue=4 |pages=1100–1117 |doi=10.1111/brv.12147 |pmid=25335651 |issn=1469-185X}}{{Cite thesis |last=Archer |first=Edward |title=Androgen controlled secondary sexual characters in the male African clawed frog, Xenopus laevis, as potential biomarkers for endocrine disruptor contaminants (with special reference to fungicides) in aquatic systems |date=2014-04-16 |publisher=Stellenbosch : Stellenbosch University |url=https://scholar.sun.ac.za/handle/10019.1/86508 |language=en-ZA}}{{Cite journal |last1=Wallace |first1=H. |last2=Wallace |first2=B. M. |date=2000-10-01 |title=Sex reversal of the newt Triturus cristatus reared at extreme temperatures |url=https://pubmed.ncbi.nlm.nih.gov/11128575/ |journal=The International Journal of Developmental Biology |volume=44 |issue=7 |pages=807–810 |issn=0214-6282 |pmid=11128575}}

File:AcarolbrIVI.JPG

In reptiles, particularly species with temperature-dependent sex determination (TSD) such as certain turtles and lizards, researchers have observed the impact of hormonal exposure on the masculinization of females.{{Cite journal |last1=Holleley |first1=Clare E. |last2=Sarre |first2=Stephen D. |last3=O'Meally |first3=Denis |last4=Georges |first4=Arthur |date=2016-10-29 |title=Sex Reversal in Reptiles: Reproductive Oddity or Powerful Driver of Evolutionary Change? |url=https://karger.com/sxd/article-abstract/10/5-6/279/296441/Sex-Reversal-in-Reptiles-Reproductive-Oddity-or?redirectedFrom=fulltext |journal=Sexual Development |volume=10 |issue=5–6 |pages=279–287 |doi=10.1159/000450972 |pmid=27794577 |issn=1661-5425}} Although the primary determinant of sex in these species is temperature, additional exposure to androgens can masculinize females, influencing both physical traits and behavior. In some species of lizards, females exposed to elevated androgen levels during their development may exhibit male-typical behaviors, including increased aggression and altered mating strategies.{{Cite journal |last1=Ehl |first1=Jan |last2=Vukić |first2=Jasna |last3=Kratochvíl |first3=Lukáš |date=2017-11-01 |title=Hormonal and thermal induction of sex reversal in the bearded dragon (Pogona vitticeps, Agamidae) |url=https://linkinghub.elsevier.com/retrieve/pii/S0044523117300931 |journal=Zoologischer Anzeiger |volume=271 |pages=1–5 |doi=10.1016/j.jcz.2017.11.002 |bibcode=2017ZooAn.271....1E |issn=0044-5231}}{{Cite journal |last1=Milnes |first1=Matthew R. |last2=Robinson |first2=Christopher D. |last3=Foley |first3=Alexis P. |last4=Stepp |first4=Charleigh |last5=Hale |first5=Matthew D. |last6=John-Alder |first6=Henry B. |last7=Cox |first7=Robert M. |date=2024-01-15 |title=Effects of testosterone on urogenital tract morphology and androgen receptor expression in immature Eastern Fence lizards (Sceloporus undulatus) |url=https://linkinghub.elsevier.com/retrieve/pii/S001664802300223X |journal=General and Comparative Endocrinology |volume=346 |pages=114418 |doi=10.1016/j.ygcen.2023.114418 |pmid=38036014 |issn=0016-6480}}

When studying the interaction between genetic, environmental, and hormonal factors in sexual differentiation, TSD species provide a unique context in which the female masculinization hypothesis can be tested.{{Cite journal |last1=Stubbs |first1=Jessica L. |last2=Kearney |first2=Michael R. |last3=Whiting |first3=Scott D. |last4=Mitchell |first4=Nicola J. |date=2014-10-22 |title=Models of primary sex ratios at a major flatback turtle rookery show an anomalous masculinising trend |journal=Climate Change Responses |language=en |volume=1 |issue=1 |pages=3 |doi=10.1186/s40665-014-0003-3 |doi-access=free |bibcode=2014ClChR...1....3S |issn=2053-7565}} The role of hormonal exposure in these species also raises concerns about how climate change might influence sex ratios and behavioral patterns within reptile populations.{{Cite journal |last1=Dutton |first1=P. H. |last2=Whitmore |first2=C. P. |last3=Mrosovsky |first3=N. |date=1985-01-01 |title=Masculinisation of leatherback turtle Dermochelys coriacea hatchlings from eggs incubated in styrofoam boxes |url=https://linkinghub.elsevier.com/retrieve/pii/0006320785900709 |journal=Biological Conservation |volume=31 |issue=3 |pages=249–264 |doi=10.1016/0006-3207(85)90070-9 |bibcode=1985BCons..31..249D |issn=0006-3207}}

File:Standing female Drosophila melanogaster.jpg

The female masculinization hypothesis has been explored in some invertebrate species, particularly in relation to hormonal control of sexual differentiation. In certain species of insects, such as the fruit fly (Drosophila melanogaster), exposure to sex hormones during development can influence the expression of male-typical traits in females.{{Cite journal |last1=Slee |first1=Roger |last2=Bownes |first2=Mary |date=1990-06-01 |title=Sex Determination in Drosophila melanogaster |url=https://www.journals.uchicago.edu/doi/abs/10.1086/416718 |journal=The Quarterly Review of Biology |volume=65 |issue=2 |pages=175–204 |doi=10.1086/416718 |pmid=2117298 |issn=0033-5770}} In some cases, female fruit flies exposed to elevated levels of testosterone or other androgens during development may exhibit male-like behaviors, such as increased aggression or mating attempts toward other females.{{Cite journal |last1=Monyak |first1=Rachel E. |last2=Golbari |first2=Nicole M. |last3=Chan |first3=Yick-Bun |last4=Pranevicius |first4=Ausra |last5=Tang |first5=Grace |last6=Fernández |first6=Maria Paz |last7=Kravitz |first7=Edward A. |date=2021-03-25 |title=Masculinized Drosophila females adapt their fighting strategies to their opponent |journal=Journal of Experimental Biology |volume=224 |issue=6 |pages=jeb238006 |doi=10.1242/jeb.238006 |issn=0022-0949 |pmc=8015213 |pmid=33568440|bibcode=2021JExpB.224B8006M }}

Research in invertebrates like fruit flies has also been instrumental in uncovering the genetic and molecular pathways through which hormones influence sexual differentiation.{{Cite journal |last=Langston |first=W. J. |date=2020-06-30 |title=Endocrine disruption and altered sexual development in aquatic organisms: an invertebrate perspective |url=https://www.cambridge.org/core/journals/journal-of-the-marine-biological-association-of-the-united-kingdom/article/endocrine-disruption-and-altered-sexual-development-in-aquatic-organisms-an-invertebrate-perspective/34130127B54F310D4C4D02D77FCE6D48 |journal=Journal of the Marine Biological Association of the United Kingdom |language=en |volume=100 |issue=4 |pages=495–515 |doi=10.1017/S0025315420000533 |bibcode=2020JMBUK.100..495L |issn=0025-3154}} These studies offer a more detailed understanding of the mechanisms underlying masculinization and how they might be conserved across evolutionary lines.{{Cite journal |last1=Juchault |first1=P |last2=Louis |first2=C |last3=Martin |first3=G |last4=Noulin |first4=G |date=1991-12-01 |title=Masculinization of female isopods (Crustacea) correlated with non-Mendelian inheritance of cytoplasmic viruses. |journal=Proceedings of the National Academy of Sciences |volume=88 |issue=23 |pages=10460–10464 |doi=10.1073/pnas.88.23.10460 |doi-access=free |pmc=52948 |pmid=11607243|bibcode=1991PNAS...8810460J }}{{Cite journal |last1=Ford |first1=Alex T. |last2=Fernandes |first2=Teresa F. |last3=Rider |first3=Sebastien A. |last4=Read |first4=Paul A. |last5=Robinson |first5=Craig D. |last6=Davies |first6=Ian M. |date=2004-08-01 |title=Endocrine disruption in a marine amphipod? Field observations of intersexuality and de-masculinisation |url=https://linkinghub.elsevier.com/retrieve/pii/S0141113604000443 |journal=Marine Environmental Research |series=Twelfth International Symposium on Pollutant Responses in Marine Organisms |volume=58 |issue=2 |pages=169–173 |doi=10.1016/j.marenvres.2004.03.013 |pmid=15178030 |bibcode=2004MarER..58..169F |issn=0141-1136}}

The challenge hypothesis, first proposed by Wingfield et al. in 1990, posits that testosterone levels in males are modulated by reproductive effort and social challenges, such as competition for mates or dominance.{{Cite journal |last1=Wingfield |first1=John C. |last2=Hegner |first2=Robert E. |last3=Dufty |first3=Alfred M. |last4=Ball |first4=Gregory F. |date=1990-12-01 |title=The "Challenge Hypothesis": Theoretical Implications for Patterns of Testosterone Secretion, Mating Systems, and Breeding Strategies |url=https://www.journals.uchicago.edu/doi/10.1086/285134 |journal=The American Naturalist |volume=136 |issue=6 |pages=829–846 |doi=10.1086/285134 |bibcode=1990ANat..136..829W |issn=0003-0147}} While traditionally applied to male behaviors, this hypothesis has increasingly been explored in the context of females, particularly in species where masculinized traits are observed.{{Cite journal |last=Wingfield |first=John C. |date=2017-06-01 |title=The challenge hypothesis: Where it began and relevance to humans |url=https://linkinghub.elsevier.com/retrieve/pii/S0018506X16303488 |journal=Hormones and Behavior |series=Hormones and Human Competition |volume=92 |pages=9–12 |doi=10.1016/j.yhbeh.2016.11.008 |pmid=27856292 |issn=0018-506X}}

In females, the principles of the challenge hypothesis suggest that androgen levels may rise in response to competitive or reproductive pressures, leading to behaviors typically associated with males, such as aggression or territoriality. In meerkats, dominant females exhibit elevated testosterone levels during periods of intense social competition, a pattern consistent with predictions from the challenge hypothesis.{{Cite journal |last1=Drea |first1=Christine M. |last2=Davies |first2=Charli S. |date=2022-09-01 |title=Meerkat manners: Endocrine mediation of female dominance and reproductive control in a cooperative breeder |url=https://linkinghub.elsevier.com/retrieve/pii/S0018506X22001398 |journal=Hormones and Behavior |volume=145 |pages=105245 |doi=10.1016/j.yhbeh.2022.105245 |pmid=35988450 |issn=0018-506X}} These hormonal surges may enhance traits like aggression and dominance, enabling females to maintain control over group dynamics and access to reproductive opportunities.{{Cite journal |last1=Goymann |first1=Wolfgang |last2=Moore |first2=Ignacio T. |last3=Oliveira |first3=Rui F. |date=2019 |title=Challenge Hypothesis 2.0: A Fresh Look at an Established Idea |url=https://www.jstor.org/stable/26732562 |journal=BioScience |volume=69 |issue=6 |pages=432–442 |doi=10.1093/biosci/biz041 |jstor=26732562 |issn=0006-3568}}

The female masculinization hypothesis extends this idea by examining how androgen exposure during prenatal or early developmental stages influences female behavior and physiology throughout life. The interaction between these two hypotheses provides a framework for understanding how short-term hormonal changes (via the challenge hypothesis) and long-term developmental effects (via the female masculinization hypothesis) shape female behavior and reproductive strategies.{{Cite journal |last1=Grebe |first1=Nicholas M. |last2=Sheikh |first2=Alizeh |last3=Drea |first3=Christine M. |date=2022-03-01 |title=Integrating the female masculinization and challenge hypotheses: Female dominance, male deference, and seasonal hormone fluctuations in adult blue-eyed black lemurs (Eulemur flavifrons) |url=https://linkinghub.elsevier.com/retrieve/pii/S0018506X22000022 |journal=Hormones and Behavior |volume=139 |pages=105108 |doi=10.1016/j.yhbeh.2022.105108 |pmid=35033896 |issn=0018-506X}}

Some challenges arise when applying the challenge hypothesis to females. Unlike males, whose testosterone levels often show clear peaks during mating seasons or competitive encounters, the patterns in females can be more variable and context dependent. Environmental factors, such as resource availability and social hierarchy, may play a more significant role in modulating androgen levels in females.{{Cite journal |last1=Wingfield |first1=John C. |last2=Ramenofsky |first2=Marilyn |last3=Hegner |first3=Robert E. |last4=Ball |first4=Gregory F. |date=2020-07-01 |title=Whither the challenge hypothesis? |url=https://linkinghub.elsevier.com/retrieve/pii/S0018506X19302508 |journal=Hormones and Behavior |series=30th Anniversary of the Challenge Hypothesis |volume=123 |pages=104588 |doi=10.1016/j.yhbeh.2019.104588 |pmid=31525343 |issn=0018-506X}}

The organizational-activational hypothesis is a foundational framework in behavioral endocrinology, first proposed by Phoenix et al. in 1959.{{Cite journal |last=Phoenix |first=Charles H. |date=2009-05-01 |title=Organizing action of prenatally administered testosterone propionate on the Tissues mediating mating behavior in the female guinea pig |url=https://linkinghub.elsevier.com/retrieve/pii/S0018506X0900018X |journal=Hormones and Behavior |series=50th Anniversary of the Publication of Phoenix, Goy, Gerall & Young 1959: Organizational Effects of Hormones |volume=55 |issue=5 |pages=566 |doi=10.1016/j.yhbeh.2009.01.004 |pmid=19302826 |issn=0018-506X}} It posits that the effects of hormones on behavior occur in two distinct phases: an organizational phase during early development, when hormones shape the structure and function of the nervous system, and an activational phase later in life, when hormones influence behavior through transient effects.{{Cite journal |last=Arnold |first=Arthur P. |date=2009-05-01 |title=The organizational–activational hypothesis as the foundation for a unified theory of sexual differentiation of all mammalian tissues |journal=Hormones and Behavior |series=50th Anniversary of the Publication of Phoenix, Goy, Gerall & Young 1959: Organizational Effects of Hormones |volume=55 |issue=5 |pages=570–578 |doi=10.1016/j.yhbeh.2009.03.011 |issn=0018-506X |pmc=3671905 |pmid=19446073 }}{{Cite journal |last=Wallen |first=Kim |date=2009-05-01 |title=The Organizational Hypothesis: Reflections on the 50th anniversary of the publication of Phoenix, Goy, Gerall, and Young (1959) |url=https://linkinghub.elsevier.com/retrieve/pii/S0018506X09000671 |journal=Hormones and Behavior |series=50th Anniversary of the Publication of Phoenix, Goy, Gerall & Young 1959: Organizational Effects of Hormones |volume=55 |issue=5 |pages=561–565 |doi=10.1016/j.yhbeh.2009.03.009 |pmid=19446072 |issn=0018-506X}}

This hypothesis provides a critical context for understanding the female masculinization hypothesis, as the female masculinization hypothesis emphasizes the lasting impact of androgen exposure during early developmental windows on female physiology and behavior. According to the female masculinization hypothesis, prenatal or neonatal exposure to elevated androgen levels can masculinize female traits, both physically and behaviorally. These organizational effects are thought to prime the individual for subsequent activational influences during adulthood, when hormonal changes further modulate behavior and reproductive strategies.{{Cite journal |last=Hines |first=Melissa |date=2006-11-01 |title=Prenatal testosterone and gender-related behaviourThis paper was presented at the 4th Ferring Pharmaceuticals International Paediatric Endocrinology Symposium, Paris (2006). Ferring Pharmaceuticals has supported the publication of these proceedings. |url=https://academic.oup.com/ejendo/article-abstract/155/Supplement_1/S115/6695781?redirectedFrom=fulltext |journal=European Journal of Endocrinology |volume=155 |issue=Supplement_1 |pages=S115–S121 |doi=10.1530/eje.1.02236 |pmid=17074984 |issn=0804-4643}}{{Cite journal |last=Arnold |first=Arthur P. |date=2009-05-01 |title=The organizational–activational hypothesis as the foundation for a unified theory of sexual differentiation of all mammalian tissues |journal=Hormones and Behavior |series=50th Anniversary of the Publication of Phoenix, Goy, Gerall & Young 1959: Organizational Effects of Hormones |volume=55 |issue=5 |pages=570–578 |doi=10.1016/j.yhbeh.2009.03.011 |issn=0018-506X |pmc=3671905 |pmid=19446073}}

In rodents, exposure to testosterone in utero has been shown to masculinize brain structures and neural circuits associated with aggression and mating behavior.{{Citation |last1=Krebs-Kraft |first1=Desiree L. |title=Chapter 1 - Sexual Differentiation of the Mammalian Brain |date=2011-01-01 |work=Hormones and Reproduction of Vertebrates |pages=1–24 |editor-last=Norris |editor-first=David O. |url=https://linkinghub.elsevier.com/retrieve/pii/B978012374928410001X |access-date=2025-01-02 |place=London |publisher=Academic Press |doi=10.1016/b978-0-12-374928-4.10001-x |isbn=978-0-12-374928-4 |last2=McCarthy |first2=Margaret M. |editor2-last=Lopez |editor2-first=Kristin H.}} These organizational changes are later activated by hormonal surges during the reproductive phase, leading to behaviors typically associated with males, such as increased territoriality or dominance. In spotted hyenas, androgen exposure during fetal development results in masculinized genitalia and increased aggression in females, behaviors that are further reinforced during adulthood by hormonal cycles.{{Cite journal |last=Arnold |first=Arthur P. |date=2020-04-01 |title=Sexual differentiation of brain and other tissues: Five questions for the next 50 years |journal=Hormones and Behavior |volume=120 |pages=104691 |doi=10.1016/j.yhbeh.2020.104691 |issn=0018-506X |pmc=7440839 |pmid=31991182 }}

The Organizational-Activational Hypothesis also highlights the importance of timing and context in hormonal effects. Not all individuals exposed to androgens during development exhibit masculinized traits, suggesting that genetic background, environmental factors, and the presence of other hormones play significant roles in modulating these effects.{{Cite journal |last=Diamond |first=Milton |date=2009-05-01 |title=Clinical implications of the organizational and activational effects of hormones |url=https://linkinghub.elsevier.com/retrieve/pii/S0018506X09000609 |journal=Hormones and Behavior |series=50th Anniversary of the Publication of Phoenix, Goy, Gerall & Young 1959: Organizational Effects of Hormones |volume=55 |issue=5 |pages=621–632 |doi=10.1016/j.yhbeh.2009.03.007 |pmid=19446079 |issn=0018-506X}} This aligns with critiques of the female masculinization hypothesis, which emphasize the complexity of hormonal environments and the need to consider broader ecological and social contexts.{{Cite journal |last=Swaab |first=Dick F. |date=2007-09-01 |title=Sexual differentiation of the brain and behavior |url=https://linkinghub.elsevier.com/retrieve/pii/S1521690X07000334 |journal=Best Practice & Research Clinical Endocrinology & Metabolism |series=Normal and Abnormal Sex Development |volume=21 |issue=3 |pages=431–444 |doi=10.1016/j.beem.2007.04.003 |pmid=17875490 |issn=1521-690X}}

The concept of maternal androgen effects highlights the influence of hormones produced by mothers during gestation on the development of their offspring. In many species, androgens such as testosterone can cross the placental barrier, affecting the growth, behavior, and reproductive strategies of the developing fetus.{{Cite journal |last1=Makieva |first1=Sofia |last2=Saunders |first2=Philippa T.K. |last3=Norman |first3=Jane E. |date=2014-07-01 |title=Androgens in pregnancy: roles in parturition |url=https://academic.oup.com/humupd/article/20/4/542/2952640 |journal=Human Reproduction Update |volume=20 |issue=4 |pages=542–559 |doi=10.1093/humupd/dmu008 |issn=1355-4786 |pmc=4063701 |pmid=24643344}} This phenomenon provides an important lens for understanding the female masculinization hypothesis, particularly regarding the prenatal androgen exposure believed to drive the masculinization of female traits.{{Cite journal |last1=Parsons |first1=Agata M. |last2=Bouma |first2=Gerrit J. |date=2021-07-01 |title=A Potential Role and Contribution of Androgens in Placental Development and Pregnancy |journal=Life (Basel, Switzerland) |language=en |volume=11 |issue=7 |pages=644 |doi=10.3390/life11070644 |doi-access=free |issn=2075-1729 |pmc=8305703 |pmid=34357016|bibcode=2021Life...11..644P }}

Research on maternal androgen effects suggests that elevated maternal androgen levels can have far-reaching developmental consequences.{{Cite journal |last1=Abruzzese |first1=Giselle Adriana |last2=Ferreira |first2=Silvana Rocio |last3=Ferrer |first3=Maria José |last4=Silva |first4=Aimé Florencia |last5=Motta |first5=Alicia Beatriz |date=2023-01-01 |title=Prenatal Androgen Excess Induces Multigenerational Effects on Female and Male Descendants |journal=Clinical Medicine Insights: Endocrinology and Diabetes |language=en |volume=16 |pages=11795514231196461 |doi=10.1177/11795514231196461 |issn=1179-5514 |pmc=10496475 |pmid=37705939}} In species such as rodents, increased maternal androgens during pregnancy have been linked to the development of masculinized traits in daughters, including altered genital morphology, increased aggression, and enhanced competitive behaviors. These traits often align with the predictions of the female masculinization hypothesis, which posits that such masculinization can confer adaptive advantages under certain ecological or social conditions.{{Cite journal |last1=Wang |first1=Yuqi |last2=Riedstra |first2=Bernd |last3=Hulst |first3=Ronja |last4=Noordhuis |first4=Roy |last5=Groothuis |first5=Ton |date=2023-03-01 |title=Early conversion of maternal androgens affects the embryo already in the first week of development |journal=Biology Letters |volume=19 |issue=3 |pages=20220593 |doi=10.1098/rsbl.2022.0593 |pmc=9975654 |pmid=36855858 }}{{Cite journal |last1=Udry |first1=J. Richard |last2=Morris |first2=Naomi M. |last3=Kovenock |first3=Judith |date=1995-07-01 |title=Androgen effects on women's gendered behaviour |url=https://www.cambridge.org/core/journals/journal-of-biosocial-science/article/abs/androgen-effects-on-womens-gendered-behaviour/C3073D4DA7DD05FF1178B7A6508F4883 |journal=Journal of Biosocial Science |language=en |volume=27 |issue=3 |pages=359–368 |doi=10.1017/S0021932000022884 |pmid=7650053 |issn=1469-7599}}

In spotted hyenas, high maternal androgen levels are associated with the development of masculinized genitalia and social dominance in female offspring.{{Cite journal |last1=Dloniak |first1=S. M. |last2=French |first2=J. A. |last3=Holekamp |first3=K. E. |date=2006-04-27 |title=Rank-related maternal effects of androgens on behaviour in wild spotted hyaenas |url=https://www.nature.com/articles/nature04540 |journal=Nature |language=en |volume=440 |issue=7088 |pages=1190–1193 |doi=10.1038/nature04540 |pmid=16641996 |bibcode=2006Natur.440.1190D |issn=1476-4687}} These traits are thought to enhance the daughters’ reproductive success in the species’ highly competitive social structure.{{Cite journal |last1=Goymann |first1=Wolfgang |last2=East |first2=Marion L. |last3=Hofer |first3=Heribert |date=2001-02-01 |title=Androgens and the Role of Female "Hyperaggressiveness" in Spotted Hyenas (Crocuta crocuta) |url=https://linkinghub.elsevier.com/retrieve/pii/S0018506X00916349 |journal=Hormones and Behavior |volume=39 |issue=1 |pages=83–92 |doi=10.1006/hbeh.2000.1634 |pmid=11161886 |issn=0018-506X}} In birds, maternal androgens deposited into egg yolks have been shown to influence offspring growth rates, aggression, and survival, demonstrating a broader relevance of maternal androgen effects beyond mammals.{{Cite journal |last=Ruuskanen |first=Suvi |date=2015-12-01 |title=Hormonally-mediated maternal effects in birds: Lessons from the flycatcher model system |url=https://linkinghub.elsevier.com/retrieve/pii/S0016648015002609 |journal=General and Comparative Endocrinology |volume=224 |pages=283–293 |doi=10.1016/j.ygcen.2015.09.016 |pmid=26393309 |issn=0016-6480}}

• Sexual differentiation
• Sexual dimorphism
• Virilization
• Challenge hypothesis
• Organizational-Activational Hypothesis
  

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