lactation#Human

File:White Cat Nursing Four Kittens HQ.jpgs nursing]]

File:Lactating Female Coyote - cropped.jpg with visible teats]]

Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals.{{cite journal | vauthors = Capuco AV, Akers RM | title = The origin and evolution of lactation | journal = Journal of Biology | volume = 8 | issue = 4 | pages = 37 | year = 2009 | pmid = 19439024 | pmc = 2688910 | doi = 10.1186/jbiol139 | doi-access = free }} The process of feeding milk in all female creatures is called nursing, and in humans it is also called breastfeeding. Newborn infants often produce some milk from their own breast tissue, known colloquially as witch's milk.

In most species, lactation is a sign that the female has been pregnant at some point in her life, although in humans and goats, it can happen without pregnancy.{{Cite web|url=https://sites.google.com/a/babiekids.com/babiekids/lactating-without-pregnancy | archive-url = https://web.archive.org/web/20210114034426/https://sites.google.com/a/babiekids.com/babiekids/lactating-without-pregnancy | archive-date = 14 January 2021 |title=Lactating Without Pregnancy |website=sites.google.com}}{{unreliable source?|date=October 2022}}{{Cite web|url=https://www.berryemporium.com/precocious-udder-maiden-milker/|archive-url=https://web.archive.org/web/20210114032429/https://www.berryemporium.com/precocious-udder-maiden-milker/ | work = berryemporium.com |title=Goats with Precocious Udder Syndrome |archive-date=January 14, 2021}} Nearly every species of mammal has teats; except for monotremes, egg-laying mammals, which instead release milk through ducts in the abdomen. In only a handful of species of mammals, certain bat species, is milk production a normal male function.

Galactopoiesis is the maintenance of milk production. This stage requires prolactin. Oxytocin is critical for the milk let-down reflex in response to suckling. Galactorrhea is milk production unrelated to nursing. It can occur in males and females of many mammal species as result of hormonal imbalances such as hyperprolactinaemia.

Purpose

The chief function of a lactation is to provide nutrition and immune protection to the young after birth. Due to lactation, the mother-young pair can survive even if food is scarce or too hard for the young to attain, expanding the environmental conditions the species can withstand. The costly investment of energy and resources into milk is outweighed by the benefit to offspring survival.{{cite book | vauthors = Power ML, Schulkin J |title=Milk: the biology of lactation |date=2016 |location=Baltimore, Maryland | publisher = Johns Hopkins University Press|isbn=978-1-4214-2042-4 }} In almost all mammals, lactation induces a period of infertility (in humans, lactational amenorrhea), which serves to provide the optimal birth spacing for survival of the offspring.{{cite journal | vauthors = McNeilly AS | title = Lactation and fertility | journal = Journal of Mammary Gland Biology and Neoplasia | volume = 2 | issue = 3 | pages = 291–298 | date = July 1997 | pmid = 10882312 | doi = 10.1023/A:1026340606252 | s2cid = 30817565 }}

Human

File:Lactation.jpg

=Hormonal influences=

From the eighteenth week of pregnancy (the second and third trimesters), a woman's body produces hormones that stimulate the growth of the milk duct system in the breasts:

Progesterone influences the growth in size of alveoli and lobes; high levels of progesterone inhibit lactation before birth. Progesterone levels drop after birth; this triggers the onset of copious milk production.{{cite book | vauthors = Mohrbacher N, Stock J | title = The Breastfeeding Answer Book | edition = 3rd (revised) | publisher = La Leche League International | year = 2003 | isbn = 978-0-912500-92-8 }}
Estrogen stimulates the milk duct system to grow and differentiate. Like progesterone, high levels of estrogen also inhibit lactation. Estrogen levels also drop at delivery and remain low for the first several months of breastfeeding. Breastfeeding mothers should avoid estrogen based birth control methods, as a spike in estrogen levels may reduce a mother's milk supply.
Prolactin contributes to the increased growth and differentiation of the alveoli, and also influences differentiation of ductal structures. High levels of prolactin during pregnancy and breastfeeding also increase insulin resistance, increase growth factor levels (IGF-1) and modify lipid metabolism in preparation for breastfeeding. During lactation, prolactin is the main factor maintaining tight junctions of the ductal epithelium and regulating milk production through osmotic balance.
Human placental lactogen (HPL) – from the second month of pregnancy, the placenta releases large amounts of HPL. This hormone is closely associated with prolactin and appears to be instrumental in breast, nipple, and areola growth before birth.
Follicle stimulating hormone (FSH), luteinizing hormone (LH), and human chorionic gonadotropin (hCG), through control of estrogen and progesterone production, and also, by extension, prolactin and growth hormone production, are essential.
Growth hormone (GH) is structurally very similar to prolactin and independently contributes to its galactopoiesis.
Adrenocorticotropic hormone (ACTH) and glucocorticoids such as cortisol have an important lactation inducing function in several animal species, including humans. Glucocorticoids play a complex regulating role in the maintenance of tight junctions.
Thyroid-stimulating hormone (TSH) and thyrotropin-releasing hormone (TRH) are very important galactopoietic hormones whose levels are naturally increased during pregnancy.
Oxytocin contracts the smooth muscle of the uterus during and after birth, and during orgasm(s). After birth, oxytocin contracts the smooth muscle layer of band-like cells surrounding the alveoli to squeeze the newly produced milk into the duct system. Oxytocin is necessary for the milk ejection reflex, or let-down, in response to suckling, to occur.

It is also possible to induce lactation without pregnancy through combinations of birth control pills, galactagogues, and milk expression using a breast pump.

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File:Asymmetric breastfeeding latch.jpg

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==Secretory differentiation==

During the latter part of pregnancy, the woman's breasts enter into the Secretory Differentiation stage. This is when the breasts make colostrum (see below), a thick, sometimes yellowish fluid. At this stage, high levels of progesterone inhibit most milk production. It is not a medical concern if a pregnant woman leaks any colostrum before her baby's birth, nor is it an indication of future milk production.

==Secretory activation==

At birth, prolactin levels remain high, while the delivery of the placenta results in a sudden drop in progesterone, estrogen, and HPL levels. This abrupt withdrawal of progesterone in the presence of high prolactin levels stimulates the copious milk production of Secretory Activation.

When the breast is stimulated, prolactin levels in the blood rise, peak in about 45 minutes, and return to the pre-breastfeeding state about three hours later. The release of prolactin triggers the cells in the alveoli to make milk. Prolactin also transfers to the breast milk. Some research indicates that prolactin in milk is greater at times of higher milk production, and lower when breasts are fuller, and that the highest levels tend to occur between 2 a.m. and 6 a.m.{{cite journal | vauthors = Cregan MD, Mitoulas LR, Hartmann PE | title = Milk prolactin, feed volume and duration between feeds in women breastfeeding their full-term infants over a 24 h period | journal = Experimental Physiology | volume = 87 | issue = 2 | pages = 207–214 | date = March 2002 | pmid = 11856965 | doi = 10.1113/eph8702327 | doi-access = free }}

Other hormones—notably insulin, thyroxine, and cortisol—are also involved, but their roles are not yet well understood. Although biochemical markers indicate that Secretory Activation begins about 30–40 hours after birth, mothers do not typically begin feeling increased breast fullness (the sensation of milk "coming in the breast") until 50–73 hours (2–3 days) after birth.

Colostrum is the first milk a breastfed baby receives. It contains higher amounts of white blood cells and antibodies than mature milk, and is especially high in immunoglobulin A (IgA), which coats the lining of the baby's immature intestines, and helps to prevent pathogens from invading the baby's system. Secretory IgA also helps prevent food allergies.{{cite book | vauthors = Sears M, Sears W |author-link2=William Sears (physician) | title = The Breastfeeding Book | url = https://archive.org/details/isbn_9780316779241 | url-access = registration | publisher = Little, Brown | year = 2000 | isbn = 978-0-316-77924-1 }} Over the first two weeks after the birth, colostrum production slowly gives way to mature breast milk.

==Autocrine control - Galactopoiesis==

The hormonal endocrine control system drives milk production during pregnancy and the first few days after the birth. When the milk supply is more firmly established, autocrine (or local) control system begins.

During this stage, the more that milk is removed from the breasts, the more the breast will produce milk.{{cite journal | vauthors = deCarvalho M, Anderson DM, Giangreco A, Pittard WB | title = Frequency of milk expression and milk production by mothers of nonnursing premature neonates | journal = American Journal of Diseases of Children | volume = 139 | issue = 5 | pages = 483–485 | date = May 1985 | pmid = 3984973 | doi = 10.1001/archpedi.1985.02140070057033 }}{{cite journal | vauthors = Hopkinson JM, Schanler RJ, Garza C | title = Milk production by mothers of premature infants | journal = Pediatrics | volume = 81 | issue = 6 | pages = 815–820 | date = June 1988 | pmid = 3368280 | doi = 10.1542/peds.81.6.815 | s2cid = 36906244 }} Research also suggests that draining the breasts more fully also increases the rate of milk production.{{cite journal | vauthors = Daly SE, Owens RA, Hartmann PE | title = The short-term synthesis and infant-regulated removal of milk in lactating women | journal = Experimental Physiology | volume = 78 | issue = 2 | pages = 209–220 | date = March 1993 | pmid = 8471241 | doi = 10.1113/expphysiol.1993.sp003681 | doi-access = free }} Thus the milk supply is strongly influenced by how often the baby feeds and how well it is able to transfer milk from the breast. Low supply can often be traced to:

not feeding or pumping often enough
inability of the infant to transfer milk effectively caused by, among other things:
jaw or mouth structure deficits
poor latching technique
premature birth
drowsiness in the baby, due to illness, medication or recovery from medical procedures
rare maternal endocrine disorders
hypoplastic breast tissue
inadequate calorie intake or malnutrition of the mother

=Milk ejection reflex=

File:2922 Let Down Reflex-new.jpg

This is the mechanism by which milk is transported from the breast alveoli to the nipple. Suckling by the baby stimulates the paraventricular nuclei and supraoptic nucleus in the hypothalamus, which signals to the posterior pituitary gland to produce oxytocin. Oxytocin stimulates contraction of the myoepithelial cells surrounding the alveoli, which already hold milk. The increased pressure causes milk to flow through the duct system and be released through the nipple. This response can be conditioned e.g. to the cry of the baby.

Milk ejection is initiated in the mother's breast by the act of suckling by the baby. The milk ejection reflex (also called let-down reflex) is not always consistent, especially at first. Once a woman is conditioned to nursing, let-down can be triggered by a variety of stimuli, including the sound of any baby. Even thinking about breastfeeding can stimulate this reflex, causing unwanted leakage, or both breasts may give out milk when an infant is feeding from one breast. However, this and other problems often settle after two weeks of feeding. Stress or anxiety can cause difficulties with breastfeeding. The release of the hormone oxytocin leads to the milk ejection or let-down reflex. Oxytocin stimulates the muscles surrounding the breast to squeeze out the milk. Breastfeeding mothers describe the sensation differently. Some feel a slight tingling, others feel immense amounts of pressure or slight pain/discomfort, and still others do not feel anything different. A minority of mothers experience a dysphoric milk ejection reflex immediately before let-down, causing anxiety, anger or nausea, amongst other negative sensations, for up to a few minutes per feed.

A poor milk ejection reflex can be due to sore or cracked nipples, separation from the infant, a history of breast surgery, or tissue damage from prior breast trauma. If a mother has trouble breastfeeding, different methods of assisting the milk ejection reflex may help. These include feeding in a familiar and comfortable location, massage of the breast or back, or warming the breast with a cloth or shower.

==Milk ejection reflex mechanism==

This is the mechanism by which milk is transported from the breast alveoli to the nipple. Suckling by the baby innervates slowly adapting{{cite journal | vauthors = Grachev II, Alekseev NP, Velling VA | title = [Slowly-adapting mechanoreceptor units of the guinea pig mammary nipple] | journal = Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova | volume = 63 | issue = 3 | pages = 391–400 | date = March 1977 | pmid = 863036 }} and rapidly-adapting{{cite journal | vauthors = Grachev II, Alekseev NP, Velling VA | title = [Properties of the mechanoreceptors of the nipple of the guinea pig mammary gland. (Rapidly adapting mechanoreceptor units)] | journal = Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova | volume = 62 | issue = 6 | pages = 885–892 | year = 1976 | pmid = 1010088 }} mechanoreceptors that are densely packed around the areolar region. The electrical impulse follows the spinothalamic tract, which begins by innervation of fourth intercostal nerves. The electrical impulse then ascends the posterolateral tract for one or two vertebral levels and synapses with second-order neurons, called tract cells, in the posterior dorsal horn. The tract cells then decussate via the anterior white commissure to the anterolateral corner and ascend to the supraoptic nucleus and paraventricular nucleus in the hypothalamus, where they synapse with oxytocinergic third-order neurons. The somas of these neurons are located in the hypothalamus, but their axon and axon terminals are located in the infundibulum and pars nervosa of the posterior pituitary, respectively. The oxytocin is produced in the neuron's soma in the supraoptic and paraventricular nuclei, and is then transported down the infundibulum via the hypothalamo-neurohypophyseal tract with the help of the carrier protein, neurophysin I, to the pars nervosa of the posterior pituitary, and then stored in Herring bodies, where they are stored until the synapse between second- and third-order neurons.

Following the electrical impulse, oxytocin is released into the bloodstream. Through the bloodstream, oxytocin makes its way to myoepithelial cells, which lie between the extracellular matrix and luminal epithelial cells that also make up the alveoli in breast tissue. When oxytocin binds to the myoepithelial cells, the cells contract. The increased intra-alveolar pressure forces milk into the lactiferous sinuses, into the lactiferous ducts (a study found that lactiferous sinuses may not exist.{{cite journal | vauthors = Ramsay DT, Kent JC, Hartmann RA, Hartmann PE | title = Anatomy of the lactating human breast redefined with ultrasound imaging | journal = Journal of Anatomy | volume = 206 | issue = 6 | pages = 525–534 | date = June 2005 | pmid = 15960763 | pmc = 1571528 | doi = 10.1111/j.1469-7580.2005.00417.x }} If this is true then milk simply enters the lactiferous ducts), and then out the nipple.

=Afterpains=

A surge of oxytocin also causes the uterus to contract. During breastfeeding, mothers may feel these contractions as afterpains. These may range from period-like cramps to strong labour-like contractions and can be more severe with second and subsequent babies.{{cite book | vauthors = Mohrbacher N | title = Breastfeeding Answers Made Simple: A Guide for Helping Mothers | publisher = Hale Publishing | date = 2010 | isbn = 978-0-9845039-0-2 }}{{cite book | vauthors = Fray K | title = Oh Baby...Birth, Babies & Motherhood Uncensored | publisher = Random House NZ | year = 2005 | isbn = 978-1-86941-713-0 }}

{{anchor|Lactation without pregnancy, induced lactation, relactation}}

Without pregnancy, induced lactation, relactation

In humans, induced lactation and relactation have been observed frequently in some cultures, and demonstrated with varying success in adoptive mothers and wet nurses.{{cite book | vauthors = Goljan E | title = Pathology | edition = 2nd | publisher = Mosby Elsevier | series = Rapid Review Series | isbn = 978-0-323-04414-1 | date = 2007 }}{{cite book | vauthors = Wilson-Clay B | date = 1996 | url = http://www.surrogacy.com/medres/article/lac.html | title = Induced Lactation | archive-url = https://web.archive.org/web/20100209070720/http://www.surrogacy.com/medres/article/lac.html | archive-date=9 February 2010 | publisher = The American Surrogacy Center }} It appears plausible that the possibility of lactation in women (or females of other species) who are not biological mothers does confer an evolutionary advantage, especially in groups with high maternal mortality and tight social bonds.{{cite journal | vauthors = Sobrinho LG | title = Prolactin, psychological stress and environment in humans: adaptation and maladaptation | journal = Pituitary | volume = 6 | issue = 1 | pages = 35–39 | year = 2003 | pmid = 14674722 | doi = 10.1023/A:1026229810876 | s2cid = 1335211 }}{{cite journal | vauthors = Bose CL, D'Ercole AJ, Lester AG, Hunter RS, Barrett JR | title = Relactation by mothers of sick and premature infants | journal = Pediatrics | volume = 67 | issue = 4 | pages = 565–569 | date = April 1981 | pmid = 6789296 | doi = 10.1542/peds.67.4.565 | s2cid = 12991397 }} The phenomenon has been also observed in most primates, in some lemurs, and in dwarf mongooses.{{cite journal | vauthors = König B | title = Cooperative care of young in mammals | journal = Die Naturwissenschaften | volume = 84 | issue = 3 | pages = 95–104 | date = March 1997 | pmid = 9112240 | doi = 10.1007/s001140050356 | s2cid = 23240724 | bibcode = 1997NW.....84...95K }}{{cite journal | vauthors = Creel SR, Monfort SL, Wildt DE, Waser PM | title = Spontaneous lactation is an adaptive result of pseudopregnancy | journal = Nature | volume = 351 | issue = 6328 | pages = 660–662 | date = June 1991 | pmid = 2052092 | doi = 10.1038/351660a0 | s2cid = 4336672 | bibcode = 1991Natur.351..660C }}

Lactation can be induced in humans by a combination of physical and psychological stimulation, by drugs, or by a combination of those methods.{{cite journal | vauthors = Patwari AK, Satyanarayana L | title = Relactation: an effective intervention to promote exclusive breastfeeding | journal = Journal of Tropical Pediatrics | volume = 43 | issue = 4 | pages = 213–216 | date = August 1997 | pmid = 9283123 | doi = 10.1093/tropej/43.4.213 | doi-access = free }} Several protocols for inducing lactation were developed by Jack Newman and Lenore Goldfarb and are commonly called the Newman-Goldfarb protocols. The "regular protocol" involves the use of birth control pills to mimic the hormone levels of pregnancy with domperidone to stimulate milk production, followed by discontinuing the birth control and the introducing use of a double electric breast pump to induce milk production.{{cite web | vauthors = Goldfarb L, Newman J |title=The Newman Goldfarb Protocols for Induced Lactation |url=https://www.asklenore.info/breastfeeding/induced_lactation/protocols4print.shtml |website=www.asklenore.info}} Additional protocols exist to support an accelerated timeline and to support induced lactation in menopausal parents.

Some couples may stimulate lactation outside of pregnancy for sexual purposes.

Rare accounts of male lactation (as distinct from galactorrhea) exist in historical medical and anthropological literature.{{Cite web|url=https://www.scientificamerican.com/article/strange-but-true-males-can-lactate/|title=Strange but True: Males Can Lactate| vauthors = Swaminathan N |website=Scientific American}} Most recently a subject of transgender health care, multiple case reports have described transgender women successfully inducing lactation.{{cite journal | vauthors = Reisman T, Goldstein Z | title = Case Report: Induced Lactation in a Transgender Woman | journal = Transgender Health | volume = 3 | issue = 1 | pages = 24–26 | date = December 2018 | pmid = 29372185 | pmc = 5779241 | doi = 10.1089/trgh.2017.0044 }}{{cite journal | vauthors = Wamboldt R, Shuster S, Sidhu BS | title = Lactation Induction in a Transgender Woman Wanting to Breastfeed: Case Report | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 106 | issue = 5 | pages = e2047–e2052 | date = April 2021 | pmid = 33513241 | doi = 10.1210/clinem/dgaa976 | s2cid = 231755160 | doi-access = free }} Research has indicated that such breast milk is nutritionally comparable to both the milk of naturally lactating and induced lactating cisgender women.{{cite journal | vauthors = Weimer AK | title = Lactation Induction in a Transgender Woman: Macronutrient Analysis and Patient Perspectives | journal = Journal of Human Lactation | volume = 39 | issue = 3 | pages = 488–494 | date = August 2023 | pmid = 37138506 | doi = 10.1177/08903344231170559 | s2cid = 258485541 | url = https://escholarship.org/uc/item/9k3189h7 }}

Domperidone is a drug that can induce lactation.{{Cite web | vauthors = Glenza J |url= https://www.theguardian.com/science/2018/feb/14/transgender-woman-breastfeed-health|title = Transgender woman able to breastfeed in first documented case|website = TheGuardian.com|date = 14 February 2018}}{{cite journal | vauthors = Reisman T, Goldstein Z | title = Case Report: Induced Lactation in a Transgender Woman | journal = Transgender Health | volume = 3 | issue = 1 | pages = 24–26 | year = 2018 | pmid = 29372185 | pmc = 5779241 | doi = 10.1089/trgh.2017.0044 }}

Evolution

Charles Darwin recognized that mammary glands seemed to have developed specifically from cutaneous glands, and hypothesized that they evolved from glands in brood pouches of fish, where they would provide nourishment for eggs. The latter aspect of his hypothesis has not been confirmed; however, more recently the same mechanism has been postulated for early synapsids.{{cite journal | vauthors = Oftedal OT | title = The mammary gland and its origin during synapsid evolution | journal = Journal of Mammary Gland Biology and Neoplasia | volume = 7 | issue = 3 | pages = 225–252 | date = July 2002 | pmid = 12751889 | doi = 10.1023/A:1022896515287 | s2cid = 25806501 }}

As all mammals lactate, lactation must have evolved before the last common ancestor of all mammals, which places it at a minimum in the Middle or Late Triassic when monotremes diverged from therians.{{cite journal | vauthors = van Rheede T, Bastiaans T, Boone DN, Hedges SB, de Jong WW, Madsen O | title = The platypus is in its place: nuclear genes and indels confirm the sister group relation of monotremes and Therians | journal = Molecular Biology and Evolution | volume = 23 | issue = 3 | pages = 587–597 | date = March 2006 | pmid = 16291999 | doi = 10.1093/molbev/msj064 | doi-access = free }} O. T. Oftedal has argued that therapsids evolved a proto-lacteal fluid in order to keep eggs moist, an adaptation necessitated due to synapsids’ parchment shelled eggs which are more vulnerable to evaporation and dehydration than the mineralized eggs produced by some sauropsids.{{cite journal | vauthors = Oftedal OT | title = The origin of lactation as a water source for parchment-shelled eggs | journal = Journal of Mammary Gland Biology and Neoplasia | volume = 7 | issue = 3 | pages = 253–266 | date = July 2002 | pmid = 12751890 | doi = 10.1023/A:1022848632125 | s2cid = 8319185 }} This protolacteal fluid became a complex, nutrient-rich milk which then allowed a decline in egg size by reducing the dependence on a large yolk in the egg. The evolution of lactation is also believed to have resulted in the more complex dentition seen in mammals, as lactation would have allowed the prolonged development of the jaw before the eruption of teeth.

Oftedal also proposed that the protolacteal fluid was initially secreted through pilosebaceous glands on mammary patches, analogous to the areola, and that hairs on this patch transported the fluid to the hatchlings as is seen in monotremes. In monotremes, they are said to have evolved from apocrine sweat glands.{{cite web | url=https://truthinscience.uk/platypus-a-darwinian-cautionary-tale/?amp=1 | title=Platypus: A Darwinian Cautionary Tale | date=6 April 2021 }} This would have occurred in the mammal lineages that diverged after monotremes, metatheria and eutheria. In this scenario, some genes and signaling pathways involved in lactation evolved from ancient precursors which facilitated secretions from spiny structures, which themselves evolved from odontodes.{{cite journal |vauthors=Oftedal OT, Dhouailly D |date=June 2013 |title=Evo-devo of the mammary gland |journal=Journal of Mammary Gland Biology and Neoplasia |volume=18 |issue=2 |pages=105–120 |doi=10.1007/s10911-013-9290-8 |pmid=23681303 |s2cid=6608975}}

Occurrence outside Mammalia

Recent research, as documented in the journal Science, has shed light on the behavior of certain species of caecilians. These studies reveal that some caecilians exhibit a phenomenon wherein they provide their hatchlings with a nutrient-rich substance akin to milk, delivered through a maternal vent. Among the species investigated, the oviparous nonmammalian caecilian amphibian Siphonops annulatus stood out, indicating that the practice of lactation may be more widespread among these creatures than previously thought. As detailed in a 2024 study, researchers collected 16 mothers of the Siphonops annulatus species from cacao plantations in Brazil's Atlantic Forest and filmed them with their altricial hatchlings in the lab. The mothers remained with their offspring, which suckled on a white, viscous liquid from their cloaca, experiencing rapid growth in their first week. This milk-like substance, rich in fats and carbohydrates, is produced in the mother's oviduct epithelium's hypertrophied glands, similar to mammal milk. The substance was released seemingly in response to tactile and acoustic stimulation by the babies. The researchers observed the hatchlings emitting high-pitched clicking sounds as they approached their mothers for milk, a behavior unique among amphibians. This milk-feeding behavior may contribute to the development of the hatchlings' microbiome and immune system, similar to mammalian young. The presence of milk production in caecilians that lay eggs suggests an evolutionary transition between egg-laying and live birth.{{Cite news |last=Quaglia |first=Sofia |date=2024-03-07 |title=Bizarre, Wormlike and Oozing Milk for Their Babies |url=https://www.nytimes.com/2024/03/07/science/caecilians-milk-amphibians-worms.html |access-date=2024-03-11 |work=The New York Times |language=en-US |issn=0362-4331}}{{Cite journal |last1=Wake |first1=Marvalee H. |date=7 Mar 2024 |title=Amphibian hatchlings find mother's milk |journal=Science |language=en |volume=383 |issue=1060-1061 (2024) |pages=1060–1061 |doi=10.1126/science.ado2094|pmid=38452095 |bibcode=2024Sci...383.1060W }}{{Cite journal |last1=Mailho-Fontana |first1=Pedro L. |last2=Antoniazzi |first2=Marta M. |last3=Coelho |first3=Guilherme R. |last4=Pimenta |first4=Daniel C. |last5=Fernandes |first5=Lígia P. |last6=Kupfer |first6=Alexander |last7=Brodie |first7=Edmund D. |last8=Jared |first8=Carlos |date=2024-03-08 |title=Milk provisioning in oviparous caecilian amphibians |url=https://www.science.org/doi/10.1126/science.adi5379 |journal=Science |language=en |volume=383 |issue=6687 |pages=1092–1095 |doi=10.1126/science.adi5379 |pmid=38452082 |bibcode=2024Sci...383.1092M |issn=0036-8075}}

Another well known example of nourishing young with secretions of glands is the crop milk of certain birds such as columbiform birds (pigeons and doves), among others. As in mammals, this also appears to be directed by prolactin.{{cite journal | vauthors = Horseman ND, Buntin JD | title = Regulation of pigeon cropmilk secretion and parental behaviors by prolactin | journal = Annual Review of Nutrition | volume = 15 | pages = 213–238 | year = 1995 | pmid = 8527218 | doi = 10.1146/annurev.nu.15.070195.001241 }} Other birds such as flamingos and penguins utilize similar feeding techniques.{{Cite web|url=https://web.stanford.edu/group/stanfordbirds/text/essays/Bird_Milk.html|title=Bird Milk|website=web.stanford.edu}}

The discus fish (Symphysodon) is known for (biparentally) feeding their offspring by epidermal mucus secretion.{{cite journal | vauthors = Buckley J, Maunder RJ, Foey A, Pearce J, Val AL, Sloman KA | title = Biparental mucus feeding: a unique example of parental care in an Amazonian cichlid | journal = The Journal of Experimental Biology | volume = 213 | issue = Pt 22 | pages = 3787–3795 | date = November 2010 | pmid = 21037057 | doi = 10.1242/jeb.042929 | doi-access = free | hdl = 10026.1/12620 | hdl-access = free }}{{cite journal | vauthors = Chong K, Joshi S, Jin LT, Shu-Chien AC | title = Proteomics profiling of epidermal mucus secretion of a cichlid (Symphysodon aequifasciata) demonstrating parental care behavior | journal = Proteomics | volume = 6 | issue = 7 | pages = 2251–2258 | date = April 2006 | pmid = 16385477 | doi = 10.1002/pmic.200500591 | s2cid = 37973363 }} A closer examination reveals that, as in mammals and birds, the secretion of this nourishing fluid may be controlled by prolactin.{{cite journal | vauthors = Khong HK, Kuah MK, Jaya-Ram A, Shu-Chien AC | title = Prolactin receptor mRNA is upregulated in discus fish (Symphysodon aequifasciata) skin during parental phase | journal = Comparative Biochemistry and Physiology. Part B, Biochemistry & Molecular Biology | volume = 153 | issue = 1 | pages = 18–28 | date = May 2009 | pmid = 19272315 | doi = 10.1016/j.cbpb.2009.01.005 }} Similar behavior is seen in at least 30 species of cichlids.

Lactation is also the hallmark of adenotrophic viviparity – a breeding mechanism developed by some insects, most notably tsetse flies. The single egg of the tsetse develops into a larva inside the uterus where it is fed by a milky substance secreted by a milk gland inside the uterus.{{cite journal | vauthors = Attardo GM, Lohs C, Heddi A, Alam UH, Yildirim S, Aksoy S | title = Analysis of milk gland structure and function in Glossina morsitans: milk protein production, symbiont populations and fecundity | journal = Journal of Insect Physiology | volume = 54 | issue = 8 | pages = 1236–1242 | date = August 2008 | pmid = 18647605 | pmc = 2613686 | doi = 10.1016/j.jinsphys.2008.06.008 }} The cockroach species Diploptera punctata is also known to feed their offspring by milky secretions.{{cite journal | vauthors = Williford A, Stay B, Bhattacharya D | title = Evolution of a novel function: nutritive milk in the viviparous cockroach, Diploptera punctata | journal = Evolution & Development | volume = 6 | issue = 2 | pages = 67–77 | year = 2004 | pmid = 15009119 | doi = 10.1111/j.1525-142x.2004.04012.x | s2cid = 31048064 }}

Toxeus magnus, an ant-mimicking jumping spider species of Southeast Asia, also lactates. It nurses its offspring for about 38 days, although they are able to forage on their own after 21 days. Blocking nursing immediately after birth resulted in complete mortality of the offspring, whereas blocking it 20 days after birth resulted in increased foraging and reduced survival. This form of lactation may have evolved from production of trophic eggs.{{cite journal | vauthors = Chen Z, Corlett RT, Jiao X, Liu SJ, Charles-Dominique T, Zhang S, Li H, Lai R, Long C, Quan RC | display-authors = 6 | title = Prolonged milk provisioning in a jumping spider | journal = Science | volume = 362 | issue = 6418 | pages = 1052–1055 | date = November 2018 | pmid = 30498127 | doi = 10.1126/science.aat3692 | doi-access = free | bibcode = 2018Sci...362.1052C }}