Carrier's constraint
{{Short description|Movement that makes breathing difficult}}
File:Eublepharis macularius 2009 G6.jpg]]
Carrier's constraint is the observation that air-breathing vertebrates with two lungs that flex their bodies sideways during locomotion find it difficult to move and breathe at the same time, because the sideways flexing expands one lung and compresses the other, shunting stale air from lung to lung instead of expelling it completely to make room for fresh air.{{cite journal
| last=Carrier
| first=D.R.
| title=The evolution of locomotor stamina in tetrapods: circumventing a mechanical constraint
| journal=Paleobiology
| issue=3
| pages=326–341
| year=1987
| volume=13|doi=10.1017/s0094837300008903
| s2cid=83722453
}}
It was named by English paleontologist Richard Cowen for David R. Carrier, who wrote his observations on the problem in 1987.{{cite book |last= Cowen|first= Richard |chapter= Locomotion and Respiration in Aquatic Air-Breathing Vertebrates |title= Evolutionary Paleobiology |editor= Jablonski, David |location= Chicago |publisher= University of Chicago Press |year= 1996 |page= 337+ |isbn= 0-226-38911-1|display-editors=etal}}{{cite web |url= http://mygeologypage.ucdavis.edu/cowen/~GEL107/carrierconstraint.html |author= Cowen, Richard |date= 2003 |title= Respiration, Metabolism, and Locomotion |publisher= Richard Cowen, University of California, Davis |quote= If the animal is walking, it may be able to breathe between steps, but sprawling vertebrates cannot run and breathe at the same time. I shall call this problem Carrier's Constraint. |accessdate= October 21, 2014 |archive-url= https://web.archive.org/web/20141021212932/http://mygeologypage.ucdavis.edu/cowen/~GEL107/carrierconstraint.html |archive-date= October 21, 2014 |url-status= dead }}{{cite journal |url= http://www.americanscientist.org/issues/postComment.aspx?id=3663&y=2008&no=1&page=4 |last= Shipman |first= Pat |title= Freed to Fly Again |journal= American Scientist |location= Research Triangle Park |publisher= Sigma Xi |volume= 96 |issue= 1 |date= January 2008 |page= 20 |doi= 10.1511/2008.69.20 |quote= Carrier's constraint is named for David R. Carrier at the University of Utah in Salt Lake City, who observed that the typical sprawling gait of a lizard restricts the animal's ability to breathe while running or walking. |accessdate= October 21, 2014 |archive-date= March 4, 2016 |archive-url= https://web.archive.org/web/20160304055508/http://www.americanscientist.org/issues/postComment.aspx?id=3663&y=2008&no=1&page=4 |url-status= dead |url-access= subscription }}
Consequences
Most lizards move in short bursts, with long pauses for breath.{{citation needed|date=April 2023}}
Around the Late Triassic period, animals with Carrier's constraint were preyed on by bipedal species that evolved a more efficient stride.{{citation needed|date=April 2023}}
Solutions
=Workarounds=
Most snakes have only one lung, so Carrier's constraint does not apply.
Monitor lizards increase their stamina by using bones and muscles in the throat and floor of the mouth to "gulp" air via gular pumping.{{cite journal |url= https://www.questia.com/read/1P3-635880221 |last= Summers |first= Adam |title= Monitor Marathons |journal= Natural History |volume= 112 |issue= 5 |date= 2003 |page= 32 |accessdate= October 21, 2014 }}{{Dead link|date=August 2024 |bot=InternetArchiveBot |fix-attempted=yes }}
Some other lizards, mainly agamids, use bipedal locomotion for running and avoid sideways flexing. Bipedality in modern lizards is rare, but it is an effective way to run without pausing to breathe, and is advantageous for catching active prey or evading predators.
Crocodilians use a "high walk" with a more erect limb posture that minimizes sideways flexing to cross long distances. However, as they evolved from upright walkers with limited bipedality, this may simply be a remnant of past behavior rather than a specific adaptation to overcome this difficulty. Todd J. Uriona of the University of Utah hypothesized that costal ventilation may have aided the upright posture in overcoming the constraint.{{cite book |url= https://books.google.com/books?id=_WYGA8NXmbIC&pg=PA38 |author= Uriona, Todd J. |date= 2008 |title= The Function of the Crocodilean Diaphragmaticus |isbn= 9780549977285 |accessdate= October 21, 2014}}
=Avoiding the constraint=
Birds have erect limbs and rigid bodies, and therefore do not flex sideways when moving. In addition many of them have a mechanism which pumps both lungs simultaneously when the birds rock their hips.{{cn|date=July 2017}}
Most mammals have erect limbs and flexible bodies, which makes their bodies flex vertically when moving quickly. This aids breathing, as it expands or compresses both lungs simultaneously.{{cn|date=July 2017}}
Contrary evidence
Contrary to the above model, breathing is maintained in lizards during movement, even above their aerobic scope, and arterial blood remains well oxygenated.{{cite encyclopedia
| last=Bennett
| first=Albert F.
| url=http://compphys.bio.uci.edu/bennett/pubs/120.pdf
| title=Exercise performance of reptiles
| pages=113–138
| encyclopedia=Comparative Vertebrate Exercise Physiology: Phyletic Adaptations
| series=Advances in Veterinary Science and Comparative Medicine
| volume=38B
| editor1-last=Jones
| editor1-first=James H.
| editor2-last=Cornelius
| editor2-first=Charles E.
| editor3-last=Marshak
| editor3-first=R. R.
| publisher=Academic Press
| location=New York
| year=1994
| isbn=0120392399
| access-date=2009-12-03
| archive-date=2010-06-10
| archive-url=https://web.archive.org/web/20100610182449/http://compphys.bio.uci.edu/bennett/pubs/120.pdf
| url-status=dead
}}
In popular culture
Paleontologist Richard Cowen wrote a limerick to explain and celebrate Carrier's rule:
{{poemquote|The reptilian idea of fun
Is to bask all day in the sun.
A physiological barrier,
Discovered by Carrier,
Says they can't breathe, if they run.}}
See also
References
{{reflist}}