Carrier's constraint is the observation that air-breathing vertebrates which have two lungs and flex their bodies sideways during locomotion find it very 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.
Most lizards move in short bursts, with long pauses for breath.
Around the Late Triassic period, animals with Carrier's constraint were preyed on by bipedal species that evolved a more efficient stride.
Most snakes have only one lung, so Carrier's constraint does not apply. It is not known how this lung operates during locomotion, other than that snakes can indeed move and breathe at the same time.
Some other lizards, mainly agamids, use bipedal locomotion for running and avoid sideway flexing. Bipedality in modern lizards is very rare, but it is effective way to run without pause and breath, 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 (University of Utah) hypothesizes that costal ventilation may have aided the upright posture in overcoming the constraint.
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.
Most mammals have erect limbs and flexible bodies, which makes their bodies flex vertically when moving quickly. This aids breathing, as it expands and compresses both lungs simultaneously.
Contrary to the above model, breathing is maintained in lizards during movement, even above their aerobic scope, and arterial blood remains well oxygenated.
In popular culture
Paleontologist Richard Cowen wrote a limerick to explain and celebrate Carrier's rule:
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.
- Carrier, D.R. (1987). "The evolution of locomotor stamina in tetrapods: circumventing a mechanical constraint". Paleobiology (13): 326–341.
- Cowen, Richard (1996). "Locomotion and Respiration in Aquatic Air-Breathing Vertebrates". In Jablonski, David; et al. Evolutionary Paleobiology. Chicago: University of Chicago Press. p. 337+. ISBN 0-226-38911-1.
- Cowen, Richard (2003). "Respiration, Metabolism, and Locomotion". Richard Cowen, University of California, Davis. Retrieved October 21, 2014.
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.
- Shipman, Pat (January 2008). "Freed to Fly Again". American Scientist. Research Triangle Park: Sigma Xi. 96 (1): 20. Retrieved October 21, 2014.
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.
- Summers, Adam (2003). "Monitor Marathons". Natural History. 112 (5): 32. Retrieved October 21, 2014.
- Uriona, Todd J. (2008). "The Function of the Crocodilean Diaphragmaticus". ProQuest. Retrieved October 21, 2014.
- Bennett, Albert F. (1994). "Exercise performance of reptiles" (PDF). In Jones, James H.; Cornelius, Charles E.; Marshak, R. R. Comparative Vertebrate Exercise Physiology: Phyletic Adaptations. Advances in Veterinary Science and Comparative Medicine. 38B. New York: Academic Press. pp. 113–138. ISBN 0120392399.