Carrier's constraint
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.[1]
It was named, by English paleontologist Richard Cowen, for David R. Carrier, who wrote his observations on the problem in 1987.[2][3][4]
Consequences
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.
Solutions
Workarounds
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.[citation needed]
Monitor lizards increase their stamina by using bones and muscles in the throat and floor of the mouth to "gulp" air via gular pumping.[5]
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 the active prey or escaping the 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.[6]
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.[citation needed]
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.[citation needed]
Further, a moment's contemplation will yield the notion that glycolysis (rapid but anerobic ATP production), and the resulting lactic acid accumulation in tissues and blood during strenuous exercise, constitute a partial avoidance of the need to breathe during such exercise. However, this soon results in adicification of the blood (metabolic acidosis and development of an "oxygen debt"), leading to obligatory cessation of movement until the lactate is removed from the blood (i.e., the repaying of the "oxygen debt"). Even among land mammals (which show little side-to-side flexing during running), tolerance of extreme metabolic acidosis allowed by high blood concentrations of buffering bicarbonate ion has evolved, allowing longer periods of running and longer-term oxygen debt "loans". Thus, loping canids such as wolves have high blood bicarbonate concentrations,[citation needed] allowing them to run down and kill deer, caribou, elk and even moose by simply following them at a running pace for an hour or more.[citation needed] Interestingly, groups of San (=Bushman) hunters in Africa used a similar strategy to run down and kill large antelopes over a period of a few days, alternating the chase among themselves. [citation needed]
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.[7]
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.[3]
See also
References
- ^ 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. (eds.). Evolutionary Paleobiology. Chicago: University of Chicago Press. p. 337+. ISBN 0-226-38911-1.
- ^ a b 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. 96 (1). Research Triangle Park: Sigma Xi: 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. (eds.). Comparative Vertebrate Exercise Physiology: Phyletic Adaptations. Advances in Veterinary Science and Comparative Medicine. Vol. 38B. New York: Academic Press. pp. 113–138. ISBN 0120392399.