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.
Contents
It was named, by English paleontologist Richard Cowen, for David R. Carrier, who wrote his observations on the problem in 1987.
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.
Partial solutions
Most snakes have only one lung. It is not known how this lung operates during locomotion, other than that they can indeed move and breathe at the same time.
Monitor lizards increase their stamina by using bones and muscles in the throat and floor of the mouth to "gulp" air via gular pumping.
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.
Complete solutions
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 evidence
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.