Carrier's constraint is the observation that air-breathingvertebrates with twolungs that flex their bodies sideways duringlocomotion 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.[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]
Most lizards move in short bursts, with long pauses for breath.[citation needed]
Around the Late Triassic period, animals with Carrier's constraint were preyed on by bipedal species that evolved a more efficient stride.[citation needed]
Mostsnakes 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 viagular pumping.[5]
Some other lizards, mainlyagamids, usebipedal 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 theUniversity of Utah hypothesized thatcostal ventilation may have aided the upright posture in overcoming the constraint.[6]
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]
Mostmammals 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.[citation needed]
Contrary to the above model, breathing is maintained in lizards during movement, even above their aerobic scope, and arterial blood remains well oxygenated.[7]
Paleontologist Richard Cowen wrote alimerick to explain and celebrate Carrier's rule:[3]
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.
If the animal is walking, it may be able to breathe between steps, butsprawling vertebrates cannot run and breathe at the same time. I shall call this problem Carrier's Constraint.
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.