Carrier's constraint

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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.
  • this shunts stale air from lung to lung instead of expelling it completely to make room for fresh air.[1]

Consequences of Carrier's constraint[edit]

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.

Ways of avoiding Carrier's constraint[edit]

Partial solutions[edit]

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.[2][dead link]

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.

Complete solutions[edit]

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[edit]

Contrary to the above model, breathing is maintained in lizards during movement, even above their aerobic scope, and arterial blood remains well oxygenated.[3]

See also[edit]


  1. ^ Carrier, D.R. (1987). "The evolution of locomotor stamina in tetrapods: circumventing a mechanical constraint". Paleobiology (13): 326–341. 
  2. ^ Summers, A (2003). "Monitor Marathons". 
  3. ^ Bennett, Albert F. (1994). "Exercise performance of reptiles". 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.