- Carrier's constraint
Carrier's constraint is the observation that air-breathing
vertebrateswhich have two lungsand flex their bodies sideways during locomotionfind 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. [cite journal
title=The evolution of locomotor stamina in tetrapods: circumventing a mechanical constraint
Consequences of Carrier's constraint
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
Sea snakeshave only one lung. Monitor lizards increase their stamina by using bones and muscles in the throat and floor of the mouth to "gulp" air. [cite web | url=http://biomechanics.bio.uci.edu/_html/nh_biomech/monitor/monitor.htm | title=Monitor Marathons | date=2003 | last=Summers | first=A] Crocodilianshave three modes of locomotion: crawling for short distances (this sprawling gait is subject to Carrier's Constraint); "high walk" for longer distances (the erect limb posture minimizes sideways flexing); "gallop" in emergencies (avoids Carrier's Constraint but they can only gallop for a few seconds).
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.
mammals have erect limbs and flexible bodies, which makes their bodies flex vertically when moving fast. This aids breathing as it expands and compresses both lungs simultaneously.
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