Carrier's constraint is the observation that air-breathing vertebrates which have two lungs and flex their bodies sideways during locomotion

Animal locomotion

Animal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, swimming, jumping and flying. Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators...

 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.

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.

Partial solutions

Snake

Snake

Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s 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 lizard

Monitor lizard

Monitor lizards are usually large reptiles, although some can be as small as in length. They have long necks, powerful tails and claws, and well-developed limbs. Most species are terrestrial, but arboreal and semiaquatic monitors are also known...

s increase their stamina by using bones and muscles in the throat and floor of the mouth to "gulp" air via buccal pumping

Buccal pumping

Buccal pumping is a method of respiration in which the animal moves the floor of the mouth in a rhythmic manner that is externally apparent.This method has several stages. These will be described for an animal starting with lungs in a deflated state: First, the glottis is closed, and the...

.

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

Bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s 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 mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s 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.