Carrier's constraint is the observation that air-breathing

vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

which have two

lungs The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either si ...

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. 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.

Solutions

Workarounds

Most

snake Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more ...

s have only one lung, so Carrier's constraint does not apply.

Monitor lizard Monitor lizards are lizards in the genus ''Varanus,'' the only extant genus in the family Varanidae. They are native to Africa, Asia, and Oceania, and one species is also found in the Americas as an invasive species. About 80 species are rec ...

s increase their stamina by using bones and muscles in the throat and floor of the mouth to "gulp" air via gular pumping. Some other lizards, mainly

agamid Agamidae is a family of over 300 species of iguanian lizards indigenous to Africa, Asia, Australia, and a few in Southern Europe. Many species are commonly called dragons or dragon lizards. Overview Phylogenetically, they may be sister to the ...

s, use bipedal locomotion for running and avoid sideways flexing. Bipedality in modern lizards is very rare, but it is an effective way to run without pausing to breathe, and is advantageous for catching active prey or evading predators.

Crocodilia Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest livi ...

ns 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 the

University of Utah The University of Utah (U of U, UofU, or simply The U) is a public research university in Salt Lake City, Utah. It is the flagship institution of the Utah System of Higher Education. The university was established in 1850 as the University of De ...

hypothesized that costal ventilation may have aided the upright posture in overcoming the constraint.

Avoiding the constraint

Bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

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 Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...

s 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.

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 Limerick ( ; ga, Luimneach ) is a western city in Ireland situated within County Limerick. It is in the province of Munster and is located in the Mid-West which comprises part of the Southern Region. With a population of 94,192 at the 2 ...

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.

References

{{reflist Respiration