Bodies in Motion

Land animals have developed countless solutions to a single challenge: how best to get from one place to another. Modes of travel vary depending on an animal’s size and environment, with the goal of moving efficiently toward resources and away from danger.

By Jason Treat, Brad Scriber, Patricia Healy and Kennedy Elliott

Illustrations by Bryan Christie Design

Published April 23, 2020

Fins and feet

Some modern fish can move overland by pushing their bodies up with their fins and flopping forward. The limbs of tetrapods—four-legged vertebrates, including amphibians—evolved from the fins of ancestral fish species. Feet developed underwater, but the search for food and safety may have coaxed some early walkers onto dry ground. Natural selection drove arms, legs, feet, and hands to diversify and specialize.

Tiger salamander

Ambystoma tigrinum

A salamander’s life cycle transition—from aquatic larva to terrestrial adult with sprawled legs—reflects one way that walking evolved.

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Graceful mudskipper

Periophthalmus gracilis

Less refined than their name suggests, mudskippers can use their front fins on land in a lurching style of locomotion called crutching.

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Quadrupeds

From skittering shrews to lumbering elephants, quadruped bodies are shaped by physics and physiology. Larger animals have more powerful muscles, but their skeletons have to support much greater weight. Smaller animals typically move more quickly but use energy less efficiently. Leg differences reflect certain trade-offs.

Cheetah

Acinonyx jubatus

A flexible spine that arches and then extends gives cheetahs their extremely long stride. Their light, furry tails and sharp claws offer stability during turns.

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African elephant

Loxodonta africana

Titans on tiptoes, elephants can amble quickly but can’t trot or gallop. A pad behind the toes lets their raised-heel, digitigrade bone structure work like humans’ flat feet.

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Zebra-tailed lizard

Callisaurus draconoides

All lizards bend their bodies from side to side, elongating their four-legged steps. But this type can extend its ankles and run on its toes, traveling 50 body lengths a second.

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Giraffe

Giraffa camelopardalis

Top-heavy giraffes carry about 10 percent of their weight above the shoulders. Raising and lowering their long necks while walking keeps their eyes level and maintains balance.

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Eastern gray squirrel

Sciurus carolinensis

When climbing down, squirrels swivel their hind feet at the ankles so their toes point up and their claws are positioned to carry their weight. Splayed limbs also enhance their grip.

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African spurred tortoise

Sciurus carolinensis

Plant-eaters with rigid defensive shells, tortoises favor stability over speed. They can climb steep slopes, and their well-spaced feet keep them from toppling over.

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Bipedalism

Humans and birds use forelimbs for grasping or flying but rely on their two legs for walking. Other animals move bipedally only when needed. Kangaroos forage pentapedally—using all four legs and their tails—but hop with just their hind legs for speed.

Human

Homo sapiens

More efficient when walking than when running, human legs function like inverted pendulums. Part of each step uses momentum and gravity to move the body forward.

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Theropod

Tyrannosaurus rex

Tyrannosaurs may have been kings, but it’s unlikely they could run fast. Their muscles were too small, and the load of their multi-ton bodies would have broken their leg bones.

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Ostrich

Struthio camelus

An ostrich’s heel matches the height of a human knee. Large muscles around short femurs plus long, light bones in the rest of the leg help the ostrich take big, quick steps.

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Red kangaroo

Macropus rufus

Thanks to elastic Achilles tendons that store and release energy with each hop, kangaroos can increase speed without burning more calories.

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Lar gibbon

Hylobates lar

Gibbons can swing like pendulums below close handholds, a movement called brachiation. At high speeds they release both hands to go completely airborne between supports.

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No legs

Snakes use muscles, skin, and a flexible body to propel themselves over a variety of surfaces. Some techniques use less friction and are better for loose, level terrain. Others use points of contact along the pliable body to push off of bumps on the ground, the sides of passages, or tree bark.

Sidewinder

Crotalus cerastes

On loose sand, the head lifts into place first. The rest follows in a whipping motion.

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Many legs

Most invertebrates spread their body weight across multiple legs, which lessens the load on each limb. Moving requires coordination or even varied leg lengths so they don’t trip over their own feet.

Atlantic ghost crab

Ocypode quadrata

Sideways-running ghost crabs pause frequently while fleeing threats. This slows the buildup of lactate and allows them to travel farther before tiring.

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House centipede

Scutigera coleoptrata

Undulating steps begin with the rear legs, which are longer to avoid tangling. Each stride exceeds total body length.

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Sources: Parvez Alam, University of Edinburgh; Miriam A. Ashley-Ross, Wake Forest University; Andrew Biewener, Harvard University; S. Tonia-Hsieh, Temple University; John Hutchinson, Royal Veterinary College, University of London; Bruce Jayne, University of Cincinnati; Melissa Merrick and John Koprowski, University of Arizona; Scott Stahl, Stahl Exotic Animal Veterinary Services, Naomi Wada, Yamaguchi University