Mammals move by swinging their limbs in a front-to-back plane beneath their bodies, a posture that sets them apart from reptiles and amphibians, whose legs splay out to the sides. This fundamental arrangement, called parasagittal locomotion, lets mammals walk, run, swim, fly, dig, and climb with remarkable efficiency. But the specifics vary wildly: a cheetah sprinting at 29 meters per second and a mole tunneling through soil share the same basic skeletal blueprint, reshaped by evolution for completely different tasks.
Limbs Under the Body, Not Out to the Sides
The single biggest difference between how mammals move and how most reptiles move comes down to limb orientation. Reptiles hold their legs out horizontally, pushing against the ground in a side-to-side motion that involves a lot of rotational twisting at the shoulder and hip. Mammals tuck their legs directly underneath the torso. This vertical alignment means the limbs swing forward and backward in a single plane, like pendulums.
This shift didn’t happen overnight. It took roughly 200 million years, progressing through early mammal ancestors that used slow, forceful limb movements combining rotation and retraction. The final pieces fell into place with three key changes: a shoulder blade that could slide freely along the ribcage, a shoulder socket that faced downward instead of sideways, and an elbow joint that worked like a simple hinge. Together, these features let mammals channel muscle force into powerful forward-and-back motion rather than wasting energy on twisting. The result is a limb with lower inertia and a center of mass positioned closer to the body, meaning less energy is needed to swing each leg through a stride.
Walking, Trotting, and Galloping
Most land mammals use a predictable sequence of gaits as they speed up. At low speeds, they walk, keeping at least one foot on the ground at all times. As speed increases, they shift to a trot, where diagonal pairs of legs move together. Push faster still, and many mammals break into a gallop, where the body launches into brief airborne phases between footfalls.
The transition to a gallop is driven by a simple mechanical reality. At higher speeds, the body’s center of mass is moving so fast horizontally that each ground contact only needs to redirect its path by a small angle. This makes the brief, powerful collisions of a gallop more efficient than the continuous ground contact of a trot. It also explains why galloping only works at higher speeds: at slow speeds, the deflection angles required at each footfall would be too large, and the energy losses from those collisions would be enormous.
Not all gallops are the same. Horses use a transverse gallop, where the front legs initiate the upward redirection of the body between airborne phases. Cheetahs use a rotary gallop, where the hind legs take on that role. The distinction matters because it changes how the spine flexes and extends, and it influences top speed. Cheetahs exploit their highly flexible spines to increase stride length, reaching about 29 meters per second (roughly 65 mph). At top speed, they cycle through an estimated 4 strides per second. Even at a moderate 9 meters per second, they manage 2.4 strides per second, ramping up to 3.2 strides per second at 18 meters per second.
Muscles Built for Speed or Endurance
What a mammal can do with its legs depends heavily on the type of muscle fiber packed into them. Slow-twitch fibers contract with less force but resist fatigue, making them ideal for sustained activity like long-distance travel. Fast-twitch fibers generate explosive power but tire quickly.
Body size plays a surprisingly large role in which fibers dominate. Small mammals like mice have muscles composed mostly of fast-twitch fibers loaded with oxygen-processing enzymes. Large mammals like humans rely more heavily on slow-twitch and moderately fast fibers. This pattern holds across species and helps explain performance differences. A cheetah’s muscles, rich in the fastest fiber types, can produce enormous power over a very short burst, letting it accelerate explosively. A gazelle, by contrast, is built to cruise efficiently at high speed for longer periods but with less raw acceleration. The tradeoff between sprint power and endurance efficiency is written directly into muscle composition.
How the Brain and Spinal Cord Coordinate Gait
Walking forward is so fundamental that it doesn’t require much brain involvement at all. The spinal cord contains networks of neurons called central pattern generators that produce the rhythmic, alternating signals needed for basic forward locomotion. These circuits can drive a walking pattern even when higher brain regions are shut down. In experiments where the brain’s movement-planning centers were fully inactivated, mice could still walk forward on a treadmill normally.
The brain becomes essential for more complex movements. Walking backward, for instance, requires input from the motor cortex and a deep brain structure called the striatum, which integrates signals from multiple brain regions. The striatum is also critical for selecting between different gaits, adjusting stride width and length, and controlling the vigor of movement. So while the spinal cord handles the basic rhythm of putting one foot in front of another, the brain acts as the conductor, choosing when to start, stop, speed up, change direction, or switch gaits entirely.
Swimming With an Up-and-Down Stroke
Whales and dolphins are mammals that returned to the ocean, and their swimming style reflects their terrestrial ancestry. Fish swing their tails side to side, but cetaceans oscillate their tail flukes up and down, mirroring the spinal flexion pattern their land-dwelling ancestors used for running. The fluke itself is a crescent-shaped lifting surface, not unlike an airplane wing turned sideways, that generates thrust on both the upstroke and downstroke.
This approach is remarkably efficient. Dolphins and whales convert over 75% of their muscular effort into forward propulsion, with some estimates reaching 90%. For comparison, undulatory swimmers like trout achieve roughly 60 to 80% efficiency, while drag-based swimmers (animals that paddle, like muskrats or humans) manage only 20 to 35%. The high efficiency holds across an enormous size range. Baleen whales weighing tens of thousands of kilograms maintain the same 75%-plus efficiency as much smaller dolphins, suggesting the underlying physics of their tail-driven propulsion scales well regardless of body size.
Powered Flight in Bats
Bats are the only mammals capable of true powered flight, and they achieve it with a wing design unlike anything else in the animal kingdom. Their wings are made of thin skin membranes stretched between enormously elongated finger bones, the forearm, and the hind legs. The section between the fingers, called the dactylopatagium, is the core propelling surface.
What makes bat flight so agile is the number of independently controllable joints in each wing. Because the membrane is anchored to individual finger bones, bats can reshape their wing profile in real time. During the downstroke, the membrane stretches taut between the fingers to maximize lift. During the upstroke, the membrane’s natural elasticity allows it to deform and fold slightly, reducing drag. This dynamic reshaping is something gliding mammals (like flying squirrels) cannot do. Their membranes act as passive surfaces, good for coasting but useless for generating thrust. Bats, by contrast, adjust wing shape continuously to optimize airflow, giving them the tight turning ability and hovering capacity that makes them effective nocturnal hunters.
Digging and Climbing Specialists
Some mammals have reshaped the standard limb plan for movement through soil or trees. Moles are among the most extreme diggers. Their upper arm bone is so widened for muscle attachment that its width equals roughly two-thirds of its length, a proportion that would look bizarre on any other mammal. This stocky, almost disc-like humerus provides massive leverage for the chest and shoulder muscles that power each digging stroke, letting moles push through compacted earth with surprising force.
Tree-dwelling primates face a different challenge: gripping branches while moving in three dimensions. Several lineages of New World monkeys have evolved prehensile tails that function as a fifth limb, strong enough to support the animal’s entire body weight. These tails work because of structural changes to the individual vertebrae. Compared to non-grasping tails, prehensile tails have a longer proximal section with more joints, giving greater overall flexibility even though each joint has a limited range of motion. The vertebrae further from the body are shorter and wider, built to resist the bending and twisting forces of suspension. The bony ridges where the tail’s flexor muscles attach are also significantly more developed, providing the grip strength needed to hang from a branch while the hands are free to reach for food.