One of the key features of an animal is that it moves. It moves parts of its body when it opens its mouth, bends its neck or curls its tentacles. Most creatures also move about in their surroundings. They run, walk, jump, hop, slither, swim or fly. All animal movement is made possible by muscles. A gorilla has about 640 muscles, making up about two-fifths of its body weight. A few animals do not move about, at least as adults. Barnacles and mussels are stuck to seashore rocks. But they were mobile during their young, or larval, stages.

Moving through water

Water is much denser than air and so resists movement more. To travel through water quickly, creatures must be smooth and streamlined so the water slips past them easily. Fish swish their tails from side to side to provide the forward propulsive force for swimming. A whale or dolphin moves its fluke tail up and down to produce the same effect. A fish’s fins and a dolphin’s flippers provide control for steering, slowing and going up or down. Penguins flap their wings and turtles their flippers to “fly” through the water.

Moving through air

Only three groups of living animals truly fly in a sustained, controlled way: birds, bats and insects. Air is so thin it produces little resistance to movement, but it provides very little buoyancy either. Fliers flap their wings down to create a lifting force as well as back to push themselves forwards.

Birds and bats have very thin, light bones to reduce body weight and so save on the energy needed to stay airborne. Even so, they must take in almost twice the amount of energy as food, compared to their ground-dwelling counterparts. The down-flapping wing muscles in the chest are by far the largest muscles in the body of a bird or bat.

Insects and bats control their movement by tilting or twisting their wings. Tiny insects like gnats and midges flap their wings nearly 1000 times each second to stay aloft. Birds control their flight by fanning and twisting their feathers.

Moving on land

Large land animals move in a way suited to their habitat. On open plains, cheetahs, gazelles and antelopes have long legs and run very fast. Forest animals like deer are slower in straight-line speed, but more agile as they zigzag between trees.

Limbs are not vital for movement. Many snakes are limbless, yet they can slither on the ground, swim in water, climb trees—and even glide. The sidewinder, a kind of rattlesnake, has a winding method of moving, which means that it only has one small patch of its body in contact with the hot sand at any one time. More importantly, allows it to move rapidly over sand, a surface that offers little grip.