A hindlimb, also commonly written as two words, hind limb, is one of the back legs on any four-legged animal. This includes the entire limb, from the hip to the toes. The hindlimbs may be matched in size to the front limbs, but in many animals the hindlimbs become larger than the forelimbs, providing extra power for the animal to hunt or to escape. There is evidence that some animals that don’t have a hindlimb once had these appendages and walked the earth, instead of crawling or swimming as they do today.
A powerful hindlimb structure is a necessity for many animals. Hunters such as the cheetah have very muscular rear legs that allow them to attain bursts of speed in order to capture their prey. The cheetah’s back limbs appear proportional to its body, but for some animals the hindlimb has developed to the point that even a fleeting glance shows a distinct difference between the hindlimb and the forelimb.
Rabbits and hares have very large, muscular rear legs. They use these legs to provide the power to escape predators, covering the ground in large, hopping strides that often take them to safety. Kangaroos and wallabies have even more pronounced differences between the hindlimb and the forelimb structures, with very large, heavy rear legs and small, delicate forelegs that serve them almost as hands and arms, though there are times when they use their front legs for walking, as well.
Whales and other sea mammals have evidence of having had hindlimbs at some point in their evolutionary past, but the evidence is not usually visible in the adult animals. During their development the embryos of many of these mammals display actual external rear legs, but by the time the animal is born there is no longer any visible sign of it. The limbs either disappear completely or remain as vestigial appendages, visible only when the skeleton of the animal is exposed.
Scientists have found fossil evidence that the prehistoric ancestors of snakes also had hindlimbs. Snakes have developed to the point that an adult snake does not show any visible evidence of having a hindlimb, but in many cases the developing snake has very pronounced signs of hindlimbs while it is still in the egg. These disappear as the snake becomes more fully developed, and there is little or no sign of them by the time the snake hatches.