Adaptations of Bones in the Mammalian Foot

By Renn Tumlison,

bones of a human foot

 This photograph shows the bones of a human foot (left side). The large bone on the bottom right is the heel bone (calcaneus). Toes are comprised of three bones, with the exception of the "big toe," which has only two skeletal elements (top left). The longer bones in the middle of the foot are metatarsals, and the remaining bones are tarsals (the two bones at the top right are the tibia and fibula of the lower leg).

Besides humans, mammals such as raccoons, bears, and armadillos walk with the heel on the ground. This is called plantigrade posture. Such posture is good for ambulation (walking) but is not so efficient for running. Human sprinters are supposed to "run on their toes." In nature, running mammals tend to be built to run on their toes.

The top photo is of the bones of the left hind foot of a cat and the bottom photo is of the left hind foot of a live dog. The feet are naturally held in this position - with the heel off the ground - and only the toes maintain contact with the substrate. Because such mammals are running on their toes (digits), this is called digitigrade posture. Besides cats and dogs, other predators are adapted for the chase through this posture.

bones of the left hind foot of a cat
Live Dog Foot

If running on the toes increases speed, the same adaptation would be useful for potential prey animals to escape predators. Pictured here is the foot of a pig. The big toe is completely absent, leaving four toes (two of which are large and two reduced in size - one small toe is visible from this angle). Because the weight of the animal is borne by two toes, this is referred to as a paraxonic foot, and the posture is unguligrade. The artiodactyls (pigs, deer, bison, elk, etc.) all have this type of foot, balancing on the last bone of the toe, much like a ballet dancer. Notice that the two enlarged metatarsals are not fused.

Pig Foot
Deer Foot Blackback

The metatarsals are fused in the foot of a deer. A longer foot provides greater spring for running. As an analogy, consider the effect of playing racquetball with a tennis racquet. The extra length of the tennis racquet would result in greater strike force on the ball. Similarly, a longer foot would allow greater speed in an animal such as a deer. However, when the metatarsals are lengthened, they would be broken more easily. The adaptation to circumvent this is called a cannon bone. A cannon bone is formed when the two elongated (and thus relatively thinned) metatarsals fuse together to restore strength to a longer foot. Compare the pig and the deer feet to see these relationships. Note: the front feet of mammals with a cannon bone are similarly adapted with fused metacarpals forming a cannon bone.

 Still, not all fast running is based on the paraxonic foot. Horses and their relatives run on one toe. Because this is the middle toe of the five toes typical of the basic mammalian plan, it is referred to as a mesaxonic foot. All other digits have been lost, and all of the weight is carried by the middle metatarsal and the three bones of the middle toe. Other relatives of horses, such as the tapir and the rhinoceros, also use the middle toe to support their weight, but smaller toes remain on the foot.

Horse Foot Blackback
Bat Foot Blackback

At the other extreme, some mammalian feet are adapted merely to "hang around." Bats spend the day or the winter hanging in their roosts by use of the claws of their feet. All five toes are present and terminate in long, curved claws, which allow the bat to hang effortlessly.



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