Upper Limb - Evolutionary Variation

Evolutionary Variation

The skeletons of all mammals are based on a common pentadactyl ("five-fingered") template but optimised for different functions. While many mammals can perform other tasks using their forelimbs, their primary use in most terrestrial mammals is one of three main modes of locomotion: unguligrade (hoof walkers), digitigrade (toe walkers), and plantigrade (sole walkers). Generally, the forelimbs are optimised for speed and stamina, but in some mammals some of the locomotion optimisation have been sacrificed for other functions, such as digging and grasping.

In primates, the upper limbs provide a wide range of movement which increases manual dexterity. The limbs of chimpanzees, compared to those of humans, reveal their different lifestyle. The chimpanzee primary uses two modes of locomotion: knuckle-walking, a style of quadrupedalism in which the body weight is supported on the knuckles (or more properly on the middle phalanges of the fingers), and brachiation (swinging from branch to branch), a style of bipedalism in which flexed fingers are used to grasp branches above the head. To meet the requirements of these styles of locomotion, the chimpanzee's finger phalanges are longer and have more robust insertion areas for the flexor tendons while the metacarpals have transverse ridges to limit dorsiflexion (stretching the fingers towards the back of the hand). The thumb is small enough to facilitate brachiation while maintaining some of the dexterity offered by an opposable thumb. In contrast, virtually all locomotion functionality has been lost in humans while predominant brachiators, such as the gibbons, have very reduced thumbs and inflexible wrists.

In ungulates the forelimbs are optimised to maximize speed and stamina to the extent that the limbs serve almost no other purpose. In contrast to the skeleton of human limbs, the proximal bones of ungulates are short and the distal bones long to provide length of stride; proximally, large and short muscles provide rapidity of step. The odd-toed ungulates, such as the horse, use a single third toe for weight-bearing and have significantly reduced metacarpals. Even-toed ungulates, such as the giraffe, uses both their third and fourth toes but a single completely fused phalanx bone for weight-bearing. Ungulates whose habitat does not require fast running on hard terrain, for example the hippopotamus, have maintained four digits.

In species in the order carnivora, some of which are insectivores rather than carnivores, the cats are some of the most highly evolved predators designed for speed, power, and acceleration rather than stamina. Compared to ungulates, their limbs are shorter, more muscular in the distal segments, and maintain five metacarpals and digit bones; providing a greater range of movements, a more varied function and agility (e.g. climbing, swatting, and grooming). Some insectivorous species in this order have paws specialised for specific functions. The sloth bear uses their digits and large claws to tear logs open rather than kill prey. Other insectivorous species, such as the giant and red pandas, have developed large sesamoid bones in their paws that serve as an extra "thumb" while others, such as the meerkat, uses their limbs primary for digging and have vestigial first digits.

The arboreal two-toed sloth, a South American mammal in the order pilosa, have limbs so highly adapted to hanging in branches that it is unable to walk on the ground where it has to drag its own body using the large curved claws on its foredigits.