Like any other adaptation, locomotor adaptations are influenced by the ancestry of the species being considered. Consequently, primate species have evolved a wide number of solutions to the the same basic problem. In the case of locomotion, this has lead to a wide and varied locomotor repertoire. In previous posts I looked at how the competing demands of food and locomotion can affect the primate skeleton and discussed vertical clinging and leaping. In this post I will look at quadrupedalism.
Primate quadrupeds can be, more or less, divided into three broad groups.
Arboreal quadrupeds are, perhaps, the most common type of quadrupeds among primates. Arboreal quadrupeds face a number of unique challenges such as relative size of the support compared to the size of the animal and differences in support size depending on vertical level within the forest (as well as the diversity of tree species in the forest). Arboreal quadrupeds have short, somewhat robust, limbs that are roughly the same length. The hindlimbs provide the propulsion, while the forelimbs and tail steer and provide stability. One of the diagnostic areas is in the elbow. The medial epicondyle is large and directed medially. Below is a picture of a human epicondyle for reference:
One of the requirements for arboreal life is a strong grip, and the large medial epicondyle provides an attachment area for the wrist and some finger flexors. The olecranon process of the ulna is long ( to provide leverage for the triceps ) and shallow (limits extension of the elbow). The bones of the wrist – particularly the hamate (see picture below) are broad to support weight
The fingers are intermediate in length between those of terrestrial and those of suspensory species. The hindlimbs also display some interesting features. The neck of the femur is highly angled, and the femoral condyles are asymmetric – features that enhance abduction (motion away from the median plane).
Terrestrial quadrupeds are relatively rare among the primates. Baboons, some macaques, and the patas are about it. They come in two varities, digitigrade and palmigrade. In digitigrade quadrupeds the animal supports its weight on its digits rather than its palm. Terrestrial primates have narrow, deep chests, long limbs and a short or missing tail. The limbs display a number of adaptations for speed. For example, the morphology of the shoulder joint allows for rapid fore-aft movement. In the elbow, the olecranon extends dorsally (towards the back) rather than proximally as in aboreal primates (which aides in extending the elbow) and the olecranon fossa is deep. The medial epicondyle is short and posteriorly directed (again, unlike the aboreal quadrupeds) which helps when the forearm is pronated (medially rotated – that is rotated towards the body). The wrist bones are broad and the metacarpals and fingers are short and robust. The hindlimbs are long and the ankel and toe bones are short and robust.
The final type of quadruped – really two types – are the knuckle-walkers and fist-walkers. When I originally conceived the idea for this series, my intention was to discuss them along with the other quadrupeds. After thinking about it, I think they deserve a separate post of their own.