It’s all in the toes – Why Old World monkeys change their limb posture to run
Just by looking at its limbs, you can tell that a cheetah is born to run. Not only does this felid have non-retractable claws which act like cleats on a runner's shoe - a unique feature among big cats - but it also has the familiar tip-toe limb posture which allows the carnivore to reach further with each stride during the chase. Many other fast-moving terrestrial mammals share this limb posture, technically known as "digitigrade", but at least one group employs it in an unexpected way.
Variation in cercopithecine forelimb posture. The darker-colored bones represent full digitigrade posture, while the grey ones show an approach to the platigrade condition. From Patel 2009.
Although monkeys are often depicted as inhabitants of the tropical forest canopy, in many parts of Africa and Asia cercopithecine primates, or "Old World monkeys" such as baboons and macaques, walk on the ground. As they do so they adopt a digitigrade posture, with the palms of their hands and soles of their feet held off the ground. (If their palms or soles did contact the ground, their limb posture would be called "plantigrade.") According to studies of other digitigrade mammals, it would be expected that this would help the terrestrial monkeys speed up quickly. By walking on their toes, these monkeys would be extending their limb length for longer strides and more efficient locomotion at high speed.
Yet, as scientist Biren Patel describes in a recent American Journal of Physical Anthropology paper, at least three species of ground-dwelling cercopithecine monkeys don't run with a digitigrade limb posture. When Patel observed individual olive baboons (Papio anubis), Rhesus macaques (Macaca mulatta), and patas monkeys (Erythrocebus patas) walking along a runway at varying speeds, he observed that the monkeys held their palms and soles closer to the ground as they moved faster. Contrary to what is seen in other mammals, these monkeys adopted a less digitigrade limb posture when running, and the reason why may have everything to do with the anatomy of their hands and feet.
The hands and feet of primates can be useful for grabbing, grasping, manipulation of objects, and other activities, but they are not especially well-suited to running. There is an apparent trade-off between dexterity and adaptation to moving quickly - a horse can run very fast, but is ill-equipped to manipulate objects with its single, hoof-tipped toe. Thus, among the cercopithecines studied by Patel, it appears that they bring their palms and soles closer to the ground to lessen the physical stresses involved with running. The highly mobile joints of the primate arm, wrist, and fingers cannot cope with the forces associated with running as well as those of an animal - like a cheetah or horse - which lack such flexibility, and so the cercopithechine monkeys change their limb posture to compensate.
Stick figures showing forelimb movement of a male baboon at slow (top), intermediate (middle), and fast (bottom) speeds. The red lines represent the bones of the hand, excluding the fingers. Notice how these bones of the palm come closer to the ground at higher speeds. From Patel 2009.
The significance of this locomotor shifting in Old World monkeys is that their habitual tip-toe posture is probably not an adaptation for moving fast. This strongly contrasts with what has been hypothesized for other groups of fast-moving, digitigrade animals, and it would seem that the flexibility of the primate arm is one of the chief reasons for the difference. A digitigrade limb posture may help a baboon or macaque move farther, faster when walking at relatively slow speeds, but when it comes time to run for it, bolting on tip-toe just doesn't work as well if you are going to use those hands and fingers for things like grasping branches or picking the nits off your friend's back.
References:
Patel, B. (2009). Not so fast: Speed effects on forelimb kinematics in cercopithecine monkeys and implications for digitigrade postures in primates American Journal of Physical Anthropology, 140 (1), 92-112 DOI: 10.1002/ajpa.21039