Know Your Primate: Vertical Clingers and Leapers

Primatologists divide primate locomotion into a number of different categories. In this post I am going to look at vertical clinging and leaping (VCL). Below are two videos to give you a good idea of what I am talking about.

This first one demonstrates the simple act of leaping.

This one demonstrates a lemur determined to get somewhere (it’s only a couple of seconds long so pay attention):

So, how do they do it? To answer that question I need to talk about cursorial animals (i.e. animals that run fast or far) and their adaptations for speed. First, a few definitions. Locomotion is broken down into strides. A stride is, loosely, defined as a full cycle of motion. An animals speed is determined by two things, stride length and rate of stride. Some animals, such as the giraffe, get their speed from stride length while other animals get it through rate of stride. To get really fast, you need both.

Cheetahs are really fast, so let’s look at the skeletal adaptations that make them fast. As mentioned above, one way to achieve high speeds is to have long legs, but there is a catch. The legs have to be long relative to other parts of the body. Additionally, the distal limb segments (the radius and tibia) are usually longer than the proximal limb segments (humerus and femur). The metatarsals (the bones in the area between the wrist and fingers in the hand – in humans called metacarpals – and the bones in the area between the heel and toes) are also much longer and the animal walks on these rather than flat footed (or plantigrade). Ungulates, for example are named after their style of locomotion (unguligrade, which means hoof walking). In the shoulder the clavicle, which normally acts as a strut and somewhat immobilizes the shoulder, is reduced and the whole joint is rotated so that the forelimb works in the same plane as the hindlimb. This has the additional effect of increasing the length of the leg. Finally, the spine is flexible in a vertical plane – which makes the animal longer when the back is extended and has an impact on speed. Let’s see how this works out to increase stride length in one locomotion cycle. A cheetah gains part of its stride length when it first pushes off with it’s hindlimb (technically called rotation of the hindlimbs), it gets an additional amount of stride length from extension of its back and it gets a third amount from the period when it is unsupported – that is when all four limbs are off the ground. Some animals, bears for example, never have an unsupported phase. Others have one unsupported pahse, while the Cheetah has two (so it gets a fourth amount added to stride length). The second component of speed – the rate of stride – is gained through things like the rate of muscle contraction and having the origins and insertions of muscles closer to the joints. Further discussion of this aspect is outside the scope of this post.
So, what does this have to do with primates and leaping? Quite simply, we see a lot of these same adaptations in primates which engage in VCL. VCL primates have long hindlimbs, although unlike cheetahs and such, their forelimbs are shorter because they are not as heavily involved in locomotion. They have long trunks with highly flexible vertebrae – particularly in the lumbar region. They also have a longer ischium with the extension depending on whether the animal is a vertical clinger or a quadrupedal leaper. In vertical clingers the ischium is elongated posteriorly, while in quadrupedal leapers it is elongated distally. The head of the femur is usually short and thick – to help restrict motion to extension and flexion – and the hip as a whole does not display the simple ball-and-socket joint seen in other primates. The patellar groove has a pronounced lip to prevent displacement of the knee and the tibia and tarsals are long (although this varies). Interestingly enough, amongst the VCL primates these adaptations cause some difficulty for traveling on the ground:

Although the quadrupedal leapers – such as ring tailed lemurs – get along okay…