Tuesday, March 29, 2016

Primate Locomotion

We all know that one thing that sets us apart from other primates is that as adults, the majority of our locomotion is upright on two legs.  The scientific term for this is "obligate bipedalism" -- obligate here meaning that our anatomy does not allow us to walk any other way, and bipedal meaning on two (bi) feet (ped).  We may be special in walking upright, but that definitely doesn't mean that all other primates move about in the same way!  Let's take a look at how some of the other primates we've seen on Chimp & See get around.  For each species below, we'll look at their most common form of locomotion.

Starting with the primates most distantly related to us, we have galagos and pottos.  These are both small, nocturnal, arboreal, solitary primates, but they have surprisingly different modes of locomotion.  Pottos generally move by climbing slowly and deliberately, using their arms and legs about equally.  Most galagos (though it differs somewhat by species) are great leapers, sometimes traveling more than 2.5 meters (8.2 ft) in one jump!  They get most of their power from strong legs, which are much longer than their arms.  Since they're a bit hard to see in the black and white videos, here are a couple pictures to help familiarize you with these primates.

 Potto photo credit: Josh More                                                Galago photo credit: Wegmann

This video shows a clip of a slow-moving potto, then a couple leaping galagos.
Original clips: ACP00058omACP000cb2b

Before we look at the next clips, a very quick side lesson in skeletal anatomy.  When humans walk, they contact the ground with the heel bone: the calcaneus, shown in yellow in the image on the left below.  Some animals, like your dog or cat, only contact the ground with the bones that make up the toes.  Their heel bone, shown again in yellow in the wolf skeleton on the right, remains off the ground throughout their stride.


Human foot image credit: DBCLS                                                                                   Wolf image credit: Royal Natural History Volume 1

That sets the stage for the the African monkeys.  We have seen over a dozen different species of monkey, but we mostly see them using one of two subtly different types of terrestrial locomotion.  The first is called palmigrade quadrupedalism -- palmigrade meaning that they place the whole palm on the ground as they walk, and quadrupedal meaning on four (quad) feet (ped).  This is equivalent to the way humans place their whole foot on the ground.  The other type of locomotion we see in African monkeys on Chimp & See is called digitigrade quadrupedalism, where digitigrade means on the fingers/toes (digits).  This is equivalent to the way your dog or cat would walk, without touching the heel bone to the ground and instead supporting the weight on the fingers/toes.  At first glance, both of these types of locomotion look quite similar in C&S clips, but when you look closely, you can see how the wrist/ankle are in different positions, either contacting the ground, or held above it.

The first clip in this video is a green monkey walking palmigrade.  With each step, the whole hand and foot contact the ground.  The second clip shows a patas monkey walking digitigrade.  Like your cat or dog, the palm and heel remain raised off the ground as the monkey walks.

Original clips: ACP0006fnmACP000638c

Last we have the chimps and gorillas.  As adults, both gorillas and chimps knuckle-walk for the majority of their locomotion, but there is some debate as to whether this evolved independently in each lineage, or just once in an ancestor.  Either way, they both curl their fingers under, contacting the ground with the knuckles.  This is actually a behavior that has to develop in infant chimps and gorillas.  When they take their first quadrupedal steps, they walk palmigrade, just like the green monkey in the previous video.  Gorillas make the switch to knuckle-walking earlier than chimps do: at 10 months of age about 70% of quadrupedalism is knuckle-walking in gorillas, while only about 20% is knuckle-walking in chimps.

This video shows two adult chimps knuckle-walking, followed by an infant chimp taking a few palmigrade steps.  Last, a handsome silverback gorilla knuckle-walks past the camera.

As you can see, we've been able to watch quite a variety of primates on Chimp & See, moving about in many different ways!  Join us and see if you can spot some of them yourself!

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Butynski, Thomas M., and Yvonne A. de Jong. "Natural history of the Somali lesser galago (Galago gallarum)." Journal of East African Natural History 93.1 (2004): 23-38.
Schaefer, Melissa S., and Leanne T. Nash. "Limb growth in captive Galago senegalensis: getting in shape to be an adult." American journal of primatology 69.1 (2007): 104-112.
Gebo, Daniel L. "Locomotor diversity in prosimian primates." American Journal of Primatology 13.3 (1987): 271-281.
Patel, Biren A. "Functional morphology of cercopithecoid primate metacarpals." Journal of human evolution 58.4 (2010): 320-337.
Isbell, Lynne A., et al. "Locomotor activity differences between sympatric patas monkeys (Erythrocebus patas) and vervet monkeys (Cercopithecus aethiops): implications for the evolution of long hindlimb length in Homo." American Journal of Physical Anthropology 105.2 (1998): 199-207.
Kivell, Tracy L., and Daniel Schmitt. "Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor." Proceedings of the National Academy of Sciences 106.34 (2009): 14241-14246.
Doran, D. M. "Ontogeny of locomotion in mountain gorillas and chimpanzees." Journal of Human Evolution 32.4 (1997): 323-344.

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