Primatology.net

Primatology.net has a sister site, if you don’t know already, and that’s Anthropology.net. This evening I put up a post there that may interest any one keen to primate locomotion and/or human evolution. The post is specifically on orangutan locomotion, specifically on how some researchers have observed a set of Sumatran Orangutans exhibit bipedal tendencies.

If this sounds kinda sorta familiar to this very recent post, “On the biophysics of Sumatran orangutan swaying,” it is!  The same researchers that observed orangutans cheat the system and choose limbs that help them sway the best have published a new finding on bipedalism amongst the same population.

In almost one year of blogging here, we haven’t yet covered how the study of biophysics intersects with primatology. (Update: I realized I kinda lied, I forgot about this post on gibbon gait. Oops.)

Biophysics, in a sense, is the study of the form and function of bodies, and is formally defined as the application of the laws of physics to life processes. The ways a primate body plan is structured dictate some of the primary functions and forms of mobility. For example, humans have very robust lower-limb skeletal structures. From really dense femurs, to joints heavily padded with cartilage, our lower limbs are made to take a beating. This feature in our bodies, correlate to how we move about — bipedally.

A new study, published in Biology Letters, studies how orangutans use the sway of branches to propel themselves from tree to tree. They already have longer arms than legs, a useful adaptation to reach from branch to branch. Also their lower limbs are as flexible as our upper ones (if not more)… Isn’t that crazy?

Swaying, as their primary form of movement is way more energetically cost effective than climbing down one tree and up the next and interestingly is seen in only Sumatran orangutans. According to a Nature news report on this study,

“These are the only primates known to live exclusively in the tree canopy, in part because of the Sumatran tiger and other predators that await them on the ground.”

Pretty, interesting tangent between this form of movement and the ecological context the orangutans live in, if you ask me.

The authors of the paper used video footage of Sumatran orangutans in movement to calculate the energy orangutans use in swings, by estimating the weight, mass, and angles observed in the films and plugging them into some classical torque equations.

After all these calculations, another interesting observation was made, about orangutans selecting for ‘stronger vertical branches nearer the tree trunk,’ so that they are less likely to snap under the weight of these big guys.

If you are interested in this publication, it’s published under this title and link, “Orangutans use compliant branches to lower the energetic cost of locomotion.”