Humans belong to a group of primates called the great apes. This family also includes chimps, gorillas, and orang-utans. And we're a weird branch of primates, separated from the others by our big brains and big bodies. Our big bodies are especially bizarre when one considers how we're all climbers (or at least, were climbers up until recently). Having to haul a huge body into the canopy seems like it would be detrimental to our survival.
Beyond simple energy costs, being big causes apes a whole host of other climbing related problems. For instance, it would surely limit what branches will support their weight. This is an issue that continues to impact chimps, with larger males prevented from reaching the tops of trees. Plus, the larger a body becomes the smaller the muscles are, relatively speaking. So a larger ape would be relatively weak. Not a good combination for climbing.
However, the fact remains we did still evolve big bodies. And for humans, those big bodies have proven very useful. It set us up to evolve the long legs which help make bipedalism efficient over long distances. So how could our ape-y ancestors have evolved this paradoxical trait in the first place?
Parkour for science!
The existence of this paradox is hardly a revelation. It's been known about for decades. However, it still remains unsolved because it's ruddy hard to study apes as they climb. Studying the impact of body size on the energetic efficiency of climbing apes would first require studying how much energy apes actually use in this activity. Doing so is typically done by measuring the oxygen
Studying the impact of body size on the energetic efficiency of climbing apes would first require studying how much energy apes actually use in this activity. Doing so is typically done by measuring the oxygen consumption of something. The more oxygen is needed by an individual, the more metabolism and energy they are using. As you might imagine, it's rather difficult to strap all of the breathing apparatus needed to measure this onto a free climbing chimp. So our ability to study how much energy climbing takes is very limited.
This has forced scientists to rely on other methods. Some have made computer models, others tried to make extrapolations based on observations of living primates. Both of these haven't been able to shed light on the matter. The viability of large bodied climbing still seemed as impossible as ever.
However, some researchers realised that humans are willing to be masked up for science. And some of them can be quite good climbers. Enter parkour athletes, helping scientists study the energetic demands of climbing. Already research in these climbers is shedding light on how early members of the human family may have moved through the trees.
Why the big body?
Now that same group of researchers have turned their parkour guinea-pigs to another question. How could apes survive with a big body? So they strapped their test subjects up with the oxygen measuring equipment and had them run through arboreal-like obstacle courses. Was there some behavioural trick that could reduce the energetic demand of being a big-bodied climber?
It turns out the secret is memory.
Climbing with a big body did turn out to be energetically costly. As expected. However, the more people ran the course, the less energy it cost. This wasn't because of improving skill leading to better climbing ability. Instead, simple changes in hand placement, stride length, and other tweaks that come with familiarity served to dramatically decrease the energy cost of climbing. And huge amounts of rehearsal weren't needed either. If an ape took the same branch only twice, that second trip would be 6% more efficient than before.
Memory, combined with a good climbing anatomy could really take the energetic sting out of climbing with a big body.
The go on to speculate that planning could offer some of the same benefits as memory. Being able to look ahead and plot the fastest, easiest route - even in unfamiliar territory - could further reduce energy consumption. Unfortunately, their course wasn't flexible enough to really test this idea.
Implications
However, this research has implications beyond simply explaining an evolutionary paradox. It suggests the planning and memory could mitigate a large body. Might this then link body size and our relatively big brains? Or at least, our ability to use them for great planning. How many key human attributes have their origin deep back in the early evolution of apes and their bodies?
References
Halsey, L.G., Coward, S.R., Crompton, R.H. and Thorpe, S.K., 2017. Practice makes perfect: Performance optimisation in 'arboreal'parkour athletes illuminates the evolutionary ecology of great ape anatomy. Journal of Human Evolution, 103, pp.45-52.
Hunt, K.D., 1994. Body size effects on vertical climbing among chimpanzees. International journal of primatology, 15(6), pp.855-865.
