King of the swingers
In the world of zoology, some things are trickier than others to study, such as the energy consumption of an orangutan swinging through trees, for example! But Roehampton University's Lewis Halsey has solved the problem in an ingenious way, by engaging some "human-utans" to help him, as he explained to Connie Orbach...
Lewis - I had a sort of epiphany. I think it was about 2009, I was at the Society for Experimental Biology annual conference. Dr Susannah Thorpe at the University of Birmingham was talking about locomotion in arboreal primates. She mentioned that there is very limited information on the energy costs for arboreal primates to move around in an energetically challenging environment. I thought well, um, why don't we measure energy expenditure in human beings moving around and kind of get them to do things that are ape-like. And that could be a first insightful step into the this world of the energetics of arboreal primates.
Connie - Arboreal primates. Those are the types that spend some or most of their time in the trees, swinging from the branches. And Lewis had some new, exciting kit that could measure oxygen consumption through a mask whilst not impeding movement. But humans and apes, they seem quite different to me.
Lewis - There's always going to be a limit to how much you can infer about a specific species from a referential model. The trade off it this - to try and gain an understanding of the energetic costs experienced by these animals in the wild directly is almost impossible. It's very, very hard to even see an orangutan in the wild, let alone capture or restrain or whatever an orangutan and then put equipment on it, that's never going to happen. The natural first step towards gaining some sort of insight into the energetics of their movements up in the trees is to work with a tractable model. Very obviously, the most tractable model for this situation is a human being and that's why I turned to our own species, if you like, to try and get some insight into these wonderful arboreal animals.
Connie - So what sort of people would you use to simulate them? Well, parkour athletes of course.
Lewis - They're basically street gymnasts who like to use, typically, the urban environment to move around with and they'll practice, and practice, and practice moving around certain routes within their environment, within their urban habitat. It's acrobatic and very impressive to watch. It's quite a new sport, but it's fairly hip and happening as it were, and so we thought these would be the people that could best emulate a tree dwelling primate.
Conne - I like the idea of anyone who actually practices parkour listening to us trying to talk about it in a hip and happening way!
Lewis - Indeed.
Connie - Potentially cringing.
Lewis - Yes - my apologies to all those parkour athletes out there.
Connie - So what were you getting these guys to do; what were they doing to mimic apes?
Lewis - We created several scenarios that were simulating a forest environment. It didn't look like a forest but it created situations somewhat akin to what arboreal primates will face, and then were able to measure the energy expenditure of these parkour athletes.
Connie - Pole swinging, rope climbing, high level jumps. Lewis and his team created a veritable assault course for the athletes with the idea that these are big apes on sparse diets, and they are probably looking to use as little energy as possible.
Lewis - They're moving through the trees and they come to a gap in the canopy, and basically they've got three options. One is to jump the gap, another one is to sway the tree they're on and bend it across the gap and the third one, which is the most simplest in a sense, is to climb down the tree they're on, walk across the canopy floor and climb up the next tree. What we found was that something like an order of magnitude more energy for them to climb down a tree, walk across a forest floor and climb up the next tree than if they find some way to bridge that gap staying up in the trees. We can infer from that that they're not going to want to lose height unless they've got to.
Connie - Well sure. I'd rather take the path of least resistance too. But where does this sort of information fit into the bigger picture?
Lewis - We need to go further now in looking at how an arboreal ape's total daily energy budget is split between its background costs just to exist, and it's costs to move, the costs to digest its food and then we can have a better idea about how their environments affect their energy expenditure, which of course drives how much food they need to eat. And then, in turn, we can have an idea about how their changing environment, so either indirectly due to climate change or more directly due to tree felling, how environmental change will affect their energy costs.
So it's likely there is a breakpoint in terms of how much more energy they can can be expending, for example going up and down trees because the gaps in the tree canopy have become bigger or the only trees around are too small to climb across easily, but there's not going to be enough fruits available to service that energy cost. Where that point is I certainly couldn't say but I think it's reasonable to suggest that it may not be too far off.