Cracking the Tough Nut of Chimp Tool Use

A paper just out in PLoSOne reports that chimpanzees, given some experience and enough of a weight difference, prefer to use heavier hammer stones when cracking hard nuts. This is apparently quite exciting: this is the first study to isolate weight as a property relevant to the task of cracking open a nut.

This caught my attention because weight is not actually the only key property that determines nut cracking success. A heavy hammer is great, but it will eventually become too heavy to lift, and for a given size stone there may very well be an optimum weight (similar to how people choose very specific combinations of size and weight when asked to throw objects to a maximum distance; Zhu & Bingham, 2011). In the current experiment, when the only difference between the objects was weight, the chimpanzees often went to the heaviest stone because it took the fewest strikes and least time to crack the nut. But is this just an artefact of the current experiment? And if so, can an ecological approach find ways to find out just how chimps choose their tools?

Did language emerge from the neural systems supporting aimed throwing?

Aimed throwing is surprisingly uncommon in the animal kingdom. Humans do it par excellence, and otherwise it only shows up occasionally, even in our closest relatives. Chimpanzees will throw things (often faeces) but unlike humans don't throw things when hunting or trying to get food; when non-human animals throw things, it's usually part of a social encounter.

Throwing is a fascinating task for many reasons; I hope to blog some about the perception-action aspects of this task in the future as I prepare a couple of papers on the topic with my colleagues Qin Zhu and Geoff Bingham (who have previously done some excellent work on throwing to a maximum distance and the size-weight illusion; various papers available here). There are many fascinating questions about the perception of the affordances of throwing and distances to targets which we're starting to tackle empirically.

Biomechanically, throwing an object accurately over any distance requires the precise transmission of force from the large trunk muscles along a kinetic chain formed by the segments of the arm. The large trunk muscles generate forces the arm cannot, and this force is then transmitted by the motion of the arm; each segment weighs progressively less and so the force accelerates each one faster than the last. The end result is a hand moving at high speed. This requires careful timing; if the motion of the segments aren't coordinated carefully you will waste energy moving the limbs in ways that aren't helping the throw.

There has been some speculation for a while now that the neural mechanisms that help support this fine tuned coordination and control for throwing might also be just the kind of resources that could support the development of spoken language. Speech is a complex action that requires exquisite control over the coordination and timing of numerous elements, just like throwing. One hypothesis is that our ancestors began to develop the ability to throw long distances (this being favoured by natural selection processes because it enabled us to hunt and kill huge prey with much less physical risk to ourselves; e.g. Calvin, 1983). Evolution selected for neural resources that supported this activity, and this then opened the door to the possibility of complex spoken language. So do we speak the way we do because we throw the way we do?