The latest puzzling chimpanzee behavior

Young chimpanzee. Photo credit Sabine Bresse

Of the animal kingdom, chimpanzees, Pan troglodytes, are one of the well-known tool users. All populations of wild chimpanzees studied have been observed using leaves to obtain food and some populations have been observed modifying branches to forage for termites and even fashioning spears to hunt bushbabies (Shumaker et al, 2011; Boesch, 2012).

As is the case with studying many primates, our knowledge of tool use and how it is passed from generation to generation is limited by the number of field sites where long-term research occurs. Chimpanzees must first become used to human presence (or habituated) before their natural behavior can be studied, and this can take a great deal of time.

Theorizing that the repertoire of chimpanzee behavior is much wider than what is currently known, Kühl and colleagues used camera traps to study multiple populations of chimpanzees in West Africa that are not habituated. This is when they noticed some very interesting behavior. Researchers first noted unusual piles of rocks in trees and used camera traps to determine what was behind this puzzle. They observed chimpanzees picking up rocks from piles around or in trees and then throwing them at the tree.

Through the camera trap footage, researchers were able to determine that chimpanzees would throw the stones at the trees and vocalize a long-distance pant-hoot at the same time. The stones are mainly thrown by adult males and they seem to be independent of any sort of foraging behavior. The same chimpanzee was often observed returning to the tree and engaging in the behavior. This strange accumulative stone-throwing has only observed in West Africa and was seen in four populations of chimpanzees.

Stone throwing may be a type of male display, during which the individual attempts to draw attention to one's strength and impress other chimps. Kühl and colleagues theorize that using the stones in this manner may produce a loud sound resulting in a greater display. Another theory the authors present is that this may be a symbolic behavior. In either case, obviously more data is needed. Stone-throwing chimpanzees raises more questions than it does answers in terms of behavior and possibly cognition in the case of the latter theory.
 
This work goes to show that you can still learn a great deal about a comparatively well-studied primate species even if you're not Jane Goodall and haven't been working at the same research site for decades. Kühl and colleagues have captured some truly fascinating behavior that raise a series of new questions using technology to their benefit.

Links of potential interest:
Video of behavior
Pan African Programme: The Cultured Chimpanzee
How human are chimps?
Females more likely to use tools when hunting



Works cited:

Boesch, C. Wild Cultures: A Comparison between Chimpanzee and Human Cultures. (Cambridge University Press, 2012).

Kühl, H. S. et al. Chimpanzee accumulative stone throwing. Sci. Rep. 6, 22219; doi: 10.1038/srep22219 (2016).

Shumaker, R. W., Walkup, K. R. & Beck, B. B. Animal Tool Behavior: The Use and Manufacture of Tools by Animals. (JHU Press, 2011).