|Fuku shelters under leafy rain cover|
The study compared Wamba's bonobos with chimpanzees at Goualougo in northern Republic of Congo. Good for comparison, because they selected sites with similar enough climate and habitat. Asking why forest-dwelling bonobos are different from savannah chimpanzees would be a very different question. The downside of this comparative study is that the two sites were using quite different methods, and habituation for Wamba was far superior. Bearing in mind these methodological differences, here are the results.
The total species repertoire is 13 tool use behaviours for bonobos and 42 for chimpanzees. Bonobos at Wamba used 10 of these tools and the Goualougo chimps used 22. Ten sounds small compared to the chimp repertoires, but chimps at different sites don't all use tools at the same rate. So the bonobo repertoire is within the range for chimpanzees.
Interestingly, of the 13 bonobo tool uses only 1 is associated with feeding; at Lomako, they have been observed using leaf sponges to drink water. Compare that to the chimp repertoire, where 25 of the 42 tool uses are for feeding! What's going on?
They tested 4 hypotheses: (1) Necessity - tools should be used in areas where easily accessible food it less abundant, i.e. Wamba has more fruit than Goualougo; (2) Opportunity - tools should be used in areas where there are more opportunities, i.e. Goualougo has more termite/ant mounds than Wamba; (3) Relative profitability - tools should be used where you get more from that food source than from conventional ones, i.e. fishing for ants should provide more calories than eating fruit; and (4) Invention - tools should be more common if there is more opportunity for social learning, i.e. groups at Goualougo are bigger than at Wamba.
None of these hypotheses were supported.
The areas were equally abundant in fruiting trees. There was similar abundance of foods that would need tools, like termites and ants. They didn't actually examine the caloric costs and benefits, maybe next time. And group size was actually bigger at Wamba.
|Plenty of termite mounds at Wamba, but no termite fishing|
Here we enter the realm of speculation. Though it sounds like a pretty good guess. Maybe "these behaviours evolved in past ecological and social conditions that differ from contemporary settings". Looking at geological and paleoenvironmental evidence for each species' habitat 2.5-1 million years ago (at the point where the species diverged), ancestral chimpanzees might have been living in a much drier, patchier area that necessitated creative foraging techniques involving tools. Meanwhile those lucky bonobos were isolated on the left bank of the Congo river, which maintained its lush and abundant forest.
It's exciting to see how comparative studies and paleontology can inform each other. Only through rigorous research and (to use a buzzword) interdisciplinary cooperation, can we hope to understand our evolutionary past.
Furuichi, T., Sanz, C., Koops, K., Sakamaki, T., Ryu, H. J., Tokuyama, N., & Morgan, D. (2014). Why do wild bonobos not use tools like chimpanzees do? Behaviour. doi:10.1163/1568539X-00003226
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