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Posted by ACRG on 09 Jan 2008 at 20:58 GMT

We have two comments regarding the conclusion that chimpanzees share cultivated fruits because these are riskier to obtain and thus represent a more valuable resource. We question first, whether the chimpanzees perceive these fruits as being risky to obtain, and second, whether they are deliberately sharing these fruits more than other foods.

In terms of chimpanzees perceiving the stolen fruit as “forbidden,” as a group that studies cognition we doubt that chimpanzees possess enough of a notion of possession to understand that they are “stealing” from humans something which they “should” not. The chimpanzees did scratch more when they were in the cultivated areas, suggesting they were stressed, and it was useful that the authors provided this data. Yet we see several reasons why individuals could be stressed other than possessing a concept of theft. They may perceive the situation as dangerous because they have past experience with humans being angry at their invading this area. More simply, they might just be excited by the novelty of being in this new area. We think that it is difficult to differentiate between these possibilities with the data presented, thus we suggest that the authors temper their claim a bit about the risk factor leading to males receiving “positive attention” (since they could even just receive attention because they are holding a coveted food item).

In regard to the issue of deliberate sharing of cultivated fruits, it is possible that individuals share these fruits simply because the stressful situation of obtaining them causes them to lose their appetite or lower their defenses when others try to take the food. Thus, tiredness, rather than any sense of having obtained a valuable resource, might lead to increased sharing. In fact, it might not even be that they are deliberately sharing the cultivated fruits more than other foods because of the situation required to obtain them. Rather, it is possible that these fruits are more shareable simply because they are larger, i.e. a standard cultivated papaya is much larger than a wild fig, thus making it easier to share simply because there are more pieces which might fall off while the individual is eating it. Also, these food resources may be more valuable simply because they are nutritionally better than the standard chimpanzee diet, rather than the “forbidden” element of the fruits adding any value. So again, we suggest considering more simple alternatives before proposing these more complex explanations of what is certainly interesting data.

RE: Comments on Discussion

GregLaden replied to ACRG on 21 Jan 2008 at 22:58 GMT

As part of my thesis research in the Ituri Forest (Laden 1992, University Microfilms, if you must!) I documented the spatial behavior of ground mammals in relation to forager settlement patterns, and was able to show very clearly that your average duiker, okapi, etc. would avoid human camps (even those abandoned with food in them) and habitually used human trails. There is almost no overlap in space use in the Ituri between humans and chimps, even though there is plenty of food in the human-used territories that chimps would eat. It is well known in areas where farmers and chimps overlap, in Central and East Africa, that chimps are driven from shambas (gardens), sometimes shot at, etc.

I cannot imagine why these chimps would not be "aware" of the fact that these gardens are a totally different ball game from wild foraging areas. I think you can put this concern utterly to rest.

For people doing fieldwork in these areas (I assume W.A. is the same as East and Central Africa) the idea that chimps are not fully aware of, and reactive to, human landuse patterns on the edges of their own use areas, is probably simply something that would not occur to anyone. Thus, this probably was not mentioned in the paper not because the authors did not think of it, but because it is patently obvious (to a field worker).

The tiredness idea is not a good null hypothesis. It is a strange and weird idea that you need to provide some evidence for, don't you think!

Yes, the package size factor is a big one. That needs to be investigated more. It is addressed in the paper, but obviously remains as an open issue.