Boesch et al. have provided a highly interesting dataset on chimpanzee adoption patterns and have elaborated on potential candidate mechanisms. Two previous comments have elaborated on alternative explanations to those presented by Boesch et al. Such alternative explanations based on kin selection - and its effect on proximate mechanisms - seem especially likely for the case of adoptions made by female chimpanzees. However, Boesch et al. also provide data on a much rarer type of behaviour. As they state: "Of special interest are the adoptions by males". I very much agree. Of the ten adoptions that were performed by males, only four could be proven to be adoptions of close kin. Assuming that the remaining six adoptions were adoptions of non-related individuals (though see also the critical comment by "AfricaGomez") - could these male foster parents not gain a more indirect benefit resulting from their actions? The authors mention such a possibility: an adopted infant "could [later] become an ally of the aging male". The authors then dismiss such an explanation on the basis that female chimpanzees were also adopted by some males. I would simply like to add two further possibilities: 1. while it is true that female infants will not become later allies for these males - they might become valuable as sexual partners 2. Of the six adoptions without an immediate inclusive fitness benefit five infants were in fact males - who indeed might later turn into allies. But another possible explanation here could be group augmentation effects - "where individuals survive or reproduce better in large groups" (Kokko et al. 2001). Thus, it may pay for the males to invest into the growth of their group. And here male infants needing help would be especially likely to be helped since these will remain in the group longer than females do (since chimpanzees are a male philopatric species). In fact, Boesch et al. provide one possible factor that may lead to group augmentation effects: predation pressure. Again: under this hypothesis one would expect a preferential adoption of unrelated male infants by male chimpanzees - exactly as the observed "five to one" pattern suggest (these numbers are as of yet too small (and one might even say: luckily so) to prove this). This is not to say that females "do not count" under the group augmentation hypothesis - they simply would not be as valuable as males.

References:
Kokko, H., R. A. Johnstone, et al. (2001). "The Evolution of Cooperative Breeding Through Group Augmentation." Proceedings of the Royal Society of London B Biological Sciences 268(1463): 187-196.