Mothers' sons

I want to note the new article by Langergraber, Mitani and Vigilant on chimpanzee kinship and social behavior. The paper finds that chimpazee males tend to affiliate with their maternal brothers.

The result of the paper is different from previous work that had shown no significant association between maternal kinship and affiliations for males (e.g., Mitani et al. 2000; Mitani et al. 2002). This discrepancy is surprising, since the other studies involved the same community at Kibale. What the authors found was that the mtDNA was a poor indicator of maternal kin relationships, because haplotypes are more widely shared than within single matrilines. So previous work that typed mtDNA didn't identify maternal kin accurately.

The article creates some unnecessary confusion about kin selection. The authors note that the traditional explanation for cooperation among male chimpanzees emphasizes kin selection as a mechanism:

Cooperation in primates and other animals has frequently been attributed to kin selection, a process whereby individuals cooperate with relatives and gain indirect fitness benefits through the reproduction of kin (6-9). For example, in many Old World cercopithecine monkey species, females remain in their natal groups their entire lives and behave nepotistically toward their sisters and other maternal relatives, whereas males disperse among groups and cooperate less with each other (10). In contrast, male chimpanzees are philopatric and are much more affiliative and cooperative than female chimpanzees (11). Given these observations, it is not surprising that kin selection has historically been assumed to play a central role in the evolution of male chimpanzee cooperation (12, 13). Nevertheless, studies to date do not provide any evidence that male chimpanzees bias their social behavior toward maternal brothers (14 16), who should be readily recognizable by virtue of the life-long bonds they form with shared mothers (13) (Langergraber et al. 2007:1)

This is a bit confusing, because it is by no means necessary for chimpanzees (or other species) to directly interact with their kin for a behavior to evolve by means of kin selection. What is necessary is that their kin benefit from the behavior. Considering the long life histories of chimpanzees, the long-term fitness effects of behaviors on other individuals are not obvious, so the fact that coalitional behavior does not have a kin bias is not exclusive of kin selection as a mechanism. As an example, an individual who polices the group by suppressing aggressive interactions may benefit his kin without any direct interaction -- or even with antagonistic interactions with his kin.

In any event, the paper finds that maternal brothers do affiliate more with each other than expected by chance. It's comforting to see this result, since maternal brothers have a shared genetic interest in each other. In a game theoretical perspective, the benefits of affiliation ought to be higher for kin, and the fitness costs lower. Considering that chimpanzees recognize their maternal kin, they ought to make this distinction.

That is not to say that a male chimpanzee should always favor his maternal brothers -- they clearly do not do so in all contexts. The indirect fitness benefit from kin selection is weak, compared to the direct fitness benefit of status, and it may be more important to exchange affiliation with other males of equal or higher status or age. Since maternal brothers in chimpanzees are at least 4-5 years apart (and often much more, since sisters may come between!) they are not especially likely to be near each other in rank.

The paper spends a lot of time explaining why paternal sibling effects are not observed. The short answer is that males cannot recognize their paternal brothers. The review of paternal kin recognition and nepotism takes a couple of paragraphs, which might be valuable for those interested.


Langergraber KE, Mitani JC, Vigilant L. 2007. The limited impact of kinship on cooperation in wild chimpanzees. Proc Nat Acad Sci USA (online early) doi:10.1073/pnas.0611449104

Mitani JC, Merriwether DA, Zhang C. 2000. Male affiliation, cooperation and kinship in wild chimpanzees. Anim Behav 59:885-893.

Mitani JC, Watts DP, Pepper JW, Merriwether DA. 2002. Demographic and social constraints on male chimpanzee behavior. Anim Behav 64:727-737.