Patrilocality and genetic relatedness in chimpanzees

Lukas and colleagues (2005) studied the genetic relatedness of males in chimpanzee groups. Since males do not disperse (and females do), one might expect that the males in a group would be much more closely related to each other than the females.

But it turns out that female dispersal and large groups are quite enough to keep the average genetic relatedness of males fairly low -- barely higher than the relatedness among females.

In a comparison among many taxa for which data on relatedness were available, they found that the relatedness of the philopatric sex was negatively correlated with group size -- as the number of adult individuals of the philopatric sex increased up to around 15-20, the average relatedness decreased to zero. Also males (in species with male philopatry) were significantly less related than females (in species with female philopatry) -- in other words, the decline of relatedness was steeper for males than for females.

Lukas and colleagues confirmed this reduction in relatedness with mathematical modeling -- showing that groups with four philopatric adults shared average relatedness of around 0.25, while eight individuals brought the average relatedness below 0.125, or the relatedness of full first cousins.

Studies in other taxa in which males affiliate have produced contradictory results on the presence of significant relatedness among clusters of males. Although an influential work on relatedness and reproductive success among affiliative male lions has been widely taken as evidence for the benefits of kin association for males (Packer et al. 1991), new research on multiple prides of lions suggested that relatedness among the males is not necessary for cooperative behaviour (Spong et al. 2002). Results for dolphins have been contradictory (no influence of kinship: Moller et al. 2001; influence found: Parsons et al. 2003). However, a recent dolphin study found significantly higher average relatedness among pairs of individuals participating in long-term alliances consisting of six or fewer individuals as compared to random pairs of individuals, but they did not find this for larger super-alliances and subgrouping, suggesting that different male strategies might explain the apparent contradictions (Krützen et al. 2003) (Lukas et al. 2005:2189).

The conclusion of the paper makes a pitch for reciprocity rather than kin selection as a mechanism underlying cooperative action:

A study in Indonesia on whale hunting, which necessitates the cooperation of relatively large number of individuals per boat, found no direct choice of kin for the cooperative action, rather just a choice of individuals from the same group, and argued that 'kin selection alone cannot structure cooperation in groups larger than the nuclear family because of the ambiguous group membership it provides' (Alvard 2003). In addition, recent results from experimental economics indicate that 'biological models of self-interested cooperation' which include inclusive fitness benefits through kin selection 'are rarely plausible when they involve groups of more than a few individuals' (Gintis 2004). Instead, findings on the alternative explanation, reciprocity, converge neatly with the observation in chimpanzees, that 'cooperation within a group can make the group more lethally aggressive in its dealing with outsiders' (Seabright 2004). These results, and those presented here, suggest that indirect fitness benefits through gene sharing are not necessarily the primary mechanism driving large group actions in mammals and humans (Lukas et al. 2005:2190).

On the other hand, human bands tend to have fewer than eight adults of the same sex, and this is the category where the average relatedness is high. So kinship might well be a much more important influence on human foragers than on chimpanzees.

And, as the paper notes, even in larger groups, dyadic interactions may be shaped by relatedness, and kin groups may interact significantly even within larger groups where the average relatedness is low. In that regard, a long-lived species like chimpanzees might have more opportunities to work with kin than shorter-lived species. Chimpanzees might also have a more flexible range of options in how they interact with kin.

References:

Lukas D, Reynolds V, Boesch C, Vigilant L. 2005. To what extent does living in a group mean living with kin? Mol Ecol 14:2181-2196. DOI link