Kevin Langergraber and colleagues (2014) have undertaken a study of Y chromosome variation within chimpanzee communities of Uganda. Their goal was to estimate the ages of these chimpanzee communities, under the assumption that males very rarely transfer between groups.
Chimpanzees are patrilocal, meaning that males stay resident within the group where they are born. Females, by contrast, transfer to a different group at the time they reach sexual maturity. This pattern of male residence and female transfer establishes many of the dynamics within chimpanzee groups, including the building of male coalitions and the competitiveness between adjacent chimpanzee communities.
The patrilocal residence pattern does not by itself preclude male gene flow among groups. In West Africa there are well-documented cases of extra-group copulations by females. Males within a group are sometimes the sons of males who live in neighboring groups. Langergraber and colleagues note that such extra-group copulations have not been observed in the East African chimpanzee communities that they have studied here, and argue that the the time to the most recent Y chromosome common ancestor within a chimpanzee community gives a kind of minimum age for the community itself. That is, the present males should descend from a male ancestor in the past who lived within the same community.
They found that the times to most recent Y chromosome ancestor for five chimpanzee groups were 125 years, 462 years, 625 years, 2059 years and 2625 years. Each of those estimates is accompanied by a very large degree of error - exceeding the estimate itself - so they should not be taken too seriously. The interesting thing about them, counting the error, is that they fall within the range of hundreds to 5000 years. That is not unexpected based on the number of males within the group – a Y chromosome coalescent time will be approximately the same number of generations in the past as there are males in the group. A group with 10 adult males will probably have a most recent common ancestor something like 10 generations ago. If the reproductive success of males is skewed, with some males having high offspring number, the most recent common Y chromosome ancestor in the group should be even more recent.
Langergraber and colleagues suggest that the estimated Y chromosome common ancestor times for chimpanzee groups is similar to the age of Y chromosome ancestors within groups of humans who have historically practiced patrilocality. They compare this to the situation in primates who have matrilocal residence , emphasizing that the groups in chimpanzees seem to have a higher persistence time:
Communities of chimpanzees appear to exist as stable entities for much longer than do groups of female-philopatric primate species, where many permanent group fissions have been observed (e.g., Van Horn et al., 2007 and references therein). Empirical and theoretical work suggests that primate groups fission when group size increases to the point where the costs of group-living exceed its benefits (van Schaik, 1983). However, the factors that limit reproductive success differ between females (i.e., access to food) and males (i.e., access to mates), and may show different relationships with group size (Trivers, 1972). While it is clear that in many primate species females decrease their access to food as within-group feeding competition increases with group size (Majolo et al., 2008), male chimpanzees may actually increase their access to mates as community size increases (Boesch et al., 2006 and Langergraber et al., 2013). The positive relationship between paternity success and community size in male chimpanzees presumably occurs because communities with more males can acquire and defend larger territories, which in turn attract more females and allow them to reproduce more often (Nishida et al., 1985, Williams et al., 2004 and Mitani et al., 2010). Even if male chimpanzees were to have higher access to mates in a smaller community, this benefit can be outweighed by the costs of high levels of between-group competition: after the main community at Gombe fissioned into two communities, the males of the larger community systematically killed all of the males of the smaller community (Goodall, 1986).
That is a very interesting comparison, considering the role of group persistence in models of human origins. Archaeologists think quite a lot about the persistence of human hunter-gatherer groups. The local knowledge that they develop and share within groups helps positively with survival, equipping people with information about where resources may last during times of scarcity. It matters that human groups may last for hundreds of years, beyond the memory of any single individual lifetime. Strategies developed during a 500-year shortfall may never be useful again during the lifetimes of people who survive. But their stories may just help their grandchildren to survive the next shortfall. If a story is just passed from one individual to her children, it is likely to be forgotten. But stories that are told to an audience, a group of people, again and again over years, may survive across long spans of time. We don’t know what the thresholds are for human groups and information survival, but it seems clear that the group is important in ways that individuals are not.
Chimpanzees don’t exhibit this kind of social learning of stories and information. We don’t know whether the cultural learning that does occur within chimpanzee groups is moderated by group persistence. The strongest learning occurs between mothers and offspring, a relationship that may benefit from stability within groups but does not otherwise depend on the persistence time or size of the communities. We might even imagine that mother-offspring learning is a buffer against different community sizes and histories in chimpanzees.
Yet it is interesting that the oldest examples of chimpanzee artifacts date to around 4000-4500 years old (“Chimpanzee archaeology”). If chimpanzee groups last for a thousand years or more, these artifacts may be the products of the recent ancestors of groups that still exist today.
Langergraber KE, Rowney C, Schubert G, Crockford C, Hobaiter C, Wittig R, Wrangham RW, Zuberbühler K, Vigilant L. 2014. How old are chimpanzee communities? Time to the most recent common ancestor of the Y-chromosome in highly patrilocal societies. Journal of Human Evolution (in press) doi:10.1016/j.jhevol.2013.12.005
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