by Joseph T. Feldblum (Leakey Foundation Grantee) and Deus C. Mjungu (Leakey Foundation Baldwin Fellow)
Chimpanzees live in distinct communities, with mostly friendly relationships within groups and aggressive territorial interactions between groups. Every once in a while, a group of males (and more rarely a few females) will get together, approach a territorial boundary, and in silent single file, stopping often to stand upright and survey the valley below, go on a patrol. I never observed one and was always uncertain about whether I wanted to. It’s undoubtedly a fascinating behavior but would jeopardize the lives of some of the chimps that I grew attached to over three field seasons in Gombe National Park, Tanzania. If a patrol comes across a lone individual from a neighboring community, they often attack and kill them. But if they come across a group of similar size, they’ll usually stick to calling loudly and threateningly at each other from a safe distance. The key predictor of violence, it seems, is party size asymmetry.
This makes the behavior of the males in the Mitumba community in Gombe all the more puzzling. For more than four years, the alpha male in Mitumba has been Edgar, a ruler with an iron fist if ever there was one. Every other community member is terrified of Edgar, screaming and grunting their submission at the first sign of trouble. And strangely, other than the sickly adult male Rudi, who died after my first field season in Gombe National Park, all the other males are young adolescents.
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This unusual demographic structure has an insidious explanation. Edgar, and Rudi back when he was still alpha, are known or suspected to have killed off several of the other males in the community who could have challenged their positions. Forest, who would be 20 now and newly full size, was brutally attacked by Edgar more than four years ago, and subsequently disappeared. Now it seems that Apple, one year younger, has met the same fate, as he hasn’t been seen in more than a year following a similar attack.
So Mitumba males now number only Edgar and a couple adolescent males, certainly not intimidating numbers for border protection. Why would Edgar endanger his group by weakening it so severely? Wouldn’t such behavior be maladaptive?
Probably not. It’s important to remember that natural selection operates on individuals, not on groups. So although the group’s success could be hampered by Edgar’s brutality, his individual success is probably enhanced. From genetic analysis, we’ve found that Mitumba alpha males increased their share of paternities after killing off rivals, and Edgar is likely experiencing similar success judging by the fear he inspires.
Chimpanzees live in complex societies, and males must both compete with group-mates for access to females and cooperate against rival groups. Natural selection will hone the balance between these strategies, but no balance point will be ideal in all cases. Indeed, researchers at Mahale, another field site in Tanzania, have suggested that severe competition between males in a small community may have contributed to its dissolution and the emigration of most of its females into a neighboring group. Only time will tell if Edgar’s despotism will be his undoing, or whether selection will prove him a successful, if brutal, tactician.
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pyrrhic victory. Dictionary.com. Dictionary.com Unabridged. Random House, Inc. http://www.dictionary.com/browse/pyrrhic-victory (accessed: January 3, 2018).