, Volume 21, Issue 6, pp 375-386

The influence of intergroup competition on the survival and reproduction of female vervet monkeys

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Summary

Primate groups are often larger than might be predicted from a consideration of within-group competition alone. Wrangham (1980) has hypothesized that females live in extended kin groups in order to defend food resources against other groups. In contrast, others have argued that predation pressure, rather than intergroup competition, favors sociality. Data gathered over 10 years on a population of free-ranging vervet monkeys provide more support for the food defense hypothesis than for the predation hypothesis, and suggest than female reproductive success can be influenced strongly by intergroup competition.

  1. Of the three groups under intensive study, the smallest experienced the least predation, arguing against the hypothesis that large groups have evolved as a defense against predation.

  2. At least three different measures indicated that larger groups experienced slightly greater infant and juvenile female survival than did smaller groups.

  3. Larger groups also had larger and better quality ranges than smaller groups. Large groups were more likely to make incursions into the ranges of smaller groups than vice versa, and to expand their ranges at the expense of smaller groups. Perhaps as a result, females in small groups were more aggressive during intergroup encounters than were females in large groups.

  4. Within groups, rank reversals were influenced by the presence of female kin, and individuals with female kin were able to rise in rank over those without kin. There was no evidence that high-ranking females attempted to suppress the recruitment of daughters by low-ranking females, however, perhaps because groups with many females had a competitive advantage over groups with fewer females.

  5. Data from a small number of group fusions support the hypothesis that small groups benefit from the recruitment of additional females, particularly in populations in which the average group size is small and mortality is high.