Dynamics of intergroup competition in two neighboring chimpanzee communities.

Abstract

Among group-living animals, larger groups tend to win intergroup fights. In chimpanzees, larger groups are likewise believed to win most intergroup interactions. Because intergroup fights in chimpanzees sometimes lead to the death of individuals, larger groups are expected to eliminate smaller groups - as observers witnessed in Gombe, in the 1970s, when the Kasekela group exterminated the smaller Kahama group. However, in some cases, smaller groups continue to persist alongside larger groups. How smaller groups are able to do this is poorly known. Gombe National Park provides a unique natural settings for examining how smaller groups of chimpanzee manage to survive against difficult odds. Currently the park contains two neighboring habituated groups: the large Kasekela group and the much smaller Mitumba group. Despite the Mitumba group being sandwiched by the Kasekela group and cultivated lands, the Mitumba group has not only survived but also increased in size over the past 10 years. I explored how the smaller Mitumba group managed to survive alongside the larger group. First, I investigated the factors that caused changes in range size and population of the Mitumba group over the past 25 years. I found that, both anthropogenic habitat destruction and intergroup competition influenced the range size of the Mitumba chimpanzee group. Then, I examined whether large group size conferred competitive advantage by examining the range size and range use of two neighboring groups and outcome of territorial contests. I found that, group size influenced the range use of neighbors. Each group used more contested area when numerical strength was in its favor. Furthermore, I found that, the number of males in a party and not the location of the range determined whether the Mitumba chimpanzees would counter-call. Mitumba chimpanzees were less likely to call when in a party contained zero or one male. However, the probability of counter-calling increased when number of males in a party increased. My study suggests that, anthropogenic habitat destruction and intergroup competition could interact in a complex way to influence the survival and reproduction of individuals in this population. Furthermore, my study also suggests on the importance of numerical strength in intergroup competition in chimpanzees.