Factors influencing the formation of ground nests by eastern lowland gorillas in Kahuzi-Biega National Park: some evolutionary implications of nesting behavior.

To analyze the factors influencing nesting by gorillas on the ground, three kinds of data were collected in the Kahuzi-Biega National Park: from a single group for 3 years, from 25 groups during a population census, and from a habituated group before and after two social events. The data on the proportion of ground nests built by the single group for 3 years show no significant differences between rainy and dry seasons. Significant differences were found between some vegetation types: in particular, between bamboo forest and others (primary and secondary forests). However, these differences were not prominent or consistent across age-sex classes of gorillas. By contrast, large differences were found in this proportion between adults and immatures in both primary and secondary forests. The low proportion of ground nests built by immatures indicates their vulnerability. The present study suggests that the proportion was not influenced by group size but strongly influenced by the presence or absence of a leading silverback. Data on a habituated group that lost its leading male and acquired a new male after several months clearly show that females and immatures preferred sleeping in trees in the absence of a protector male. Immatures drastically decreased terrestrial nesting (from 54% to 6%, P< 0.001) and increased it (from 6% to 19%, P> 0.05) less dramatically than females did (from 24% to 60%, P< 0.01) after immigration of the new male. These results suggest that vulnerability of female and immature gorillas to predators, in spite of their large body size. Small body size and considerable sexual dimorphism in early hominids implies their frequent use of tree nests and the importance of the male's role as protector. Copyright 2001 Academic Press.