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 sin gle 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 prop ortion of ground nests built by the single group for 3 years show no signif icant differences between rainy and dry seasons. Significant differences we re found between some vegetation types: in particular, between bamboo fores t and others (primary and secondary forests). However, these differences we re not prominent or consistent across age-sex classes of gorillas. By contr ast, large differences were found in this proportion between adults and imm atures in both primary and secondary forests. The low proportion of ground nests built by immatures indicates their vulnerability. The present study s uggests that the proportion was not influenced by group size but strongly i nfluenced by the presence or absence of a leading silverback. Data on a hab ituated group that lost its leading male and acquired a new male after seve ral months clearly show that females and immatures preferred sleeping in tr ees in the absence of a protector male. Immatures drastically decreased ter restrial 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) afte r immigration of the new male. These results suggest that vulnerability of female and immature gorillas to predators, in spite of their large body siz e. Small body size and considerable sexual dimorphism in early hominids imp lies their frequent use of tree nests and the importance of the male's role as protector. (C) 2001 Academic Press.