Socioecological influences on the dispersal of female mountain gorillas—evidence of a second folivore paradox

Abstract

Socioecological models indicate that the group structure and female dispersal patterns of primates are determined primarily by the abundance and distribution of food, predation pressures, and infanticide risks. In response to those influences, females of folivorous primates are considered relatively free to disperse into groups with the optimal size and structure. Yet some folivores live in small groups despite a potentially higher risk of predation, an apparent inconsistency known as the folivore paradox. This paper examines the female dispersal of a folivorous primate, the Virunga mountain gorillas. Mountain gorillas currently have no natural predators, but this species presents a different version of the folivore paradox: why do 50-60% of females reside in smaller one-male groups despite a higher risk of infanticide? In this study, females left one-male groups more frequently than multimale groups, but transfer destinations were not consistently biased toward multimale groups and those groups did not have higher immigration rates. We found no evidence of dispersal to avoid feeding competition within large groups, even as they have become three to five times larger than average. Thus, the lack of a consistent bias toward multimale groups was not because they are typically larger than one-male groups. Instead, the apparent inconsistencies may reflect limited female transfer opportunities, other influences on dispersal, and possibly an evolutionary disequilibrium in which current behavior does not optimize fitness.

Infanticide as Sexual Conflict: Coevolution of Male Strategies and Female Counterstrategies

“…and mountain gorillas (G. gorilla beringei) in which there is empirical evidence that infanticide occurs at predictably lower rates in groups with more males (independently of female group size). Even so, puzzles remain, such as the apparent lack of preference among female gorillas for multimale groups as destinations for transfer (Robbins et al 2009a).…”

Sexual Conflict in Nonhuman Primates

“…Sicotte (2001) reports that male herding almost always succeeds in preventing female transfer. Indeed, Robbins, Stoinski, Fawcett, and Robbins (2009a) argue that the relative rarity of multimale group structure in the Virunga population is mostly likely due to the male-imposed costs of female transfer from unimale groups. A final possible function of male mountain gorilla aggression is considered immediately below.…”

Long-Term Research on Grauer’s Gorillas in Kahuzi-Biega National Park, DRC: Life History, Foraging Strategies, and Ecological Differentiation from Sympatric Chimpanzees

“…Infanticide by males is a major source of infant mortality in the Virunga population (Fossey 1984;Watts 1989Watts , 1991Robbins et al 2009). The contexts in which it occurs and its effects on female transfers and reproductive rates are consistent with the sexual selection hypothesis (Hrdy 1979;Sommer 1994;van Schaik 2000).…”