Proximate factors influencing dispersal decisions in male mountain gorillas, Gorilla beringei beringei

We examined demographic and behavioural influences on the dispersal decisions of 31 male mountain gorillas living in mixed-sex groups. Approximately half (45%) of silverback males dispersed from the groups in which they matured. Mean dispersal age corresponded with the age of physical maturity. For two-thirds (64%) of males, dispersal was a one time decision. The remaining individuals made repeated visits to their natal groups before permanently dispersing. Of dispersing males, 43% left with females. The remaining individuals dispersed alone, despite the presence of similar-aged male dispersal partners in their group. Dispersal decisions appeared voluntary; there was no evidence of increased aggression by the dominant silverback or peripheralization in the months prior to dispersal. Instead, dispersal decisions were influenced by limited opportunities to copulate within the group and the absence of a male's mother. Dispersing did not provide increased access to females, at least in the short term; only three dispersing males were observed to form groups whereas the remaining eight for whom data were available remained solitary (range 1-112 months) at the end of the study. The question remains why males choose to disperse rather than remain philopatric. Inbreeding avoidance seems unlikely, as all groups contained unrelated females. It may be that dispersal decisions are the result of several factors acting in combination, and thus dispersal is a more appropriate option for some males. Alternatively, dispersal decisions may actually be disadvantageous and reflect an incomplete adaptation to the ecological extremes of the Virunga region. (C) 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.