Elsevier

Animal Behaviour

Volume 77, Issue 5, May 2009, Pages 1155-1164
Animal Behaviour

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

https://doi.org/10.1016/j.anbehav.2008.12.030Get rights and content

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.

Section snippets

Study Site and Subjects

This study was conducted in the Parc National des Volcans, Rwanda. Research groups of mountain gorillas have been monitored at this site since 1967 (for description of study site and groups, see: Fossey, 1983, Watts, 1984, Robbins, 1995, Bradley et al., 2005, Robbins et al., in press). Data come from 8 of 14 groups that have been studied by the Dian Fossey Gorilla Fund International (DFGFI); the other six groups did not have males of known ages that reached dispersal age. Group composition

Characteristics of Dispersal

Of the 31 males in the study that had reached the minimum dispersal age of 13.3, 14 (45%) dispersed and 16 (52%) remained in the group in which they became a silverback (one male died and thus his dispersal status was considered unknown) (Table 1). Dispersal ages ranged from 13.3 to 21 years (mean = 15.3; median = 15.6). Nondominant, nondispersing males ranged in age from 13.5 to 21.8 years (mean = 16.8; median = 16.7). This includes the male that died at 21.1 years of age but excludes an individual

Discussion

Natal dispersal is a common, although not ubiquitous, practise in male gorillas, seen in roughly 50% of the individuals in the current study. The average age of dispersal corresponded with age of physical maturity (estimated at 15–16 years; Watts & Pusey 1993), although males begin copulating and can sire offspring before dispersal age is reached (Watts, 1991, Robbins, 1995, Bradley et al., 2005, Stoinski et al., in press). For roughly two-thirds of males, dispersal was a single event: males

Acknowledgments

We thank the l'Office Rwandais du Tourisme et des Parcs Nationaux for permission to work at the Karisoke Research Center. Special thanks to the entire Karisoke staff for their observations and protection of the gorillas and to the many researchers and staff that contributed to the demographic database (see Gerald Steklis & Steklis 2007). F. Barabwiriza, J.D. ‘Fundi’ Hategekimana, E. Hitayezu, F. Ndungutse, F. Ngaruyintwali, J. Ngirila, T. Nsengiyumva, A. Sebatware, F. Uwimana, P. Uwingeli and

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