Leave or conceive: natal dispersal and philopatry of female mountain gorillas in the Virunga volcano region

Natal dispersal is often attributed to inbreeding avoidance, competition for resources or competition for mates, but the patterns and frequency of dispersal vary considerably among and even within species. We examined the possible reasons for dispersal and philopatry of natal nulliparous female mountain gorillas, Gorilla gorilla beringei, in the Virunga volcano region, including comparisons with non-natal and/or parous females. Competition among females is generally weak and ineffective in this population, and inbreeding avoidance has been considered the ultimate cause of natal dispersal. Yet fewer than half of nulliparous females left their natal group when the dominant male was old enough to be their father, so they did not rely entirely upon dispersal to avoid such risk of inbreeding. Almost all natal nulliparous females were with at least one sexually active male who was not old enough to be their father, so the presence or absence of such mating alternatives also did not determine whether they left. Natal nulliparous females were more likely to leave groups with only one adult male, where infanticide losses have been higher than in multimale groups. Thus natal dispersal seemed to be influenced by infanticide avoidance, which has been considered the ultimate cause of secondary dispersal. Natal nulliparous females have more time to encounter suitable destinations than other females, which may be a proximate reason why they were more likely to transfer despite having the same ultimate cause of dispersal.