Dispersal and reproductive careers of male mountain gorillas in Bwindi Impenetrable National Park, Uganda

Abstract

Dispersal is a key event in the life of an animal and it influences individual reproductive success. Male mountain gorillas exhibit both philopatry and dispersal, resulting in a mixed one-male and multimale social organization. However, little is known about the relationship between male dispersal or philopatry and reproductive careers in Bwindi mountain gorillas. Here we analyze data spanning from 1993 to 2017 on social groups in Bwindi Impenetrable National Park, Uganda to examine the proportion of males that disperse, age of dispersal, pathways to attaining alpha status, fate of dispersing males and philopatric males, and male tenure length as well as make comparisons of these variables to the Virunga mountain gorilla population. We report previously undocumented cases of dispersal by immature males and old males and we also observed the only known case of a fully mature male immigrating into a breeding group. We used genetic tracking of known individuals to estimate that a minimum of 25% of males that disperse to become solitary males eventually form new groups. No differences were found between the Bwindi and Virunga population in the age of male dispersal, the proportion of males that disperse, the age of alpha male acquisition, and dominance tenure length. The lack of differences may be due to small sample sizes or because the observed ecological variability does not lead to life history differences between the populations. Males in both populations follow variable strategies to attain alpha status leading to the variable one-male and multimale social organization, including dispersal to become solitary and eventually form a group, via group fissioning, usurping another alpha male, or inheriting the alpha position when a previous group leader dies.

Lack of Rule-Adherence During Mountain Gorilla Tourism Encounters in Bwindi Impenetrable National Park, Uganda, Places Gorillas at Risk From Human Disease

“…The decision to habituate a gorilla group usually weighs costs and benefits (8)(9)(10). Benefits beyond revenue generation include protection of gorillas and their habitats, daily monitoring to identify any gorilla health issues, and facilitation of detailed research on behavior and ecology offering enhanced understanding of population dynamics including births, deaths, and dispersal patterns [e.g., (4,11,12)]. Costs of habituation include reduced avoidance of humans (13), potentially increasing the likelihood of crop raiding and/or susceptibility to poaching [e.g., (14)(15)(16)], and increased risk of disease spread by close proximity with humans and livestock (5,7,(17)(18)(19)(20)(21)(22)(23)(24)(25).…”

Variability of weaning age in mountain gorillas (Gorilla beringei beringei)

“…Such plasticity observed thus far suggests that there may also be variation between mountain gorilla populations in Bwindi versus the Virungas. Differences in ecology have been proposed for explaining some life history differences between the two mountain gorilla populations (Robbins et al, 2009;Robbins et al, 2019). Notably, Bwindi mountain gorillas have an average interbirth interval of 5 years compared to only 4 years for Virunga mountain gorillas (Robbins et al, 2009).…”

Assessing attitudes towards gorilla conservation via employee interviews

“…This study was conducted through interviews with Ugandan employees of the Bwindi Gorilla Project ( N = 11) and the Gabonese employees of the Loango Gorilla Project ( N = 10), which are both directed by Martha Robbins (see Table 1 for details of projects). Both projects focused on studying social relationships, feeding ecology, and population dynamics of the two species of gorillas (e.g., Ganas, Robbins, Nkurunungi, Kaplin, & McNeilage, 2004; Hagemann et al, 2019; Head, Boesch, Makaga, & Robbins, 2011; Robbins et al, 2019; Robbins, Gray, Kagoda, & Robbins, 2009; Seiler, Boesch, Mundry, Stephens, & Robbins, 2017; Seiler & Robbins, 2020). The data collection protocols and training are given to staff were largely the same at the two study sites.…”