Social and ecological correlates of space use patterns in Bwindi mountain gorillas

Abstract

Space use patterns determine access to resources necessary for survival and reproduction. Although it is recognized that the interplay between social and ecological variables shapes spacing patterns, few studies in group-living animals have simultaneously assessed their importance in a comprehensive approach using different spatiotemporal space use measures. In territorial species, such patterns are strongly determined by between-group competition, but its impact in non-territorial species is poorly understood. To better understand the role of social and ecological variables in non-territorial mountain gorillas (Gorilla beringei beringei), we simultaneously evaluated the impact of between-group competition (local gorilla population density and frequency of intergroup encounters) and within-group competition (group size) on space use patterns in 13 gorilla groups in Bwindi Impenetrable National Park, Uganda, while also examining variation in food availability (herbaceous food availability and level of frugivory). We found evidence of between-group competition, indicated by a decline of both monthly home range size and frequency of revisits to each part of the home range as the local gorilla population density increased and by an increase in daily travel distance on days when intergroup encounters occurred. Within-group feeding competition was inferred by a decrease in the frequency of revisits as group size increased. Lastly, food availability influenced the gorillas' spacing patterns, as indicated by a decline in monthly home range size as herbaceous food availability increased and by an increase in daily travel distance as the degree of frugivory increased. In sum, our results suggest that Bwindi gorillas adapted their space use according to the constraints of intraspecific competition and the availability of food resources. We provide some of the first evidence of between-group competition influencing spacing patterns in a non-territorial species. This study suggests a gradient between territoriality and non-territoriality, along which the level of between-group competition varies.

Western gorilla space use suggests territoriality

“…Previous hypotheses for the causes of inter-group avoidance in gorillas have included mating competition and contest competition over food 14,25,29,44,45 . However, these factors alone could not explain the importance of location, as the level of avoidance of other groups would not be expected to vary based on where within each group's home range the competition was occurring.…”

“…Mountain gorilla (Gorilla beringei beringei) groups have been found to use the core areas of their home ranges almost exclusively and avoid neighbouring groups 25,31 . They have also been found to reduce their home ranges in response to increased population density 14 , an avoidance behaviour typically observed in territorial species, demonstrating that the presence of neighbouring groups represent a social barrier constraining movement patterns. This led the authors to suggest that mountain gorillas may be intermediate on a continuum between non-territoriality and territoriality 14,25 .…”

“…A variety of reasons for the avoidance of neighbouring groups in gorillas have been suggested, including mating competition and feeding competition (both contest and scramble) 14,25 . Fundamentally, explanations for avoidance can be divided into 1) reasons to minimize interacting with other groups, and therefore avoid their actual location: mating competition and contest competition; and 2) reasons to minimize using regions within another groups home range: defended space and scramble competition.…”

Ecological correlates of space use patterns in wild western lowland gorillas

“…Both fruit availability and the amount of time spent feeding on fruit should be important determinants of western gorillas' space use. Daily travel distance increases as the amount of fruit in the gorillas' diet increases in western gorillas and Bwindi mountain gorillas (Cipolletta, 2004; Doran‐Sheehy et al, 2004; Seiler et al, 2018), suggesting that gorillas are willing to travel further to consume these foods. However, the effect of fruit availability on the daily travel distance has not been investigated, nor whether the effect of frugivory on daily travel distance might be dependent on the availability of ripe fruit.…”

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

“…2). Four groups (Ka, Mu, Ru, and Haa) ranged around Buhoma in the western section of the park, two groups (Ky and Bit) ranged near Ruhija in the eastern section of the park, and one group (Nk) ranged in the southwest (Robbins et al 2009b; Seiler et al 2018). These groups were monitored daily, allowing for accurate recording of all births, deaths, and dispersal events.…”