Mountain gorilla ranging patterns: Influence of group size and group dynamics

Abstract

Since the 1980s, the Virunga mountain gorilla population has almost doubled, now reaching 480 individuals living in a 430-km(2) protected area. Analysis of the gorillas' ranging patterns can provide critical information on the extent and possible effects of competition for food and space. We analyzed 12 years of daily ranging data and inter-group encounter data collected on 11 gorilla groups monitored by the Karisoke Research Center in Rwanda. During that period, the study population increased in size by almost 50% and the number of groups tripled. Groups had small yearly home ranges compared to other known gorilla populations, with an average 90% kernel density estimate of 8.07 km2 and large between-group variations (3.17-23.59 km2). Most groups had consistent home range location over the course of the study but for some, we observed gradual range shifts of up to 4 km. Neighboring groups displayed high home range overlap, which increased dramatically after the formation of new groups. On average, each group used only 28.6% of its 90% kernel home range exclusively, and in some areas up to six different groups had overlapping home ranges with little or no exclusive areas. We found a significant intra-group positive relationship between the number of weaned individuals in a group and the home range size, but the fitted models only explained 17.5% and 13.7% of the variance in 50% and 90% kernel home range size estimates, respectively. This suggests that despite the increase in size, the study population is not yet experiencing marked effects of feeding competition. However, the increase in home range overlap resulting from the formation of new groups led to a sixfold increase in the frequency of inter-group encounters, which exposes the population to elevated risks of fight-related injuries and infanticide.

Quadratic relationships between group size and foraging efficiency in a herbivorous primate

“…Instead, the largest group had far less home range overlap than the other study groups (~20% versus 90%), which is more consistent with an avoidance mechanism of intergroup contest competition 52,66 . Among the other groups, greater home range overlap likely caused more BGS, which may have exacerbated WGS.…”

“…The home range comparisons do not include any overlap with non-study groups, but the largest group had no known encounters with those groups during this study, so its overlap with their home ranges may have been minor. Other large groups have also had low home range overlap and/or low rates of intergroup encounters, but statistical correlations have not been reported between those parameters and group size 52,67 . Groups may avoid each other by travelling farther after encounters, by using long-distance signaling mechanisms (e.g.…”

“…Thus, both types of intragroup feeding competition (WGS and WGC) appear to be weak in this species 44 . Subsequent to most of the previous studies of group size, however, some groups have become larger and intergroup encounters have become more frequent, which increases the potential for both intragroup feeding competition and intergroup contest competition 52 .…”

Brain organization of gorillas reflects species differences in ecology

“…First, western lowland gorillas have a more frugivorous diet than mountain gorillas, which are almost entirely folivorous because of the lack of fruit trees in their high-altitude habitat, particularly in the Virunga Mountains from where the eastern gorillas in this sample originated (Tutin, 2003; Robbins, 2007), and they have larger home ranges (Doran, 1996; Caillaud et al, 2014). These two factors likely place an increased demand on spatial memory, which relies on the hippocampus (Eichenbaum et al, 1999; Burgess et al, 2002).…”

“…Mountain gorillas are more likely than western lowland gorillas to have multi-male groups (Yamagiwa et al, 2001; Robbins, 2007). Mountain gorillas in the Virungas are almost entirely folivorous (Goldsmith, 2003; Tutin, 2003; Watts, 1996, 2003; Robbins, 2007), are much more terrestrial than other subspecies, and have smaller home ranges (Doran, 1996; Taylor, 1997; Doran and McNeilage, 2001; Caillaud et al, 2014). Mountain gorillas in Bwindi and Grauer's gorillas in high altitude habitats are also highly folivorous, but eat considerable fruit when it is available and show levels of frugivory and arboreal feeding intermediate between Virunga gorillas and western lowland gorillas, which are the most frugivorous of the taxa (Yamigawa et al, 1994; Robbins and McNeilage, 2003; Yamigawa et al, 2003).…”