Monitoring temporal and spatial changes in the resource availability of endangered species contributes to their conservation. The number of critically endangered mountain gorillas (Gorilla beringei beringei) in the Virunga Volcano population has doubled over the past three decades, but no studies have examined how food availability has changed during that period. First, we assessed if the plant species consumed by the gorillas have changed in abundance and distribution during the past two decades. In 2009-2010, we replicated a study conducted in 1988-1989 by measuring the frequency, density, and biomass of plant species consumed by the gorillas in 496 plots (ca. 6 km(2)) in the Karisoke study area in Volcanoes National Park, Rwanda. We expected to observe a decreased presence of major gorilla food plants as a likely result of density-dependent overharvesting by gorillas. Among the five most frequently consumed species (composing approximately 70% of the gorilla's diet, excluding bamboo), two have decreased in availability and abundance, while three have increased. Some species have undergone shifts in their altitudinal distribution, possibly due to regional climatic changes. Second, we made baseline measurements of food availability in a larger area currently utilized by the gorillas. In the extended sampling (n = 473 plots) area (ca. 25 km(2) ), of the five most frequently consumed species, two were not significantly different in frequency from the re-sampled area, while two occurred significantly less frequently, and one occurred significantly more frequently. We discuss the potential impact of gorilla-induced herbivory on changes of vegetation abundance. The changes in the species most commonly consumed by the gorillas could affect their nutrient intake and stresses the importance of monitoring the interrelation among plant population dynamics, species density, and resource use.

Overall, despite the differences in food availability, we did not find large differences in the energetics of gorillas in the two populations, although further work is needed to more precisely quantify energy expenditure and energy balance. These results emphasize that even species with high food availability can exhibit behavioral and energetic responses to variable ecological conditions, which are likely to affect growth, reproduction, and survival.

While comparable data are scarce, our findings provide preliminary support for the prediction that mountain gorillas reach maximum body size at earlier ages compared to more frugivorous western gorillas. Data from other wild populations are needed to better understand comparative great ape development, and investigate links between trajectories of physical, behavioral, and reproductive maturation.

The effect of feeding competition on foraging efficiency is an important link between ecological factors and the social organization of gregarious species. We examined the effects of group size on daily travel distances, activity budgets, and energy intake of mountain gorillas in Rwanda. We measured daily travel distances of five groups, activity budgets of 79 gorillas in nine groups, and energy intake data for 23 adult females in three groups over a 16-month period. Travel distances and the proportion of time spent traveling increased with size for most groups, which would be expected if their foraging efficiency is limited by intragroup feeding competition. However, travel distances and times decreased for the largest group, which also had higher energy intake rates than intermediate sized groups. The improved foraging efficiency of the largest group may be explained by advantages in intergroup contest competition. The largest group had much lower home range overlap than the other study groups which may be due to groups avoiding one another as a result of male mating competition. Collectively, our results indicate that intermediate sized groups had the lowest foraging efficiency and provide a new twist on the growing evidence of non-linear relationships between group size and foraging efficiency in primates.

Socioecological models predict that contest competition will arise when high quality foods can be usurped or monopolized, leading to more favorable energy balances and higher reproductive success for high-ranking females. Gorillas are interesting species for studying such predictions due to the variety of ecological conditions that they experience in different locations. Using data from 23 female mountain gorillas in 3 social groups in the Virunga Massif, we examined food characteristics that may influence contest competition (food site residence times [FSRT]), proximate mechanisms of such competition (aggression and avoidance), and potential consequences of competition (rank-related differences in energy intake rates, travel expenditures, and activity budgets). The average FSRT of each female was significantly correlated with dominance rank, which suggests that high-ranking females may have greater access to foods that are easier to usurp (as predicted with contest competition). High-ranking females were significantly more aggressive than low-ranking females, and both aggression and avoidance were significantly higher while feeding than during other activities. Contrary to predictions for contest competition, however, rank was not significantly correlated with energy intake rates nor with the proportion of time spent traveling versus feeding. Thus, we did not find any energetic benefits to explain why high-ranking females had significantly higher reproductive success in earlier studies. We propose several alternative explanations and discuss the potential complications of assessing contest competition in species with weak dominance relationships.

Space use in mammals may be influenced not only by their primary foods, but also by localized sources of physiologically critical resources such as sodium‐rich plants. We examined how sodium acquisition influences habitat use in mountain gorillas (Gorilla beringei) in Rwanda which have increased the amount of time they forage on community land outside of Volcanoes National Park (VNP), where eucalyptus (Eucalyptus spp.) tree bark is their most frequently eaten food. We measured sodium content in samples from 34 main dietary items and quantified sodium intake by 22 gorillas in three social groups over one year. On a dry weight basis, eucalyptus bark contains 3100 mg Na/kg. In contrast, the four herbs most frequently exploited for food inside the park are relatively sodium‐poor (<70 mg/kg each). Further, sodium intake rates were highest when the gorillas were on community land. Of the two groups that fed outside of the park, one obtained 73% and the other one 45% of their sodium in that habitat despite only feeding for minimal amounts of time there. However, one group that did not feed on community land acquired 78% of its sodium in the subalpine and alpine zones through the consumption of pith of giant lobelias and groundsels. Obtaining sodium thus likely creates an incentive for the gorillas to leave the park and make forays into high‐altitude habitat. Both strategies are not without risks: exiting their natural habitat and feeding on crops may increase human‐wildlife conflict and visiting high‐altitude areas may increase the risk of hypothermia.

Two mechanisms have been proposed to explain why scramble competition can increase the travel requirements of individuals within larger groups. Firstly, individuals in larger groups may be more likely to encounter food sites where other group members have already eaten, leading to greater asynchronous “individual” travel to find fresh sites. Secondly, when food sites are aggregated into patches, larger groups may need to visit more patches to obtain the same amount of food per capita, leading to greater synchronous “group” travel between patches. If the first mechanism can be mitigated by increasing group spread, then we expect the second mechanism to be more sensitive to group size. Here, we examine the individual travel and group travel of the Virunga mountain gorillas, along with potential implications for the two mechanisms of scramble competition. Asynchronous individual travel accounted for 67% of the total travel time, and the remainder arose from group travel. Group spread increased significantly for larger groups, but not enough to prevent an increase in individual travel. Contrary to expectations, group travel decreased with size among most groups, and we found only limited evidence of patch depletion that would cause the second mechanism of scramble competition. Collectively, our results illustrate how the influence of group size can differ for individual travel versus group travel, just as it differs among species for overall travel. Studies that distinguish between the two mechanisms of scramble competition may enhance our understanding of ecological constraints upon group size, including potential differences between frugivores and folivores. Significance statement Feeding competition provides insight into how group size can influence the foraging patterns of social animals, but two key mechanisms are not typically compared. Firstly, larger groups may visit more patches to access the same amount of food per capita (group travel). Secondly, their individuals may also need to move past more spots where another member has already eaten (individual travel). Contrary to expectations, we found that group travel decreased with size for most groups of mountain gorillas, which may reflect extra travel by smaller groups to avoid larger groups. Individual travel increased with size in most groups, even though gorillas in larger groups compensated by spreading out over a broader area. The two mechanisms revealed patterns that were not apparent in our previous study of overall travel. Our approach may help to explain potential differences between folivores and frugivores.

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