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Abstract 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.

Eastern gorillas ( Gorilla beringei ) are among the few mammal species that seasonally consume large quantities of young bamboo shoots, which are a rich source of energy. Here, we document how the consumption of bamboo shoots coincides with changes in behavior of adult mountain gorillas ( Gorilla beringei beringei ) monitored by the Dian Fossey Gorilla Fund’s Karisoke Research Center in Volcanoes National Park, Rwanda. We offer a preliminary analysis of possible mechanisms underlying this behavioral change by measuring energy intake rates and the presence of cyanide and alcohol—ingredients potentially associated with increased activity levels—in fresh bamboo shoots. The percentage of bamboo shoots in the diet of gorillas was correlated positively with the rate of play behavior shown by adults in 2 of the 3 study groups. Play behavior was not the result of better weather conditions and also did not reflect the availability of spare time. Rather, there is some, but not consistent, evidence for a link between energy intake rates and play behavior of adults. Cyanide was not detected in young bamboo shoots, and the presence of alcohol remains inconclusive, albeit unlikely. In sum, our results show that consumption of a high-quality food can have a direct influence on the activity budget (and by extrapolation energy expenditure) of mountain gorillas through increased rates of play behavior. However, the physiological aspects underlying this elevated activity warrant further investigation.

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.

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.

Objectives Great apes show considerable diversity in socioecology and life history, but knowledge of their physical growth in natural settings is scarce. We characterized linear body size growth in wild mountain gorillas from Volcanoes National Park, Rwanda, a population distinguished by its extreme folivory and accelerated life histories. Methods In 131 individuals (0.09–35.26 years), we used non-invasive parallel laser photogrammetry to measure body length, back width, arm length and two head dimensions. Nonparametric LOESS regression was used to characterize cross-sectional distance and velocity growth curves for males and females, and consider links with key life history milestones. Results Sex differences became evident between 8.5 and 10.0 years of age. Thereafter, female growth velocities declined, while males showed increased growth velocities until 10.0–14.5 years across dimensions. Body dimensions varied in growth; females and males reached 98% of maximum body length at 11.7 and 13.1 years, respectively. Females attained 95.3% of maximum body length by mean age at first birth. Neonates were 31% of maternal size, and doubled in size by mean weaning age. Males reached maximum body and arm length and back width before emigration, but experienced continued growth in head dimensions. Conclusions 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.