Causes, mechanisms, and consequences of contest competition among female mountain gorillas in Rwanda

Abstract

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.

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

“…We ran a third model in which the response variable was the distance travelled between consecutive food sites. For all three models, the random effect for gorilla ID can help to control for female rank, which did not show a significant effect on these response variables in the previous study of this data 41 .…”

“…Time spent feeding was significantly longer in larger groups, but the increase was only three percentage points across a three-fold variation in group size 39 . Dominance rank did also not have a significant effect on the energy intake rates or the proportion of time spent traveling, and rank-related differences in female reproductive success might be due to female quality rather than contest competition 41,50,51 . Thus, both types of intragroup feeding competition (WGS and WGC) appear to be weak in this species 44 .…”

“…Their positive correlation could indicate an influence of group size on intragroup feeding competition, whereas our negative correlation is more consistent with theoretical predictions for intergroup contest competition 7,8 . The feeding time of mountain gorillas seems to be more sensitive to differences in energy intake rates than differences in the energy requirements from travel 36,41,72 . If so, then our negative correlation could be interpreted to suggest that larger groups had greater access to areas that facilitate higher energy intake rates.…”

“…Distances between patches are small, which limits the extra travel requirements if larger groups must visit more patches 37,38 . Food sites are numerous and densely distributed within those patches, so extra foraging costs are minimal if some sites have already been depleted, and individuals have little incentive to compete for any particular spot 39–41 . The Virunga mountain gorillas have been considered unlikely candidates for BGC because intergroup encounters have traditionally been infrequent and intergroup aggression is mainly limited to male mating competition 42–44 .…”

Competition for grooming partners and interference in affiliation among female mandrills

“…Taking over a resource from another individual is the most obvious and straightforward form of competition. Accordingly, this form of direct competition is commonly observed whenever animals compete for resources such as food or mates (Clutton‐Brock, Guinness, & Albon, ; Grueter et al., ; Janson, ; Weckerly, ). In the grooming literature, takeovers of grooming partners are often conflated with interference in others’ grooming (i.e., no information about the subsequent occurrence of grooming is given; Fairbanks, ), or it includes cases in which the disrupting individual ended up receiving, rather than giving, grooming (Sinha, ).…”