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

Cyril C Grueter,Veronica Vecellio,Tara S Stoinski,Martha M Robbins,Didier Abavandimwe,Andrew M Robbins,Felix Ndagijimana

doi:10.1038/s41598-018-35255-0

Abstract

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.

Highlights

Feeding competition has been considered one of the most important links between ecological factors and the social organization of gregarious species[1,2]
To examine the effects of group size on foraging efficiency, we looked at the daily travel distances, activity budgets, and energy intake rates of the Virunga mountain gorillas
The largest group had significantly higher energy intake rates than the intermediate sized groups, which is consistent with the quadratic pattern for our other measures of foraging efficiency

Summary

Introduction

Feeding competition has been considered one of the most important links between ecological factors and the social organization of gregarious species[1,2]. Within-group scramble competition (WGS) is predicted to create a positive correlation between group size versus daily travel times and distances[7,8]. If foraging groups advance through their habitat by “pushing forward” the members in the front, WGS and/or within-group contest competition (WGC) could contribute to a positive correlation between group size and travel[9,13,14]. WGC can involve aggressive displacement by dominant individuals, or subordinates may move preemptively to avoid such aggression[15,16] Such competition is typically associated with rank related differences in foraging efficiency and reproductive success[17,18]. Small groups may travel farther to avoid encounters with large groups, and they may be displaced from food patches where such encounters occur[21,22]. We will use the term “intergroup contest competition” to refer to BGC while acknowledging a potential influence of mating competition

Methods

Results

Conclusion