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

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.

Using long-term ranging patterns to assess within-group and between-group competition in wild mountain gorillas

“…Instead, we found evidence of a concave relationship, with the annual home range and the core area size leveling off and then declining at the larger group sizes. Similar patterns have been reported for the travel times and distances of the Virunga gorillas, which were attributed to the costs of within-group feeding competition for the smaller groups and the benefits of between-group feeding competition for the largest group [62]. Concave patterns were not observed, however, in the home ranges of those Virunga gorilla groups [52].…”

“…Our results lend support to our predictions for between-group competition, as indicated by a trend for greater exclusivity of home ranges and core areas as group size and the number of males increases. Having more exclusive access to a home range and core area should lead to more exclusive and higher access to food resources (see [ 38 , 62 ]). This should result in a difference in energetic status among groups and suggests that larger groups have a competitive advantage in intergroup competition, a pattern typically found in territorial species ([ 36 , 66 ] but see [ 44 , 45 ]).…”

Stronger between-clan than within-clan contests and their ecological correlates in a non-territorial, fission-fusion species, the Asian elephant

“…2018). In our results from piece-wise regression of rate of agonism on group size, while the absence of a positive relationship in the segment after group size of 5 could be seen as conforming to a non-linear relationship (for example, Grueter et al . 2018 on gorillas) in a strong between-clan competition regime, our sample size for large group sizes was low.…”

“…Thus, studies that account for the effects of both scramble and contest competition could add to our understanding of proximate mechanisms governing group size, which has shown different relationships with foraging efficiency, travel costs and stress in different populations (Markham et al . 2015, Grueter et al . 2018).…”

Chimpanzee ranging responses to fruit availability in a high‐elevation environment

“…Variation in primate travel effort is most commonly analyzed using daily path length (DPL, also referred to as daily travel distance or day journey length) and monthly home range size (e.g., Doran, 1997; Grueter et al, 2013; Grueter, Robbins et al, 2018; C. Johnson, Piel, Forman, Stewart, & King, 2015; Li, Chen, Ji, & Ren, 2000; Shaffer, 2013; Wrangham, Gittleman, & Chapman, 1993). DPL is calculated by summing the straight‐line distances between consecutive x and y location coordinates.…”