Optimal individual positions within animal groups

Abstract

1Animal groups are highly variable in their spatial structure, and individual fitness is 2 strongly associated with the spatial position of an animal within a group. Predation 3 risk and food gains are often higher at the group peripheries; thus, animals must trade 4 off predation costs and foraging benefits when choosing a position. Assuming this is 5 the case, we firstly use simulation models to demonstrate how predation risk and food 6 gains differ for different positions within a group. Secondly, we use the patterns from 7 the simulation to develop a novel model of the trade-off between the costs and 8 benefits of occupying different positions, and predict the optimal location for an 9 animal in a group. A variety of testable patterns emerge. As expected, increasing 10 levels of satiation and vulnerability to predators, and increasing predation risk result 11 in increased preferences for central positions, likely to lead to increased competition 12 or more tightly packed groups. As food availability increases, individuals should first 13 prefer center positions, then edge, returning to central positions under highest food 14 levels. Increasing group size and/or density lead to more uniform preferences across 15 individuals. Finally, we predict some situations where individuals differing in 16 satiation and vulnerability prefer a range of different locations, and other situations 17 where there is an abrupt dichotomy between central and edge positions, dependent on 18 the levels of monopolization of food by peripheral individuals. We discuss the 19 implications of our findings for the structure of groups and the levels of competition 20 within them, and make suggestions for empirical tests. 21 22

Spatial properties of a forest buffalo herd and individual positioning as a response to environmental cues and social behaviour

“…When resting, the spatial geometry of the herd could also be determined by the costs and benefits of an individual's spatial position within the group (Hirsch 2007). Previous studies have, in general, reported that peripheral individuals experience greater predation risk, whereas those in the centre of the group have access to fewer food resources (e.g., Hamilton 1971;Parrish and Edelstein-Keshet 1999;Focardi and Pecchioli 2005;Morrell and Romey 2008). However, since predation on the forest buffaloes in our study was never observed, antipredator behaviour seems not a good reason that influences herd size and aggregation in open habitat in this specific case.…”

“…Magurran 1993;Parrish and Hamner 1997;Ruckstuhl 1998;Krause and Ruxton 2002;Ruckstuhl and Kokko 2002;Turner et al 2005;Hay et al 2008;Korte 2008b;Morrell and Romey 2008). These factors include feeding and foraging efficiency, energy expenditure, the time spent on vigilance behaviour, predation risk, group size, sexual segregation and reproductive opportunities.…”

Aerobic capacity influences the spatial position of individuals within fish schools

“…While schooling can provide significant advantages to individual fish in terms of both predator avoidance [2] and foraging success [3], the exact benefits gained by school members depend on spatial location within the school [4][5][6][7].…”

Ecological and Social Determinants of Group Cohesiveness and Within‐Group Spatial Position in Wild Assamese Macaques

“…; Hirsch & Morrell ), access to food (Robinson ; Barta et al. ; Di Bitetti & Janson ; Morrell & Romey ) and patterns of social interaction with other group members (Baldwin & Baldwin ). The major theory used to predict the influence of within‐group spatial position on predation risk is the ‘selfish herd’ effect (Hamilton ).…”