The importance of individual variation in the dynamics of animal collective movements

Abstract

Animal collective movements are a key example of a system that links two clearly defined levels of organization: the individual and the group. Most models investigating collective movements have generated coherent collective behaviours without the inclusion of individual variability. However, new individual-based models, together with emerging empirical information, emphasize that within-group heterogeneity may strongly influence collective movement behaviour. Here we (i) review the empirical evidence for individual variation in animal collective movements, (ii) explore how theoretical investigations have represented individual heterogeneity when modelling collective movements and (iii) present a model to show how within-group heterogeneity influences the collective properties of a group. Our review underscores the need to consider variability at the level of the individual to improve our understanding of how individual decision rules lead to emergent movement patterns, and also to yield better quantitative predictions of collective behaviour.This article is part of the theme issue 'Collective movement ecology'.

From single steps to mass migration: the problem of scale in the movement ecology of the Serengeti wildebeest

“…Aerial filming and computer vision methods are now providing the tools needed to collect and process these observations. Integrating these studies of collective movement with individual tracking [54] will allow us to detect which individuals are influencing decisions [100] and to understand how wildebeest herds collectively respond to their environments.…”

The Role of Individual Heterogeneity in Collective Animal Behaviour

“…The greater the number of such individuals, the higher the groups' velocity and alignment and the lower its cohesion [7,58,[97][98][99][100]. When heterogeneity among agents is large, groups are predicted to fragment over time, leading to phenotypically assorted subgroups [97,99] and the most extreme behavioural types to be isolated [69]. Models that have considered variation in individuals' preferred movement direction, such as towards a known resource, show that goal-oriented individuals tend to end up in the front of groups and thereby obtain a larger share of potential resources [7,61,98].…”

“…Social cohesion and coordination both require individuals to possess similar movement capacities and, over time, can result in individuals acquiring similar energetic (metabolic) costs of locomotion [16]. Therefore, groups with too great a mix of physiological capacities [15], extreme variation in motivation [25,68], or low social responsiveness are expected to have low cohesion, alignment, and coordination [69], which may compromise the transfer of information. By contrast, groups of individuals with high social motivation will improve group cohesion [7,45,70], and because such individuals are generally less assertive [31], they may furthermore help to reduce conflict and improve decision-making [71] but lack directedness [7].…”

Collective movement in ecology: from emerging technologies to conservation and management

“…As empirical studies uncover the actual interactions between groups' members [10,44,47,48,57], the importance of individual heterogeneity is being revealed [47,48,54]. In this issue, del Mar Delgado et al [58] address this question directly and provide evidence that not accounting for individual variability currently impedes understanding of how group decisions are made and that predictions of collective movement that ignore individual variation are likely to fail. Through the use of aerial filming, Torney et al [48] demonstrate how interaction rules and individual variation may be quantified, in this context examining the heterogeneous nature of social interaction of migrating caribou (Rangifer tarandus groenlandicus  pearyi).…”