The temporal selfish herd: predation risk while aggregations form

Abstract

The hypothesis of the selfish herd has been highly influential to our understanding of animal aggregation. Various movement strategies have been proposed by which individuals might aggregate to form a selfish herd as a defence against predation, but although the spatial benefits of these strategies have been extensively studied, little attention has been paid to the importance of predator attacks that occur while the aggregation is forming. We investigate the success of mutant aggregation strategies invading populations of individuals using alternative strategies and find that the invasion dynamics depend critically on the time scale of movement. If predation occurs early in the movement sequence, simpler strategies are likely to prevail. If predators attack later, more complex strategies invade. If there is variation in the timing of predator attacks (through variation within or between individual predators), we hypothesize that groups will consist of a mixture of strategies, dependent upon the distribution of predator attack times. Thus, behavioural diversity can evolve and be maintained in populations of animals experiencing a diverse range of predators differing solely in their attack behaviour. This has implications for our understanding of predator -prey dynamics, as the timing of predator attacks will exert selection pressure on prey behavioural responses, to which predators must respond.

‘Selfish herds’ of guppies follow complex movement rules, but not when information is limited

“…Individual risk is defined by the 'domain of danger' (DOD), the area of space containing all points closer to the focal animal than to any other individual, and the selfish herd hypothesis suggests individuals should position themselves within groups to minimize the size of their own DOD [11]. A significant body of theoretical work has evaluated the success of various behavioural 'movement rules' in minimizing DODs and creating compact groups of individuals either once stable aggregations have formed [11][12][13][14][15] or during the process of aggregation itself [16,17].…”

“…These complex rules generate more compact aggregations in which a greater proportion of the group are able to reduce the size of their DOD. Simple rules can, however, result in more rapid initial reduction in DOD area [17], which might be particularly important when animals have little time to respond following detection of a predatory threat [16]. Simple rules have been criticized for their inability to produce the dense groups seen in nature [12,14], whereas more complex rules may be cognitively too complex for animals to follow [20,21].…”

The role of synchronized swimming as affiliative and anti-predatory behavior in long-finned pilot whales

“…Synchronization implies matching individual activity to group behavior and this could carry a cost for the individual that has to modify its optimal behavioral budget (Conradt and Roper, 2000). Generally this cost is counterbalanced by the gain of social grouping as decreased predation risk and increased prey capture (Morrell et al, 2011). However, there is no record of cooperative feeding in long-finned pilot whales.…”

Mathematical models of biological patterns: Lessons from Hamilton’s selfish herd

“…Other biologists have sided with Lima on this point. For example, Morrell et al (2011) argues that the speed with which a predator strikes will affect the best prey movement rule. Even though they measure predation risk by the areas of the cells of the Voronoi diagrams, they conclude that ''Our results suggest that the consideration of the timing of predator attacks (relative to the movement speed of the prey) is critically important in understanding the antipredator responses of prey'' (p. 610).…”