Predation is one of the main evolutionary forces driving group formation, providing various benefits related to detecting, confusing, deterring and mobbing predators (reviews in Krause & Ruxton 2002
; Caro 2005
). When a predator can only target one or a few members of a group at a given time (Foster & Treherne 1981
), an individual may be afforded a dilution of risk. However, risk is often not equally shared by group members and consequently individuals may behave to reduce their risk relative to others in the group. Hamilton's selfish herd hypothesis (1971) has arguably been the most popular model to explain how differential risk may cause loosely associated prey individuals to group up.
In Hamilton's simplest model, surface-living individuals are preyed upon by a bottom-dwelling predator. The predator attacks the nearest prey individual from the point at which it randomly appears on the surface. This creates ‘domains of danger’ for individual prey: an area within which an individual will be the ‘closest’ to a predator from its point of appearance at the surface. The larger this area, the greater the individual's predation risk relative to that of its neighbours. By moving towards neighbours, an individual can reduce this ‘domain of danger’, which hypothetically translates into lower predation risk (Hamilton 1971
). If all individuals move in this way, Hamilton argued, compact groups may form.
The elegance of the selfish herd has endeared it as a framework within which to interpret the behaviour of prey in response to predators, and more than 600 studies on a diverse array of taxa (Morton et al. 1994
) have provided support for it. While these studies have successfully quantified increased cohesiveness of groups when exposed to predators (Viscido 2003
), shown that individuals prefer central to peripheral positions within a group (e.g. Krause 1993
; Krause & Ruxton 2002
; Caro 2005
), and that centre individuals' risk are reduced relative to peripheral ones (e.g. Milinski 1977
; Uetz 1993
), this evidence is not unequivocal. The classification of centre versus edge individuals is open to bias (Stankowich 2003
; Viscido 2003
), there is ambiguity in the literature as to where animals are more at risk (Krause 1993
; Krause & Ruxton 2002
; Caro 2005
) and results at group centres and edges may be strongly confounded by other benefits and costs of grouping (Fitzgibbon 1990
; Krause 1993
; Caro 2005
). The prediction that an individual's spacing affords it differential predation risk has been largely neglected. To our knowledge, only one study has shown that predators (sparrow hawks) target more widely spaced prey (redshank), relative to non-attacked neighbours, while controlling for predator confusion, centre/edge positioning and, to some extent, vigilance (Quinn & Cresswell 2006
The critical corroboration of the selfish herd hypothesis requires empirical validation of its central concept, namely that the size of the domain of danger is proportional to predation risk and that it alone embodies differential survival probability and is subject to selection pressure. We tested this prediction in a predator–prey system involving white sharks (Carcharodon carcharias) and Cape fur seals (Arctocephalus pusillus pusillus) at Seal Island in South Africa.
Seal Island is a small rocky outcrop situated in the northwest of False Bay, which is inhabited by approximately 77 000 Cape fur seals. Adult seals leave the island to travel to the foraging grounds approximately 24 km to the south of the island (Laroche et al. 2008
) on trips lasting several days (David & Rand 1986
). In the austral winter, seals are subject to high levels of predation by white sharks (Laroche et al. 2008
). Predations are spatially and temporally predictable, typically occurring within 400 m of the south and west side of the island and within the first 2 h after sunrise (Laroche et al. 2008
). Predation success is highly dependent on the element of surprise (Laroche et al. 2008
), thus solitary hunting sharks (Le Boeuf 2004
) typically attack seals at tremendous speed from directly below, resulting in their whole bodies breaching out of the water (Laroche et al. 2008
This predator–prey relationship provides an excellent system for testing the selfish herd hypothesis for several reasons. First, there is a distinct spatial separation of foraging and predation zones: seals must traverse the ‘danger zone’ adjacent to the island, and groups which form prior to departure from the island subsequently break up, once out of the danger zone (Rand 1967
; Laroche et al. 2008
). Furthermore, predator–prey activity is spatio-temporally confined and predictable (Laroche et al. 2008
), and the system, where the predator appears by surprise within a group is one that strongly resembles Hamilton's original model. Most propitiously, sharks detect their prey using surface moving silhouettes (Laroche et al. 2008
), which allows for an opportunity to manipulate the system by constructing artificial seal silhouettes with variable domains of danger. Not only can exact distances between individual ‘seals’ be measured, but resulting domains of danger can be repeated and survival probability subsequently assigned to them. Furthermore, it allows a test of the selfish herd that controls for vigilance, the confusion effect, and phenotypical and behavioural variability within groups.
In this study, we test the prediction that animals with larger domains of danger will be at more risk than animals with smaller ones, and investigate if the size of an animal's domain of danger is proportional to its relative predation risk.