ReviewGroup-size effects on vigilance: a search for mechanisms
Introduction
In foraging animals, visual scanning of the surrounding environment is referred to as vigilance and is commonly assumed to be aimed at potential predation threats (Roberts, 1996, Treves, 2000). In several species of birds and mammals, vigilance levels have been shown to decrease with increases in group size (Elgar, 1989, Quenette, 1990). The decline in vigilance is usually interpreted as a response to the lower risk of predation in larger groups. Several mechanisms may be involved in the decline. For instance, more eyes are available to scan the surrounding environment in larger groups thus allowing individuals to detect predators more easily and decrease individual investment in vigilance without increasing predation risk. When predators attack a single prey per group, the presence of companions in a group dilutes individual risk which again may favour a decrease in individual vigilance investment. The time thus saved can be allocated to other activities such as foraging.
Despite the apparent success of behavioural studies in documenting group-size effects on vigilance, two problems have beset interpretation of the results. The first problem is related to the cause of the decline in vigilance since mechanisms unrelated to predation risk may also produce decreases in vigilance with group size. In particular, increased competition in large groups may induce a decrease in individual vigilance levels as foragers scramble for a greater proportion of limited resources (Clark and Mangel, 1986). Although the potential influence of scramble competition on vigilance has long been recognised, theoretical and empirical interest in the matter has only risen recently.
The second problem is related to the nature of the decline in vigilance. Predation factors, such as dilution or detection effects, are expected to produce an asymptotic decrease in vigilance with group size (Pulliam, 1973, Dehn, 1990). Although models have considered the relative contribution of detection and dilution effects to the decline in vigilance, the role of other behavioural mechanisms, which may interact with predation risk, has been neglected. For instance, interference among foragers can reduce individual allocation of time to feeding and vigilance (Slotow and Coumi, 2000, Blumstein et al., 2001a). Here, vigilance levels are expected to reflect a compromise between the pull of predation risk and the push of interference competition. Under these circumstances, vigilance levels have been predicted to decline in a linear, rather than non-linear fashion with group size (Blumstein et al., 2001a) or to fail to decline at all (Slotow and Coumi, 2000). Scrounging is another mechanism that could influence vigilance levels in groups of different sizes. Scanning can serve to detect the discovery of food sources by companions thus allowing foragers to scrounge resources produced by others (Giraldeau and Beauchamp, 1999). The inclusion of scrounging in vigilance models leads to changes in vigilance levels with group size and even calls into question the prediction of a decline in vigilance in large groups (Beauchamp, 2001). Therefore, unless the contribution of these factors is known, the precise pattern of changes in vigilance with group size is not easy to predict.
The purpose of this paper is to review recent theoretical and empirical research to understand the cause and the nature of changes in vigilance levels with group size with the ultimate view to assess the contribution of several mechanisms to the group-size effect.
Section snippets
Detection
Although several researchers postulated that groups could detect predators more easily than solitary animals, the first attempt to model group-size effects on vigilance, and thus to establish relevant assumptions about the predation process, was made by Pulliam (Pulliam, 1973). In Pulliam’s model, predators attack from ambush without regard to the level of vigilance maintained by the prey. The attack will be successful if the predator crosses a threshold distance undetected by any group member.
The nature of changes in vigilance with group size
Predation-related factors, such as dilution or detection effects, are expected to produce non-linear declines in vigilance with group size. What is the relative contribution of each factor and how will the shape of the expected decline in vigilance change as a result? In addition, what happens if other ecological factors interact with predation risk to determine the shape of the group-size effect?
Discussion
With respect to the causes of the decline in vigilance with group size, models indicate clearly that mechanisms based on predation effects or scramble competition effects represent equally viable alternative explanations. Therefore, the challenge for future studies is clearly to establish the relevance of each explanation to the group-size effect on vigilance. In terms of theoretical effort, models that combine the two types of effects appear most promising especially if it could be shown that
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