Prey approaching predators : a cost-benefit perspective

When faced with the presence of a predator, many species of prey initially approach rather than retreat from such a potential threat. Although seemingly paradoxical, this type of approach behaviour has been noted in a wide variety of taxa. We adopt a cost benefit perspective on the question of why prey approach potential predators, and aim to outline what is known about the putative costs and benefits of such behaviour. We identify five major types of costs associated with approach behaviour: (i) increased risk of mortality to the approacher, (ii) increased risk of mortality to relatives of the approacher, (iii) lost opportunity costs associated with approaching a predator, (iv) energetic costs of approach, and (v) potential costs of the approacher being 'exploited' by other group members. Benefits of approach behaviour are categorized as (i) acquiring information about the nature of the potential threat, (ii) informing others of the potential threat, (iii) deterring predator attack, and (iv) advertising one's quality to mates. After reviewing putative costs and benefits, we present a game theory model of prey approaching potential predators. Based on the costs and benefits of approach behaviour and group size, the model predicts the equilibrial frequency of 'approachers' and 'non approachers' and the (relative) mean approach distance (as a function of group size). To date, much of the work on prey approaching predators is either anecdotal or non experimental. We hope that the review of the costs and benefits of approaching predators presented here, in conjunction with our game theory model, will help provide direction for future studies of this interesting, but understudied, behavioural phenom