As part of a sequence of antipredator behaviors, animals sometimes approach predators when they initially encounter them. This sort of behavior has been extensively documented for vertebrates, particularly in fishes, birds, and mammals. Prey approaching predators is most often referred to as predator inspection behavior.
Among vertebrates, there is remarkable convergence in the dynamics ofapproach toward a potential predator. Prey typically approach a putative predator from a distance in a tentative, jerky manner. The approach is characterized by a series of moves toward the predator interrupted by stationary pauses, and sometimes alternating with moves away from the predator. In birds and mammals in particular, the prey may emit alarm signals or exhibit distraction or threat displays during an approach toward the potential threat. The approach may culminate in a number of possible outcomes along a continuum, ranging from the prey simply retreating to rejoin a social group of conspecifics nearby to an escalation where groups of prey attack a predator.
To understand the evolution of predator inspection, an in-depth analysis of the cost and benefits of approaching prey is necessary, and such analysis has been carried out on the predator inspection behavior ofgazelles in the Serengeti National Park (Tanzania). In the Serengeti, gazelles live in groups that can vary from fairly small (<10 individuals) to fairly large (>500) and interact with four main predators: lions, cheetahs, spotted hyenas, and wild dogs. In terms of benefits, predator inspection of cheetahs may actually decrease the current risk of predation to gazelles. In particular, cheetahs responded to gazelle inspection behavior, which is most common and most pronounced in large gazelle groups, by moving further between rest periods and between hunting periods. This in turn likely causes cheetahs to leave a particular area sooner than normal as a result of gazelle approach behavior, leading to decreased rates of mortality among potential prey.
The cost ofgazelle approach behavior is manifest primarily in terms of lost time/energy and increased risk of predation. Gazelles actually spend approximately 4% of their waking hours involved in approach behavior. This 4% could otherwise be devoted to other activities (foraging, mating, resting) and thus represents a real 'opportunity cost' to the animals. In terms ofmore direct costs, while the odds of an approaching adult being killed by a cheetah are very low (on the order of 1 in 5000), the probability of younger individuals being taken during an approach is an order of magnitude greater (about 1 in 400).
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