Search images not proven: A reappraisal of recent evidence

Abstract

It is commonly agreed that predators hunting for cryptic prey have been demonstrated to adopt search images (involving perceptual specialization) for particular prey types. However, a simple alternative strategy, that of an appropriate reduction in search rate, may in many cases have the same effect as adopting a search image. The search image and search rate hypotheses are examined, and a series of critical predictions are proposed to distinguish them. Briefly, the search rate hypothesis predicts: (1) that adjusting search rate for one cryptic prey type will enhance the ability to detect other equally cryptic types (the search image hypothesis predicts that this will interfere with the ability to detect other equally cryptic types); (2) that adjusting search rate for cryptic prey will be achieved by learning to spend longer looking at each patch of the environment, and that the more cryptic the prey the longer will be the viewing time required to effect accurate detection (the search image hypothesis makes no such predictions). Four recent studies purporting to demonstrate search images are re-examined in the light of these predictions, and it is found that the published data fail to distinguish the two hypotheses. In addition, some evidence that is better explained by the search rate hypothesis is discussed. A brief examination of the ecological consequences of the search rate hypothesis shows that, unlike the search image hypothesis, it does not predict apostatic selection. It is therefore argued that the distinction between the two mechanisms explaining how predators ‘learn to see’ cryptic prey is important, but that the critical tests have not yet been made. The existence of search images remains not proven.