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Although the stimulus situations that arouse fear in other species are not identical with those that arouse fear in humans, there is much overlap. Overlaps are especially evident, moreover, in the case of the non-human primates, to which much of this chapter is given.
Ethologists take for granted that many of the stimulus situations that arouse fear in animals can be regarded as naturally occurring clues to events that constitute a potential danger to the species in question. This applies especially to situations that arouse fear on the first occasion that an individual encounters them.
Distance receptors are commonly employed for sensing these naturally occurring clues. Depending on the species, an animal may rely mainly on visual clues and receptors, mainly on auditory ones, or mainly on olfactory ones; or on any combination of them.1 Only when the distance receptors have failed to detect potential dangers in time are the proximal receptors, those for touch and pain, called into action—and by then it may be too late. Thus, in eliciting fear behaviour, distal clues and distance receptors play a crucial role.
Of all the possible stimulus situations that could act as clues to potential danger and can be sensed at a distance, there are certain ones that are exploited by a very wide array of species. Among the best known are strangeness and sudden approach, both of which regularly evoke fear responses in birds and mammals. Another is the ‘visual cliff’ to which young mammals of all species so far tested respond by taking avoiding action.
Situations of other kinds, by contrast, arouse fear responses in animals of only a few species; and sometimes, perhaps, of only one. For example, in some species of bird the sight of mammalian fur elicits fear responses; in others, the sight of a pair of staring eyes or of something falling from the sky. In some