External Stimuli, Internal Factors and Behaviour
At the beginning of chapter 2 we stated that an animal responds selectively to relatively few changes in its environment. This “selectivity of response” is due, in part, to the complement and capabilities of its receptors. In fact, the phrase “selectivity of response” is something of an understatement; animals can be extremely specific in their responses (especially in the context of communication), with particular stimulus configurations releasing one, and only one particular response. In Stroll through the Worlds of Animals and Men, von Uexküll (Hinde, 1970) describes the behaviour of the female tick. Once she has mated, she climbs the vegetation and clings there, perhaps for months, totally unresponsive to the changes around her, except for one: if she detects butyric acid in the air she will let go her hold. Butyric acid is a component of mammalian skin secretions, and her response to it ensures a reasonable chance of landing on a host. In fact, female ticks tend to move about in the vegetation quite a lot, and there are other stimuli to which they will respond. Nevertheless, the specificity of response to butyric acid is extreme and is probably conferred largely by the possession of many chemoreceptors which respond quite specifically to butyric acid. This is an example of stimulus filtration in the olfactory modality and can be accounted for by “line coding”. In other cases, particularly when signals are in the visual modality, the filtration process cannot be ascribed so neatly to a peripheral phenomenon.
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