In ethology, a feature of a complex stimulus that triggers a specific response or fixed-action pattern. A classic example is the redness of the belly of a male stickleback fish in breeding condition, which elicits aggressive attacks from other male sticklebacks, whereas other features of the stimulus, such as its shape, size, and manner of movement, have little effect on the responses.
Similarly, the tactile sensation of prodding by a male stickleback at the base of the tail of a female in the right condition elicits egg-laying, whereas other features of the stimulus, including it colouring, are irrelevant to this response.
The Dutch-born British ethologist Niko Tinbergen (1907–88), who first identified sign stimuli in 1951, reported that even a red mail van passing outside the window of his laboratory elicited aggressive display from his male sticklebacks, and even prodding with a glass rod at the base of a female's tail elicited egg-laying. Many sign stimuli, though not all, are releasers.
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