The special scent, or pheromone, emitted by female moths is a sign stimulus that attracts males. Tactile and even electrical sign stimuli are also known. The most widespread uses of sign stimuli in the animal world are in communication, hunting, and predator avoidance. The young of most species of snake-hunting birds, for instance, innately recognize and avoid deadly coral snakes; young fowl and ducklings are born able to recognize and flee from the silhouette of hawks. Similar sign stimuli are often used in food gathering.
The bee-hunting wasp recognizes honey bees by means of a series of releasers: The odor of the bee attracts the wasp upwind; the sight of any small, dark object guides it to the attack; and, finally, the odor of the object as the wasp prepares to sting determines whether the attack will be completed. This use of a series of releasers, one after the other, greatly increases the specificity of what are individually crude and schematic cues; it is a strategy frequently employed in communication. Most animal species are solitary except when courting and rearing young. To avoid confusion, the signals that identify the sex and species of an animal's potential mate must be clear and unambiguous. For example, a minnowlike fish called the stickleback uses a system of interlocking releasers to orchestrate its mating. When its breeding season arrives, the underside of each male turns bright red. This color attracts females but also provokes attacks by other males; red objects of almost any description will trigger male stickleback aggression. A female responds to the male's red signal with a curious approach posture that displays her swollen belly full of eggs. This incites the male to perform a zigzag dance that leads the female to the tunnel-like nest he has built. The female struggles into the nest, whereupon the male touches her tail with his nose and quivers.
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