This is the 2007 version. Click here for the 2017 chapter 08 table of contents.


Species-typical behaviors are often triggered by specific stimuli in the environment. The stimulus that triggers an action pattern is called a sign stimulus or releaser . It is a specific input that activates a motor program. For example, Tinbergen found that male sticklebacks would attack anything red during mating season, when their own bellies were red. Usually this resulted in attacking other male sticklebacks that were competing for nesting territory. However, the fish would attack anything red, not just other sticklebacks.

In one experiment, Tinbergen fashioned a series of stickleback models, ranging from a very realistic but colorless model to a very unrealistic blob with a red belly. The male stickleback ignored the realistic fish without the red belly but attacked all the models with the red bellies.

What is a sign stimulus or releaser? Which of Tinbergen's stickleback models was most effective in eliciting an attack, and why?

The stickleback in mating season ignores a fish model which lacks a red belly but attacks a shapeless blob with a red belly.

Evidently the color red released the attack response. For similar reasons, fishing lures with distinctive markings can be very effective. They may release a biting response even if they do not resemble a living animal.

Why did Tinbergen's sticklebacks suddenly dart to the side of the aquarium?

The critical feature of a sign stimulus becomes especially clear when a motor program is set off by the wrong thing. One day Tinbergen's lab students noticed all the sticklebacks attacking the sides of their aquariums near a window. Across the street was a red postal van. The small patch of red visible through a window was enough to set off attack responses in the Sticklebacks.

Why does the stickleback attack anything red?

Why would the stickleback respond so vigorously to any red object? It is better to attack all red objects—wasting a little energy on red leaves—than ever to fail to attack another male stickleback during mating season. A failure to attack might allow the second stickleback to slip into the nest and fertilize the eggs. If that happened, the first stickleback would spend its time and energy aiding the reproductive success of another fish. So the fish is biased toward a strategy of attacking anything red, resulting in many false alarms but also relatively few failures to drive off competing male sticklebacks.

What do Alcock's examples have in common?

One lesson from the stickleback research and similar studies by Tinbergen and Lorenz was that complex motor stimuli are often set off by highly specific stimuli. Consider the following figure, adapted from Alcock (1975, p.65). It contains two examples of complicated movements that normally help birds pass on their genes.

Complex behaviors triggered by simple stimuli

(A) "A male red-winged blackbird copulating with a mount consisting of the tail feather of a female raised in pre-copulatory position." Even though the tail feather is on a seed pod, not a female bird, it sets off the sexual response in the male.

(B) "Willow warblers attacking the stuffed head of a cuckoo." Even though the head is on a stick, clearly not alive and not a real bird, the small warblers nevertheless attack it.

In the proper context each behavior would seem purposeful or intelligent. But the examples show that these motor responses are triggered by specific stimuli in more or less robotic fashion.

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