Sticklebacks

Sticklebacks provide a striking experimental demonstration of territorial behavior as a conditional strategy rather than a fixed trait of an individual. In Niko Tinbergen’s test-tube experiment, the same two male fish took turns attacking and retreating as their location shifted between their nesting territories, illustrating how an individual’s behavior can depend on its relation to others and the immediate setting.

Territorial behavior and the experiment

Tinbergen placed two male sticklebacks in a tank. Each had built a nest at an opposite end and defended the area around it. He then put each fish in a large glass test tube and held the tubes beside one another, allowing the fish to try to fight through the glass.

The outcome changed when Tinbergen moved the tubes. Near male A’s nest, A adopted an attacking posture while B tried to retreat. Near B’s nest, the roles reversed: B attacked and A retreated. By moving the tubes from one end of the tank to the other, Tinbergen could determine which fish acted as resident and which as intruder. The males’ opposing responses fit the conditional rule: if resident, attack; if intruder, retreat.

What the case demonstrates

The experiment gives a concrete form to the idea that animal behavior can operate like a pre-programmed strategy. In this account, a strategy is not a conscious plan worked out by the animal; it is a policy that behavior follows as if it were responding to instructions. The sticklebacks’ reversal shows how a change in the circumstances of an encounter can alter which response is expressed, even though the participants remain the same.

The example also clarifies why territorial defense may persist. When arrival order gives one individual the status of resident, a strategy favoring residents can reward remaining on a particular patch of ground and appearing to defend it. The result may look like purposeful defense of a valuable territory, but the argument presented here is that territorial behavior can arise as an evolutionarily stable strategy from the asymmetry between resident and intruder.

Place in the evolutionary argument

Sticklebacks serve as evidence in a broader discussion of how selection can shape behavioral policies. The general claim is that a strategy is evolutionarily stable when, once most members of a population adopt it, an alternative cannot do better. The Tinbergen demonstration does not by itself establish the entire theory; it makes one proposed conditional strategy visible in a simple, controlled setting.

Its force lies in the reversal. Neither fish is invariably the attacker or the retreating individual: each response depends on which nest is nearby. Territoriality thus appears not as an isolated act of aggression, but as a relation between an animal, a patch of ground, and the position of its opponent. The experiment makes that relation observable by changing only the location of the two enclosed fish.