Tinbergen’s three-spined stickleback territorial experiment showed that two male fish could switch between attacker and retreating intruder when their positions relative to a nest changed. By holding the fish in glass test-tubes and moving them from one end of a tank to the other, Niko Tinbergen made the effect of territorial status visible: each male attacked near his own nest and yielded in the other’s territory. The result illustrates how territorial defense can arise from a conditional strategy grounded in an asymmetry of arrival.

Experimental Setup

In a fish-tank, two male sticklebacks had each built a nest at opposite ends and defended the surrounding territory. Tinbergen placed each fish in a large glass test-tube, held the tubes beside one another, and watched the males attempt to fight through the glass. The tubes kept the fish apart while allowing their aggressive and retreating responses to be observed together.

Reversal of Roles

The decisive observation came when Tinbergen moved both tubes between the two nesting areas. Near male A’s nest, A took an attacking posture and male B tried to retreat. When Tinbergen moved the tubes to male B’s territory, their roles reversed: B attacked and A retreated. Neither fish consistently occupied the attacker’s role; the same pair changed behavior with the location of the encounter.

The book interprets this reversal as evidence for a simple conditional strategy: “if resident, attack; if intruder, retreat.” The experiment’s force lies in the controlled change of setting: moving the tubes from one end of the tank to the other was enough to dictate which male attacked and which yielded.

Territorial Defense and Its Interpretation

The experiment supports an account of territorial behavior based on asymmetry in the timing of arrival. In ordinary circumstances, one animal is already resident on a patch of ground while another arrives as an intruder. If resident status favors attack and intruder status favors retreat, the resulting pattern can be stable without territorial defense needing an additional, independent biological advantage to explain it.

The example also illustrates the book’s distinction between an observed behavior and a proposed explanation of its function. Biologists have offered numerous suggestions for the advantages of territoriality, but the experiment makes it possible to understand defense as an evolutionarily stable strategy arising from the resident–intruder asymmetry itself. Its significance is thus both empirical and argumentative: a deliberately simple manipulation makes a conditional behavioral rule visible and clarifies how such a rule can produce territorial defense.