Domain Defense

Territorial defence is the pattern in which an animal that occupies a place behaves as its defender while an arriving rival retreats. In the book’s game-theoretic account, this conduct need not reflect a special biological prize in the ground: it can emerge as an evolutionarily stable strategy from the simple asymmetry of who arrived first. A test-tube experiment with two male sticklebacks makes the rule visible, while a separate population-regulation theory assigns territories a different role—as limited permissions to breed.

A convention based on arrival

An encounter need not be decided by strength or by a difference in what the contestants stand to gain. The book considers the apparently arbitrary circumstance that one animal is already at the location and the other arrives later. Calling them resident and intruder, it proposes the conditional policy “If you are the resident, attack; if you are the intruder, retreat.” Even if neither position carries an intrinsic advantage, this rule can settle a contest quickly, much like a dispute settled by tossing a coin.

Either of two opposite conventions could, in principle, become stable: residents might prevail, or intruders might. Which one takes hold would depend on which strategy first became common in the population. Once most individuals followed one convention, a mutant that ignored it would be disadvantaged. Under the resident-wins convention, for example, an always-attacking animal would gain against intruders but risk injury when challenging residents. The rule thus works not because every encounter has the same immediate outcome, but because an individual’s prospects depend on how the rest of the population behaves.

The reverse convention—intruders prevail and residents yield—could also be stable in the abstract model. But it would create pressure for individuals to avoid being residents by continually moving around. Such movement would consume time and energy and would undermine the very category on which the rule depended. The resident-wins convention instead favours staying in one place and appearing to defend it. The book connects this outcome to the familiar natural behaviour called territorial defence.

The stickleback experiment

The clearest demonstration described is an experiment by Niko Tinbergen. In a tank, two male sticklebacks had built nests at opposite ends and defended the areas around them. Tinbergen placed each fish in a large glass test tube and held the tubes beside one another, so the fish could attempt to fight without reaching each other.

The result turned on where the tubes were positioned. Near male A’s nest, A adopted an attacking posture and B attempted to retreat. When Tinbergen moved the tubes to male B’s territory, their roles reversed: B attacked and A retreated. Moving the same two fish from one end of the tank to the other changed which one behaved as resident. The experiment therefore supported the conditional rule—resident attacks, intruder retreats—rather than an explanation based on a fixed difference between the individuals.

This demonstration also recasts the question of territorial behaviour’s biological advantage. If the pattern can arise from an asymmetry in arrival and persist as an evolutionarily stable strategy, it may not require a further explanation in terms of some special benefit supplied by the territory. The proposed explanation concerns how a contest is resolved and how the resulting convention resists invasion by alternative behaviour.

Territory as a breeding licence

A different explanation appears in the discussion of Wynne-Edwards’s theory of population regulation. There, territorial contests are understood as competition for a token prize: possession of a territory can determine access to reproduction, even when the territory itself contains no food. A robin’s territory may be a large woodland foraging area used by a breeding pair; in herring gulls, it may be a small area centred on a nest. In some species, females will not mate with males that lack territories, and a female may attach herself to the victor after her mate loses both a contest and his territory.

In that account, territories limit breeding because their number is finite. If population growth leaves some individuals without territories, those individuals may not reproduce. Wynne-Edwards therefore treats possession as a licence to breed: animals may struggle over who gets the available licences, while the fixed supply of territories caps the population’s total production of young. Red grouse seem, at first sight, to fit the idea: individuals that fail to secure territories may also appear to give up competing and leave successful breeders undisturbed during the breeding season.

The breeding-licence account and the arrival-based account address different explanatory questions. One treats territory as a scarce resource that regulates reproduction at the population level; the other explains territorial defence as a stable way to settle individual contests. The book’s Tinbergen example demonstrates the latter mechanism directly, while presenting the population-regulation interpretation as Wynne-Edwards’s theory rather than as an established result.