Dear enemy recognition

The dear enemy effect or dear enemy recognition is an ethological phenomenon in which two neighbouring territorial animals become less aggressive toward one another once territorial borders are well-established. As territory owners become accustomed to their neighbours, they expend less time and energy on defensive behaviors directed toward one another. However, aggression toward unfamiliar neighbours remains the same. Some authors have suggested the dear enemy effect is territory residents displaying lower levels of aggression toward familiar neighbours compared to unfamiliar individuals who are non-territorial "floaters".

The dear enemy effect has been observed in a wide range of animals including mammals, birds, reptiles, amphibians, fish and invertebrates.

The behavioural effect can be modulated by factors such as the location of the familiar and unfamiliar animal, the season, and the presence of females.

The dear enemy effect is the converse of the nasty neighbour effect in which some species are more aggressive towards their neighbours than towards unfamiliar strangers.

The ultimate function of the dear enemy effect is to increase the individual fitness of the animal expressing the behaviour. This increase in fitness is achieved by reducing the time, energy or risk of injury unnecessarily incurred by defending a territory or its resources (e.g. mate, food, space) against a familiar animal with its own territory; the territory-holder already knows about the abilities of the neighbour, and also knows that the neighbour is unlikely to try to take over the territory because it already has one.

The interaction between two neighbours can be modelled as an iterated prisoner's dilemma game. In this view, a territory owner that acts non-aggressively towards a neighbour can be thought of as cooperating, while a territory owner that acts aggressively towards its neighbour can be considered to have defected. A necessary condition for the prisoner’s dilemma game to hold is that an aggressive individual should enjoy greater benefits than a non-aggressive individual when each is faced with a non-aggressive opponent. This stipulation is plausible, as an aggressive individual might enlarge their territory or steal food or matings from a non-aggressive individual. When cooperation involves a cost, a possible mechanism for achieving stable co-operation is reciprocal altruism, where pairs of individuals trade bouts of cooperative behaviour with one another. Dear enemy cooperation could be explained by reciprocal altruism if territorial neighbours use conditional strategies such as tit-for-tat. In the tit-for-tat strategy, a subject will cooperate when its partner (neighbour) cooperates and defect when the partner defects.

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