Dear enemy effect

The dear enemy effect or dear enemy recognition is an <a href="/facts/Ethological/dM3CpysA">ethological</a> 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. It can be modulated by factors such as the location of the familiar and unfamiliar animal, the season, and the presence of females.
The effect is the converse of the <a href="/facts/Nasty_neighbour_effect/yfV6tzEw">nasty neighbour effect</a>, in which some species are more aggressive towards their neighbours than towards unfamiliar strangers.

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Dear enemy effect