The role of neighbours in territorial systems: when are they 'dear enemies'?

Many territorial animals respond less aggressively to neighbours than to strangers (the dear enemy phenomenon). The dear enemy phenomenon generally has been assumed to represent neighbour-stranger interactions for all territorial systems. Here studies of territoriality are reviewed and the occurrence of the dear enemy phenomenon is shown to depend on the kind of territory studied: it tends to be present in studies of multi-purpose, breeding territories, but absent in studies of feeding territories. Moreover, whether a territory owner interacts less strongly with a neighbour than with a stranger in studies of colonial nesting birds is a function of nearest nest distance. These results support hypotheses that explain the dear enemy phenomenon based upon the relative threat presented by neighbours versus strangers, but not those based upon familiarity with neighbours versus strangers. Hence, the role of neighbours may vary depending upon territorial conditions, and an owner's potential gains and losses from interactions with neighbours and strangers.