Conditional strategies in territorial defense: do Carolina wrens play tit-for-tat?

Neighboring territorial animals are viewed primarily as intense rivals, but there are also opportunities for cooperation among competitors. Many animals respond less aggressively toward neighbors than to strangers. This phenomenon, termed the ‘‘dear enemy’’ effect, should be stable only when the reduced aggression is reciprocal. Territory owners should use conditional strategies in territorial defense, showing reduced aggression toward neighbors who cooperate by respecting territorial boundaries but increasing aggression toward invading neighbors. In this study I examined the response of territory owners to playbacks of neighbors at shared boundaries before and after intrusions by that neighbor or by strangers. Results showed that territory owners did not increase their aggression toward defecting neighbors but did increase their aggression toward neighbors after a simulated intrusion by a stranger. This surprising result might reflect the long-term relationship between neighboring Carolina wrens and the threat posed by rare intruding strangers.