ABSTRACTTerritorial animals often exhibit the dear enemy effect, in which individuals respond less aggressively to neighbours than to other individuals. The dear enemy effect is hypothesized to be adaptive by reducing unnecessary aggressive interactions with individuals that are not a threat to territory ownership. A key prediction of this hypothesis, that individual fitness will be affected by variation in the speed and extent to which individuals reduce their aggression towards neighbours relative to strangers, has never been tested. We used a series of song playbacks to measure the change in response of male great tits on their breeding territories to a simulated establishment of a neighbour on an adjacent territory. Males reduced their approach to the speaker and sang fewer songs on later repetitions of the playback trials, consistent with a dear enemy effect through habituation learning. However, not all males discriminated between the neighbour and stranger playbacks at the end of the series of trials, and there was evidence that individuals consistently differed from one another in performing this discrimination. We monitored nests and analysed offspring paternity to determine male reproductive success. Unexpectedly, individuals that exhibited dear enemy behaviour towards the simulated neighbour did not have higher reproductive success, and in fact one measure, total offspring biomass, was lower for individuals that showed the dear enemy effect. Although the general capability to recognize neighbours is most likely adaptive, it seems that individuals who decrease their responsiveness to familiar neighbours too quickly may gain no advantage or even be at a disadvantage.
No reproductive benefits of dear enemy recognition in a territorial songbird
