No reproductive benefits of dear enemy recognition in a territorial songbird

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.

Loss of graded enemy recognition in a Whitehead population allopatric with brood parasitic Long-tailed Cuckoos

ABSTRACTMany avian hosts of brood parasitic birds discriminate between different types of threats and may respond with categorically different, specifically anti-predatory or anti-parasitic behaviors. Alternatively, hosts may adjust their responses to threat level in a graded manner, responding more aggressively to brood parasites during the laying and incubation stages of nesting, when nests are most susceptible to parasitism, and more aggressively to nest predators during the nestling and fledgling stages when predation would be more costly than parasitism. In New Zealand, endemic host Whiteheads act inconspicuously around their nests in the presence of sympatric Long-tailed Cuckoos, their obligate brood parasite, perhaps to avoid disclosing nest location. We tested behavioral responses of a Whitehead population on Tiritiri Matangi Island that has been breeding allopatrically from cuckoos for 17 years. We presented models of the allopatric parasite, a sympatric predator (Morepork Owl), and a sympatric non-threatening heterospecific (Song Thrush) during the egg and chick stages, and to groups of cooperatively breeding Whiteheads. We compared responses across nest stage and stimulus type. We found that, unlike sympatric Whiteheads elsewhere in New Zealand, Whiteheads on Tiritiri Matangi produced alarm calls in response to the cuckoo model. Furthermore, the rate of alarm calling was similar towards the cuckoo and the owl and across egg and chick stage and higher than to the control stimulus. These results are consistent with allopatric Whiteheads having lost their specific anti-parasitic defense tactics in response to brood parasitic cuckoos.

Enemy recognition is linked to soldier size in a polymorphic stingless bee

Many ant and termite colonies are defended by soldiers with powerful mandibles or chemical weaponry. Recently, it was reported that several stingless bee species also have soldiers for colony defence. These soldiers are larger than foragers, but otherwise lack obvious morphological adaptations for defence. Thus, how these soldiers improve colony fitness is not well understood. Robbing is common in stingless bees and we hypothesized that increased body size improves the ability to recognize intruders based on chemosensory cues. We studied the Neotropical species Tetragonisca angustula and found that large soldiers were better than small soldiers at recognizing potential intruders. Larger soldiers also had more olfactory pore plates on their antennae, which is likely to increase their chemosensory sensitivity. Our results suggest that improved enemy recognition might select for increased guard size in stingless bees.