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.

Identity signaling, identity reception and the evolution of social recognition in a Neotropical frog

Animals recognize familiar individuals to perform a variety of important social behaviors. Social recognition is often mediated by communication between signalers who produce signals that contain identity information and receivers who categorize these signals based on previous experience. We tested two hypotheses about adaptations in signalers and receivers that enable the evolution of social recognition using two species of closely related territorial poison frogs. Male golden rocket frogs (Anomaloglossus beebei) recognize the advertisement calls of conspecific territory neighbors and display a "dear enemy effect" by responding less aggressively to neighbors than strangers, while male Kai rocket frogs (A. kaiei) do not. Our results did not support the identity signaling hypothesis: both species produced advertisement calls that contain similar amounts of identity information. Our results did support the identity reception hypothesis: both species exhibited habituation of aggression to playbacks simulating the arrival of a new neighbor, but only golden rocket frogs showed renewed aggression when they subsequently heard calls from a different male. These results suggest that an ancestral mechanism of plasticity in aggression common among frogs has been modified through natural selection to be specific to calls of individual males in golden rocket frogs, enabling a social recognition system.

Do male Boana goiana (Lutz, 1968) (Anura: Hylidae) discriminate between the calls of neighbours and strangers?

Neighbour-stranger discrimination is an important mechanism in social behaviour, as it allows the individual to avoid the high energetic costs of repeated agonistic interactions with neighbours. In some anuran species, the owners of territories respond more aggressively towards strangers than neighbours. Here, we investigated whether gladiator frog, Boana goiana, males discriminate between neighbours and strangers. This is a lek-breeding species; therefore, it does not defend fixed territories which are associated with a specific resource. We conducted a playback field experiment with a repeated-measures design, based on three treatments: (i) neighbour’s call in initial position, (ii) stranger’s call in initial position, and (iii) neighbour’s call in new position. We did not observe any systematic modification in the acoustic properties (call duration, dominant frequency, repetition rate) of the advertisement and short aggressive calls of B. goiana males given in response to our playback treatments. Our results indicate that B. goiana males do not exhibit vocally mediated ‘dear enemy’ relationships. We discuss the possible ecological and social factors that may have impeded the evolution of ‘dear enemy’ relationships in this species.

Boosting nationalism through COVID-19 images: Multimodal construction of the failure of the ‘dear enemy’ with COVID-19 in the national press

Using a multimodal discursive approach, this study explores how the COVID-19 pandemic is constructed and used in press reportage to mobilize intergroup relations and national identities. We examine how press reporting about the development of COVID-19 in Sweden is cast as a matter of nationalism and national stereotyping in the Finnish press. The data consist of 183 images with accompanying headlines and captions published in two Finnish national newspapers between January 1 and August 31, 2020. We found three multimodal rhetorical strategies of stereotyping: moralizing, demonizing, and nationalizing. These strategies construct discourses of arrogant, immoral, and dangerous Swedes sourcing from national stereotypes. The study contributes to current knowledge about the work on national stereotypes by illustrating how they are used in media discourse to achieve certain rhetorical ends, such as to persuade, mitigate, or justify intergroup relations. Furthermore, the study offers insight into the multimodal constructions and functions of stereotypes.

The dear enemy effect drives conspecific aggressiveness in an Azteca-Cecropia system

Territoriality is costly, and the accurate identification of intruders and the decision to perform aggressive responses are key behavioral traits in social animals. We studied aggression among individuals belonging to close and distant nests of the plant-ant Azteca muelleri, which lives in stems of the pioneer tree Cecropia glaziovii. More specifically, we aim to investigate if the DE (dear-enemy effect—less aggression towards neighbors than strangers) or NN (nasty-neighbor effect—less aggression to strangers than neighbors) effects or even none of them apply for this iconic Azteca-Cecropia system. We further checked if ant aggression towards conspecifics is related to cuticular hydrocarbon profiles (CHCs), which provide chemical cues for nestmate recognition. Therefore, we sampled 46 nests of A. muelleri in three Brazilian Atlantic forest fragments and performed behavioral trials within and between sites. Consistently with the DE effect, we found higher aggression levels in ‘between sites’ versus ‘within sites’ treatments as well as a positive effect of spatial distance on ant aggressiveness. We found no effect of the overall dissimilarities on CHC blend on ant aggressiveness, but of one CHC class, the methylated alkanes. Overall, we provide key insights on nest-mate recognition in obligatory ant-plant mutualisms.

Behavioral responses to numerical differences when two invasive ants meet: the case of Lasius neglectus and Linepithema humile

Two of the world’s most invasive ants, Linepithema humile and Lasius neglectus, are destined to overlap in range as they continue to spread throughout Europe. Although L. humile arrived first, and is therefore more numerically abundant, L. neglectus is the more behaviorally dominant of the two. We performed lab trials to determine whether L. humile could use numerical abundance to overcome the behavioral dominance of L. neglectus and whether the ants’ behavioral patterns shifted when the species co-occurred. We found that L. neglectus was more aggressive when less abundant, whereas the opposite was true of L. humile. When L. neglectus was outnumbered, it employed aggressive behaviors, such as biting or chemical attacks, more frequently than L. humile; it also utilized a behavioral sequence that included mandible opening and biting. Our results for these species support the hypothesis that species modulate their behavior towards competitors, which facilitates the understanding of how multiple invasive ant species can co-occur in a given area. Moreover, our study shows that the co-occurrence of invasive species could result from the use of two strategies: (1) the Bourgeois strategy, in which aggressiveness changes based on numerical dominance and (2) the dear-enemy strategy, in which aggressiveness is reduced when competitors co-occur. Since these strategies may lead to territory partitioning, we suggest that the behavioral flexibility displayed by both species when they overlap may allow local co-occurrence and increase their likelihood of co-occurrence during their range expansion in Europe, which could have a negative cumulative impact on invaded areas.