Dear enemy effect in the Mexican volcano mouse Neotomodon alstoni: Implications of sex in the agonistic behaviour among neighbours

Modulation of dear enemy effects by male dusky warblers (Phylloscopus fuscatus) at different reproductive stages

Territorial animals show less aggression during intrusion by neighbours than by strangers, a phenomenon known as the "dear enemy effect". Recent studies have shown that the dear enemy effect is variable in some birds throughout the breeding season. However, there is limited research on changes in dear enemy effects in songbirds during the breeding season. In this study, we monitored dusky warblers (Phylloscopus fuscatus) throughout their breeding stages, conducting playback experiments simulating incursions by neighbours and strangers during their egg-laying and incubation/nestling periods, and recording physical responses and song responses from territory owners. The results showed that male dusky warblers responded similarly to neighbours and strangers during the female egg-laying period, but there was a significantly stronger response to strangers than to neighbours during the incubation/nestling period. This suggests that male dusky warblers adjust the intensity of their defences against neighbours according to the reproductive status of their own mate. This may be because neighbouring males threaten the paternity of territorial males during the egg-laying period, so territorial males demote the 'dear enemy' friendly relationships with their neighbours to defend their mate and paternity. In contrast, during the non-reproductive period of females, when neighbours are less of a threat to the paternity of territorial males, lowering defences against neighbouring males allow them to devote more time and energy to activities that improve their fitness, such as foraging and seeking extra-pair mating.

Social environment shapes female settlement decisions in a solitary carnivore

How and where a female selects an area to settle and breed is of central importance in dispersal and population ecology as it governs range expansion and gene flow. Social structure and organization have been shown to influence settlement decisions, but its importance in the settlement of large, solitary mammals is largely unknown. We investigate how the identity of overlapping conspecifics on the landscape, acquired during the maternal care period, influences the selection of settlement home ranges in a non-territorial, solitary mammal using location data of 56 female brown bears (Ursus arctos). We used a resource selection function to determine whether females' settlement behavior was influenced by the presence of their mother, related females, familiar females, and female population density. Hunting may remove mothers and result in socio-spatial changes before settlement. We compared overlap between settling females and their mother's concurrent or most recent home ranges to examine the settling female's response to the absence or presence of her mother on the landscape. We found that females selected settlement home ranges that overlapped their mother's home range, familiar females, that is, those they had previously overlapped with, and areas with higher density than their natal ranges. However, they did not select areas overlapping related females. We also found that when mothers were removed from the landscape, female offspring selected settlement home ranges with greater overlap of their mother's range, compared with mothers who were alive. Our results suggest that females are acquiring and using information about their social environment when making settlement decisions.

Stranger danger: A meta-analysis of the dear enemy hypothesis

The dear enemy hypothesis predicts that territorial individuals will be less aggressive toward known neighbors than to strangers. This hypothesis has been well studied and there is a wealth of data demonstrating its prevalence in some taxa. However, a quantitative synthesis is needed to test the generality of the phenomenon, identify key mechanisms driving the behavior, and guide future research. In this study, we conduct a meta-analysis and, we test the importance of the location of intrusion, the type of experiment conducted (field, laboratory, or neutral arenas), and the sex and breeding status of territory holders, on the occurrence of dear enemy behavior. We also test how various ecological and life history traits, such as territory type, stimulus of intrusion, and taxonomic group, affect the magnitude of dear enemy behavior. We find that this phenomenon is common and that taxonomic class and breeding status are correlated with the expression of dear enemy behaviors. Further, we found that the way authors measure aggression influences the likelihood of identifying dear enemy responses, and thus we discuss potential pitfalls of dear enemy studies. Considering this conclusion, we discuss future lines of inquiry that could more directly examine the mechanisms of the dear enemy phenomenon.