Absence of a dear enemy effect and limited evidence of neighbour-stranger vocal discrimination in Ecstatic Display Songs of nesting African penguins

Abstract: African penguins (Spheniscus demersus), a critically endangered seabird, form dense breeding colonies where nesting individuals maintain territories advertised through a loud vocalisation, the Ecstatic Display Song (EDS). This song encodes information about the caller’s identity and body size, but its potential role in distinguishing familiar from unfamiliar individuals – a key aspect of territorial defence – remains unclear. We conducted playback experiments to investigate whether African penguins exhibit the “dear enemy effect”, where territory holders show reduced aggression towards established neighbours compared to strangers. Using a mixed experimental design (independent and repeated measures), on a relatively large sample (N = 49), we observed no differences in behavioural responses, but penguins looked significantly longer towards the speaker when broadcasting a stranger’s EDS. However, in a conservative repeated-measures analysis involving a reduced sample (N = 24), this effect was no longer observed. Together, our findings do not provide clear evidence for a dear enemy effect. Nevertheless, the increased duration of the first looking towards strangers’ EDS in the larger sample suggests a potential for neighbour-stranger vocal discrimination. To further explore the absence of a dear enemy effect in the African penguin, we suggest replicating these experiments with speakers placed closer to nests to simulate greater territorial threats. Our study provides new insights into territorial behaviour, vocal discrimination, and therefore further understand vocal communication in competitive contexts in this critically endangered species. Significance statement: In territorial species, distinguishing familiar from unfamiliar individuals reduces costly conflicts, often achieved through acoustic signals encoding reliable identity cues. Many territorial birds exhibit the “dear enemy effect,” displaying heightened aggression toward unfamiliar conspecifics, but this phenomenon remains unexplored in penguins. African penguins (Spheniscus demersus), a critically endangered seabirds breeding in dense colonies, maintain territories and produce complex vocalisations, including the Ecstatic Display Song. Through playback experiments, we found no differences in African penguins’ behavioural responses between neighbours’ and strangers’ calls, suggesting an absence of the dear enemy effect consistent with the colonial ecology of this species. This study is a starting point for elucidate vocal behaviour in competitive contexts in this critically endangered species, informing acoustic-based monitoring and conservation strategies to support their declining populations.