Evidence for acoustic discrimination in lemurs: a playback study on wild indris (Indri indri)

Indris (Indri indri) are group-living lemurs which occupy stable territories over several years and perform remarkable long-distance vocal displays. Vocal exchanges between long-term territory neighbours may contribute to assessing reciprocal resource holding potentials, thus adaptively reducing the costs of territorial defence by limiting aggressive escalation. Previous work showed that indris' songs show distinctive acoustic features at individual and group level. However, the possibility that indris use such cues for individual or group-level recognition has never been investigated experimentally. We conducted a playback experiment to test whether indris discriminate between familiar and non-familiar songs. Our rationale lies in the hypothesis of the dear enemy phenomenon, which predicts that territorial animals will show reduced aggression levels towards familiar neighbours compared to novel rivals. We played back stimulus recordings to wild indris from their territory boundaries and examined their responses in terms of vocal and behavioural indicators of willingness to engage in a fight. In line with our predictions, focal animals responded more rapidly and approached more often the speaker in response to playback stimuli of non-familiar individuals than to stimuli of neighbouring groups. These results indicate that indris can discriminate between different classes of intruders based on distinctive acoustic features of their song choruses. We suggest that increased aggression directed towards unfamiliar intruders may be explained by higher threat levels associated with dispersal and group formation dynamics. We further discuss the relevance of these findings in a strepsirrhine primate model for comparative studies of vocal communication and sociality.