Who＇s your neighbor？ Acoustic cues to individual identity in red squirrel Tamiasciurus hudsonicus rattle calls

North American red squirrels Tamiasciurus hudsonicus often produce a loud territorial rattle call when conspecifics enter or invade a territory. Previous playback experiments suggest that the territorial rattle call may indicate an invader＇s identity as squirrels responded more intensely to calls played from strangers than to calls played from neighbors. This dear-enemy effect is well known in a variety of bird and mammal species and functions to reduce aggressive interactions between known neighbors. However, although previous experiments on red squirrels suggest some form of individual differentiation and thus recognition, detailed acoustic analysis of potential acoustic cues in rattle calls have not been conducted. If calls function to aid in conspecific identifi- cation in order to mitigate aggressive territorial interactions, we would expect that individual recognition cues would be acoustically represented. Our work provides a detailed analysis of acoustic cues to identity within rattle calls. A total of 225 calls across 32 in- dividual squirrels from Sheep River Provincial Park, Kananaskis, AB, Canada, were analyzed with discriminant function analysis for potential acoustic cues to individual identity. Initial analysis of all individuals revealed a reliable acoustic differentiation across individuals. A more detailed analysis of clusters of neighboring squirrels was performed and results again indicated a statistically significant likelihood that calls were assigned correctly to specific squirrels （55%-75% correctly assigned）; in other words squirrels have distinct voices that should allow for individual identification and discrimination by conspecifics [Current Zoology 58 （5）： 758-764, 2012].

Evidence for acoustic discrimination in lemurs: A playback study on wild indris Indri indri

Indris Indri indri are group-living lemurs that occupy stable territories over several years and perform remarkable long-distance vocal displays.Vocal exchanges between long-term territory neighbors may contribute to assessing reciprocal resource-holding potentials, thus adaptivelyreducing the costs of territorial defense by limiting aggressive escalation. Previous work showed that indris’ songs show distinctive acousticfeatures at individual and group level. However, the possibility that indris use such cues for individual or group-level recognition has never beeninvestigated experimentally. We conducted a playback experiment to test whether indris discriminate between familiar and nonfamiliar songs.Our rationale lies in the hypothesis of the dear enemy phenomenon, which predicts that territorial animals will show reduced aggression levelstoward familiar neighbors compared with novel rivals. We played back stimulus recordings to wild indris from their territory boundaries andexamined their responses in terms of vocal and behavioral 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 nonfamiliar individuals than to stimuliof neighboring groups. These results indicate that indris can discriminate between different classes of intruders based on distinctive acousticfeatures of their song choruses. We suggest that increased aggression directed toward unfamiliar intruders may be explained by higher threatlevels associated with dispersal and group formation dynamics. We further discuss the relevance of these findings in a strepsirrhine primatemodel for comparative studies of vocal communication and sociality.