The graded vocal repertoire of the Nosy Be sportive lemur (Lepilemur tymerlachsoni)

Descriptions of the vocal repertoires of primates are essential to understanding aspects of behaviour and social ecology, and are increasingly useful in applied conservation. Sportive lemurs (Lepilemur spp.) are highly vocal but otherwise cryptic, making them promising candidates for bioacoustics research and monitoring; however, vocal repertoires have been described for only a handful of the 25 species, and the question of gradation within and between call types remains largely unexplored. Here we describe the call types and gradedness of the vocal repertoire of the Critically Endangered Nosy Be sportive lemur (Lepilemur tymerlachsoni). We recorded wild sportive lemur vocalisations over a six-week period in early 2023. From the spectrograms of these recordings, we manually classified 14 distinct call types, distinguishable both aurally and visually, representing the largest known repertoire for the genus and among the largest reported for any nocturnal primate. Depending on the call type, calls were produced singly, in sequences and combinations, or in extended bouts. We then used an unsupervised machine learning technique, fuzzy c-means clustering, to objectively classify the repertoire and quantify its graded structure using 25 acoustic measurements extracted from the spectrograms. Fuzzy clustering identified two acoustic clusters, revealing gradation both within call types and between clusters. These patterns were visualised with Uniform Manifold Approximation and Projection ('UMAP') dimensionality reduction. We present representative spectrograms for each call type, and compare our results with the published repertoires of other sportive lemur species. Our study provides a foundation for further behavioural research and acoustic-based conservation of L. tymerlachsoni.