Quantitative Description of Indri’s Vocal Repertoire

Acoustic repertoire is characterized by sex, age and context specificity. We quantitatively investigated the vocal repertoire of ten groups of wild indris inhabiting the Andasibe-Mantadia National Park and the Mitsinjo Station Forestiére in Madagascar to increase knowledge of this specificity. We considered a sample of 1,670 vocalizations belonging to 28 individuals recorded in the field from September to December between 2004 and 2008. We distinguished 8 vocal types other than the song: roar, honk, hum, long tonal call, short tonal call, kiss, wheeze and grunt. Two of them, long tonal call and short tonal call, had never been described before. Discriminant function analyses supported the qualitative classification of vocalization groups, which correctly classified each vocal unit to its own type with a percentage of 96.4% and 96.0% for the cross-validated function. We found that indris have discrete call types in their vocal repertoire, distinguishable by ear and from analysis of the spectrograms. Some utterances were used only in particular behavioural contexts (e.g. roars and honks in alarm contexts, or long and short tonal calls in physical fights), and by individuals of specific age, whereas others were emitted under a range of situations (e.g. hums). The frequency span of all calls, except alarm calls and the song, overlapped the most prominent peaks of ambient noise, suggesting they cannot be used for long distance communication. Alarm calls showed relatively wide ranges of prominent frequency bands, and include spectral areas in which the ambient noise level is lowest. The song showed a large frequency span of prominent bands, including high-frequency regions with low amplitudes in the noise spectra. This suggests that the design of the indri song is optimised to avoid masking by ambient noise.