Vocal Tract Morphology Determines Species-Specific Features in Vocal Signals of Lemurs (Eulemur)

Abstract

The source-filter theory describes vocal production as a two-stage process involving the generation of a sound source, with its own spectral structure, which is then filtered by the resonant properties of the vocal tract. This theory has been successfully applied to the study of animal vocal signals since the 1990s. As an extension, models reproducing vocal tract resonance can be used to reproduce formant patterns and to understand the role of vocal tract filtering in nonhuman vocalizations. We studied three congeneric lemur species -Eulemur fulvus, E. macaco, E. rubriventer-using morphological measurements to build computational models of the vocal tract to estimate formants, and acoustic analysis to measure formants from natural calls. We focused on call types emitted through the nose, without apparent articulation. On the basis of anatomical measurements, we modeled the vocal tract of each species as a series of concatenated tubes, with a cross-sectional area that changed along the tract to approximate the morphology of the larynx, the nasopharyngeal cavity, the nasal chambers, and the nostrils. For each species, we calculated the resonance frequencies in 2500 randomly generated vocal tracts, in which we simulated intraspecific length and size variation. Formant location and spacing showed significant species-specific differences determined by the length of the vocal tract. We then measured formants of a set of nasal vocalizations ("grunts") recorded from captive lemurs of the same species. We found species-specific differences in the natural calls. This is the first evidence that morphology of the vocal tract is relevant in generating filter-related acoustic cues that potentially provide receivers with information about the species of the emitter.

OVERLAPPING AND SYNCHRONIZATION IN THE SONG OF THE INDRIS (Indri indri)

“…Merging textgrids of different indris we estimated the overlap between individuals, which was expressed in percentage of song duration. Durations of the overlapping and non-overlapping parts of each song and timing of the centre points of each song unit were saved within Praat and exported to a Microsoft© Excel spreadsheet (Gamba & Giacoma 2007;Gamba et al 2012). …”

Evidence of a Vocalic Proto-System in the Baboon (Papio papio) Suggests Pre-Hominin Speech Precursors

“…Previous studies have already shown certain nonhuman primate vocalizations are vowel-like through acoustic analyses revealing formants [16–22], and also that nonhuman primates can discriminate sounds differing by their formant structures [23]. A pair of studies [24,20] even reported the production of two distinct vowel-like sound in the leopard and eagle calls of Diana monkeys.…”

The vocal communication of the mongoose lemur (Eulemur mongoz): phonation mechanisms, acoustic features and quantitative analysis

“…Previous studies of primate acoustic signals have shown that both vocal fold vibration and supra-laryngeal filtering shape auditory communication in primates (FITCH 1997;RENDALL et al 2005;GAMBA & GIACOMA 2006;GAMBA et al 2012a) and other mammals (REBY & MCCOMB 2003). For each call, regardless of the vocal category to which it was assigned, we measured 11 acoustic properties using Praat (BOERSMA & WEENINK 2006, Institute of Phonetic Sciences, University of Amsterdam).…”

“…In this manner, vocal communication may regulate social interactions by informing receivers about body size and identity, as well as by indicating the ability to control territory and resources (OWINGS & MORTON 1998). Thus, the high number of grunts exchanged among the mongoose lemurs can provide conspecifics with information about emitter identity (GAMBA et al 2012a). Viewing mongoose lemur vocal communication from the perspective of motivation-structural principles (MORTON 1977;OWINGS & MORTON 1998), we conclude that the wideband (usually low-pitched) sounds serve for alert, territorial advertisement, anti-predator alarm and aggression (e.g.…”

“…Diurnal and cathemeral lemurs have vocal repertoires of comparable size (15.40 ± 7.24, MCCOMB & SEMPLE 2005) to those of other primates (16.64 ± 8.59, MCCOMB & SEMPLE 2005), but for most species we can only rely on qualitative descriptions (PAILLETTE & PETTER 1978;PETTER & CHARLES-DOMINIQUE 1979;PEREIRA et al 1988;MACEDONIA 1993;CURTIS & ZARAMODY 1999;GOSSET et al 2001). The quantitative analysis of vocalisations has shown that variability in lemur calls may provide information on sex , individual identity (ODA 2002;LELIVELD et al 2011;GAMBA et al 2012a) and the emotional state of the emitter (ZIMMERMANN 2009). The ability to recognise age, sex and individuals can be an important tool in management and conservation activities (TERRY et al 2005).…”