Retraction of the mobile descended larynx during groaning enables fallow bucks (Dama dama) to lower their formant frequencies

Abstract

A permanently descended larynx is found in humans and several other species of mammals. In addition to this, the larynx of species such as fallow deer is mobile and in males it can be retracted during vocalization. The most likely explanation for the lowered retractable larynx in mammals is that it serves to exaggerate perceived body size (size exaggeration hypothesis) by decreasing the formant frequencies of calls. In this study, we quantified for the first time the elongation of the vocal tract in fallow bucks during vocalization. We also measured the effect of this vocal tract length (VTL) increase on formant frequencies (vocal tract resonances) and formant dispersion (spacing of formants). Our results show that fallow bucks increase their VTL on average by 52% during vocalization. This elongation resulted in strongly lowered formant frequencies and decreased formant dispersion. There were minimal changes to formants 1 and 2 (À0.91 and +1.9%, respectively) during vocal tract elongation, whereas formants 3, 4 and 5 decreased substantially: 18.9, 10.3 and 13.6%, respectively. Formant dispersion decreased by 12.4%. Formants are prominent in deer vocalizations and are used by males to gain information on the competitive abilities of signallers. It remains to be seen whether females also use the information that formants contain for assessing male quality before mating.

Evidence of biphonation and source–filter interactions in the bugles of male North American wapiti (Cervus canadensis)

“…Formants drop at the beginning of the bugle and reach a minimum plateau, characteristic of the vocal tract extension already identified in red deer stags and fallow deer bucks (McElligott et al, 2006), and consistent with the laryngeal movements observed in the videos of bugling male wapitis. Analysis of the formants from the recorded vocalisations indicates that the minimum formant spacing averages 222±7.8 Hz (mean± s.d.…”

Cues to body size in the formant spacing of male koala (Phascolarctos cinereus) bellows: honesty in an exaggerated trait

“…Indeed, the 'size exaggeration' hypothesis (Ohala, 1984;Fitch, 1997;Fitch, 2002;Fitch, 2010) has been proposed to justify the permanently descended larynges found in other mammals (Fitch and Reby, 2001;Weissengruber et al, 2002;Frey and Gebler, 2003;Frey et al, 2011) and the elongated trachea present in some bird species (Fitch, 1999). More generally, the observation that several male mammals actively lower formants in their sexual calls (Fitch and Reby, 2001;Frey and Gebler, 2003;McElligott et al, 2006;Frey et al, 2007;Sanvito et al, 2007;Frey et al, 2011) suggests that selection for adaptations to exaggerate (or maximise) the acoustic impression of body size in reproductive contexts could be widespread in non-human animals (see Fitch and Hauser, 2002). Whether this is predominantly for deterring rivals and/or attracting females remains a key question for future research.…”

“…Indeed, a dissociation between female skull and body size appears to render formants poor cues to size in female giant pandas (Charlton et al, 2009), and may provide the causal basis for similar findings in other female mammals (Rendall et al, 2005;Pfefferle and Fischer, 2006). Furthermore, the presence of anatomical adaptations that allow callers to elongate their vocal tracts and lower formants (by lengthening the supra-laryngeal or nasal vocal tract) (Fitch and Reby, 2001;Frey and Gebler, 2003;McElligott et al, 2006;Frey et al, 2007;Sanvito et al, 2007;Frey et al, 2011) could also make formants unreliable cues to body size, unless physiological constraints can act as a barrier to further formant lowering (Fitch and Reby, 2001;Reby and McComb, 2003). Consequently, it cannot be assumed a priori that formants will provide reliable information on body size in a given species and, hence, it remains important to determine whether they do and, more generally, to test the prediction that formants are reliable cues to body size in a wider range of taxa.…”

Encoding Emotions in Speech with the Size Code

“…This facial expression is made by protruding the lips, which effectively lengthens the vocal tract, and would generate the impression of a larger body size during vocalization [Ohala, 1984]. Similar to that for the visual size projection, the selection pressure for acoustic size projection is strong enough for many animals to develop permanently or mobilely lengthened vocal tracts [red deer Cervus elaphus: Fitch and Reby, 2001; fallow bucks Dama dama: McElligott et al, 2006; bird of paradise Phonygammus: Clench, 1978, and 60 bird species : Fitch, 1999]. Further evidence that such lengthening of the vocal tract is for the sake of exaggerating body size is the fact that it is often found only in male animals, which suggests its importance for mating competition [Fitch, 1994;Ohala, 1984].…”