The adaptive significance of formant modulation in elephant vocalizations

Vocal tract resonances or formant frequencies (formants) are a very important means of transferring information in human speech. The perceptual relevance and functions of formants in nonhuman mammal vocal communication systems, however, remain less well understood. Formant frequency values are determined by the length and shape of the supra-laryngeal vocal tract, with longer vocal tracts producing lower, more closely spaced formants. Resent research indicates that formants may provide a universal cue to body size in vertebrates. Indeed, intriguing morphological adaptations to elongate the vocal tract in order to lower formants occur in several species with the size exaggeration hypothesis (lowered formants reflect selection pressures to sound larger) being proposed to justify most of these observations. The low-frequency rumbles of elephants may be under similar selective pressure. We (Stoeger et al. 2012*) recently documented two rumble production types based on vocal path differences: rumbles emitted orally and nasally. These rumbles vary considerably in their acoustic structure corresponding to the estimated lengths of the vocal paths. By using the nasal path, elephants potentially lower their formants by about three-fold (!) and they might do this systematically according to context or motivation. Yet, the adaptive significance of these observed formant modulation, and in particular, of the extremely low formant frequencies of nasal rumbles is still unknown: it is not known whether the very low formants of elephant rumbles reflect sexual or social selection pressures to sound larger, or (though not mutually exclusive) natural selection pressures to maximize call propagation distances. In the context of this project, we will investigate whether formants in African elephant low-frequency rumbles provide constant information on physical and motivational attributes of the caller. Since next to nothing is known about elephant bull signals and especially male vocalizations in vertebrates are often subject to sexual selection, there will be a strong focus on male elephant vocalizations and behaviour. We will then use re-synthesis techniques and playback experiments to determine the perceptual and functional relevance of specific formant characteristics of elephant rumbles; in particular, we aim to examine whether the size exaggeration hypothesis also applies to elephants, the largest terrestrial mammal. The project is proposed for an initial duration of three years and will be conducted by the Department for Cognitive Biology, University of Vienna. Filed work will be conducted in South Africa. One post-doc and one PhD student shall be employed.

Vocal tract resonances or formant frequencies (formants) are a very important means of transferring information in human speech. The perceptual relevance and functions of formants in nonhuman mammal vocal communication systems in general, and elephants in particular, however, remain less well understood. Formant frequency values are determined by the length and shape of the supralaryngeal vocal tract, with longer vocal tracts producing lower, more closely spaced formants. Elephants emit their well-known infrasonic rumbles either via the trunk (nasally) or the mouth (orally). By using the nasal path, elephants potentially lower their formants by about three- fold (!) and they might do this systematically according to context or motivation. The adaptive significance of these observed formant modulation, and in particular, of the extremely low formant frequencies of nasal rumbles was still unknown. Therefor the aim of our research project was to describe formant structure, define the information content they transmit and investigate whether the very low formants of elephant rumbles reflect sexual selection pressures to sound larger, or (though not mutually exclusive) natural selection pressures to maximize call propagation distances. Due to the fact that the vocalizations of male mammals are usually subject to sexual selection, there was a strong focus on male elephant vocal behaviour. Our results reveal that male rumbles encode information about individuality and maturity (age and size), with formant frequencies (the bigger the bull, the lower the formants) and absolute fundamental frequency values having the most informative power. Analysis further revealed that rumbles contain sufficient acoustic information to predict the sex of a vocalizing individual. Features primarily related to the vocalizer`s size, i.e. formant frequencies differed significantly between the sexes. While now some information was available on the acoustic structure and the information content of male elephant rumbles, still little was known about male vocal perception and discrimination abilities. Therefore we conducted playback experiments at the Addo Elephant National Park in South Africa. Results revealed that male elephants (1) extract social information from female vocalizations, potentially to assess mating opportunities, which may involve selection to identify individuals or kin in order to avoid inbreeding, and (2) that they use formant frequencies in rumbling vocalizations to assess the size and the maturity of other male individuals In summary, our various analysis and experiments revealed that formant frequencies are a highly relevant acoustic feature for male-male and male-female elephant communication. We further suggest that both, maximizing call propagation (due to the need to communicate over long distances) as well as sexual competition were crucial selective forces that have both shaped elephant vocal signals.