Abstract
Recent comparative data reveal that formant frequencies are cues to body size in animals, due to a close relationship between formant frequency spacing, vocal tract length and overall body size. Accordingly, intriguing morphological adaptations to elongate the vocal tract in order to lower formants occur in several species, with the size exaggeration hypothesis being proposed to justify most of these observations. While the elephant trunk is strongly implicated to account for the low formants of elephant rumbles, it is unknown whether elephants emit these vocalizations exclusively through the trunk, or whether the mouth is also involved in rumble production. In this study we used a sound visualization method (an acoustic camera) to record rumbles of five captive African elephants during spatial separation and subsequent bonding situations. Our results showed that the female elephants in our analysis produced two distinct types of rumble vocalizations based on vocal path differences: a nasally- and an orally-emitted rumble. Interestingly, nasal rumbles predominated during contact calling, whereas oral rumbles were mainly produced in bonding situations. In addition, nasal and oral rumbles varied considerably in their acoustic structure. In particular, the values of the first two formants reflected the estimated lengths of the vocal paths, corresponding to a vocal tract length of around 2 meters for nasal, and around 0.7 meters for oral rumbles. These results suggest that African elephants may be switching vocal paths to actively vary vocal tract length (with considerable variation in formants) according to context, and call for further research investigating the function of formant modulation in elephant vocalizations. Furthermore, by confirming the use of the elephant trunk in long distance rumble production, our findings provide an explanation for the extremely low formants in these calls, and may also indicate that formant lowering functions to increase call propagation distances in this species'.
Highlights
Individual and species-specific mechanisms of sound production determine the vocal characteristics accessible to receivers, and to natural and sexual selection
In order to investigate the dependency of this result on the employed classification technique, we evaluated the classification accuracy of two further classifiers (Support Vector Machine and Nearest Neighbour Classifier)
Using an acoustic camera array to visualize sound emission, we have demonstrated two types of rumbles in three sub-adult female African elephants: a nasally and an orally emitted rumble
Summary
Individual and species-specific mechanisms of sound production determine the vocal characteristics accessible to receivers, and to natural and sexual selection. This evolutionary interconnection of voice production, acoustic output and function makes it necessary to understand basic sound production mechanisms when studying animal communication [1]. Formants are reliable cues to body size in several mammals [3,4,5,6,7,8,9] due to a close relationship between the frequency spacing of the formants, the caller’s vocal tract length and overall body size. This, together with demonstrations of formant perception by nonhuman mammals in general [10,11,12,13,14] and interspecific perception [15,16] in particular, suggests that formants may provide a universal cue to body size in vertebrates [17]
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