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A. Stöger, G. Heilmann, M. Zeppelzauer, A. Ganswindt, S. Hensman, B. Charlton:
"Visualizing Sound Emission of Elephant Vocalizations: Evidence for Two Rumble Production Types";
PLoS ONE, 7(11):e48907 (2012), 8 pages.

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.

http://dx.doi.org/10.1371/journal.pone.0048907

http://publik.tuwien.ac.at/files/PubDat_211149.pdf