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’.
Since two authors (Gunnar Heilman and Sean Hensman) are employed commercial companies the authors want to declare that they did
not pay for using the equipment nor for doing research with the elephant at Bela Bela. Therefore, both companies had no financial interest concerning the
research. The fact that these authors are employed by these companies does not alter the authors’ adherence to all the PLOS ONE policies on sharing data and
Movie S1 Nasal rumble-25 fps-sound: Sound visualization
of a nasal rumble. This movie shows the sound
emission during a nasal rumble.
Movie S2 Nasal rumble-5 fps-slow-mo: Sound visualization
of a nasal rumble in slow motion. This movie shows
the sound emission during a nasal rumble in slow motion (5 frames
Movie S3 Oral rumble-25 fps-sound: Sound visualization
of an oral rumble. This movie shows the sound emission
during an oral rumble.
Movie S4 Oral rumble-5 fps-slow-mo: Sound visualization
of an oral rumble in slow motion. This movie shows the
sound emission during an oral rumble in slow motion (5 frames per
Figure S1 Spectrograms and power spectra presenting
two examples of rumbling vocalizations from a 29 year
old female African elephant (Drumbo) recorded at the
Vienna Zoo in 2003. Recordings were captured with a
condenser microphone AKG 480 B CK 62 and a DA-P1 DAT
recorder. Figures A and B show a rumble recorded during spatial
separation from a part of the group, and display the formant
structure of a typical nasal rumble. Figures C and D show a
rumble recorded during a bonding situation when the group was
reunited, and resemble an orally emitted rumble based on the
observed formant values.
Figure S2 Spectrograms and power spectra to show
examples of rumbles from a 43 year old female African
elephant (Jumbo) recorded at the Vienna Zoo in 2003
(using the same equipment as described in Figure S1).
Figures A and B also show a rumble recorded during spatial
separation from the group, again with the formant structure of a
typical nasal rumble. Figures C and D show a rumble recorded
during the bonding situation when the group was reunited, again
resembling an orally emitted rumble based on the formant values.
Jumbo died in 2004 and her oral vocal tract was measuring at
93 cm (Weissengruber, personal communication). The formants 1
and 2 of the oral vocal tract would thus be (using equation 3)
92 Hz and 277 Hz, which corresponds very well with the formant
location observed in Figures C and D.
Conceived and designed the experiments: ASS GH. Performed the
experiments: ASS GH SH AG. Analyzed the data: ASS MZ. Contributed
reagents/materials/analysis tools: ASS MZ. Wrote the paper: ASS BDC.