Visualizing Sound Emission of Elephant Vocalizations: Evidence for Two Rumble Production Types

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'.

Complex vibratory patterns in an elephant larynx

“…Whether the adductory manoeuvres performed in this study exactly resemble those of in vivo vocalization would need to be established in future studies, which will be very challenging to perform as direct endoscopic evaluation of vocal fold vibration in a live animal is virtually impossible with the current technological means. However, several arguments speak in favour of a faithful duplication of natural vocalization conditions: (1) the laryngeal configuration was created in the only way possible to easily induce phonation in our excised larynx; (2) similar adductory gestures to the ones we used in the excised larynx experiments have been documented in humans; and (3) the acoustic output of the excised larynx was closely comparable to sounds captured from in vivo vocalizations, with fundamental frequencies that were well within the range of those reported for the 'rumble' call type (Poole et al, 1988;Langbauer, 2000;Herbst et al, 2012;Stoeger et al, 2012). The mammalian larynx is a non-linear system capable of exhibiting a wide range of vibratory behaviour, such as periodic vibration, subharmonics and deterministic chaos (Titze et al, 1993a;Herzel et al, 1995;Behrman and Baken, 1997;Fitch et al, 2002;Neubauer et al, 2004;Jiang et al, 2006).…”

The elephant brain in numbers

“…The exact mechanism for the production of infrasound in elephants is currently unknown, but is presumed to involve the larynx and trunk (Herbst et al, 2012; Stoeger et al, 2012). It is thus possible that the enlarged cerebellum is related to vocalization; however, we have found that, in contrast to the elephant, the relatively large cerebellum of echolocating microchiropterans is not an enlarged cerebellum, but rather the result of a diminished cerebral cortex, possibly related to miniaturization (Herculano-Houzel et al, in preparation).…”

Contact Calls of the Northern and Southern White Rhinoceros Allow for Individual and Species Identification

“…[1]–[3]). It has been hypothesised that rhinos might be using infrasound for long-distance communication which would be similar to the communication of elephants [4], [5].…”