|Description of Methods Used to Classify Elephant Vocalisations (Electronic Supplementary Material):
In our analyses of videos of elephant behaviour, we recorded instances of elephants vocalising in response to the sound playbacks. Elephant vocalisations were categorised as one of the following: trumpets, grunts, and growls. For each elephant vocalisation, we calculated the dominant frequency of the sound by determining the darkest region on the spectrogram where the amplitude was greatest (Berg, 1983), the peak frequency of the sound which was the upper limit of the sound recorded on the spectrogram (Berg, 1983), the delay in production of the vocalisation subsequent to hearing the predator sound playback and the duration of each elephant vocalisation.
Elephant trumpets have been characterised extensively in the literature. Across characterisations, trumpets have a mean dominant frequency of 518 Hz to 1,307 Hz and a mean duration of 0.9 to 1.16 seconds (Berg, 1983; Stoeger-Horwath et al., 2007; Nair et al., 2009). Berg (1983) also described an elephant “cry” vocalisation which has since been accepted as a trumpet, based on structural similarity (Soltis, 2010). The trumpets described in this paper fit the descriptions of what has been previously characterised. Trumpets were described to occur in the context of disturbance and aggression (Berg, 1983; Nair et al., 2009), excitement (Berg, 1983; Nair et al. 2009), and mating behaviour (Berg, 1983).
Elephant growls or rumbles have also been described extensively in the literature (Berg, 1983; Stoeger-Horwath et al., 2007; Nair et al., 2009; and Soltis, 2010). In this literature, growls have been characterised as low sounds with mean dominant frequencies ranging from 37.4 to 133 Hz and mean duration of 1.6 to 5.2 seconds. Growls were described to occur in the context of communicating with conspecifics in either greeting or alerting situations (Berg 1983; Nair et al., 2009) or during intraspecific aggression (Berg, 1983).
Elephant grunts have been described in infant elephants by Stoeger-Horwath et al. (2007), who described grunts with a mean duration of 0.5 seconds and a mean dominant frequency of around 405.9 Hz. While grunts have not been described by others, they may be the same as what has been classified as “barks” by Berg, 1983 and “roars” by Nair et al. (2009). Berg (1983) described barks to have a mean dominant frequency of 629 Hz and duration range of 0.25 to 0.75 seconds. Nair et al. (2009) described roars as vocalisations with a mean dominant frequency of 656.5 Hz and mean duration of 2.0 seconds. Berg (1983) described barks to occur in the context of excitement and Nair et al. (2009) described roars to occur in the context of excitement, disturbance and aggression.
Berg, J.K. 1983. Vocalizations and associated behaviors of the African elephant (Loxodonta africana) in captivity. Z. Tierpsychol 63, 63-79. (DOI 10.1111/j.1439-0310.1983.tb00741.x)
Nair, S., Balakrishnan, R., Seelamantula, C.S., & Sukumar, R. 2009. Vocalizations of wild Asian elephants (Elephas maximus): structural classification and social context. J Acoust Soc Am 126, 2768-2778.
Soltis, J. 2010. Vocal communication in African elephants (Loxodonta africana). Zoo Biol. 29, 192-209.
Stoeger-Horwath, A.S., Stoeger, S., Schwammer, H.M., & Kratochvil, H. 2007. Call repertoire of infant African elephants: first insights into the early vocal ontogeny. J Acoust Soc Am 121, 3922-3931