Information content and acoustic structure of male African elephant social rumbles

Abstract

Until recently, the prevailing theory about male African elephants (Loxodonta africana) was that, once adult and sexually mature, males are solitary and targeted only at finding estrous females. While this is true during the state of ‘musth’ (a condition characterized by aggressive behavior and elevated androgen levels), ‘non-musth’ males exhibit a social system seemingly based on companionship, dominance and established hierarchies. Research on elephant vocal communication has so far focused on females, and very little is known about the acoustic structure and the information content of male vocalizations. Using the source and filter theory approach, we analyzed social rumbles of 10 male African elephants. Our results reveal that male rumbles encode information about individuality and maturity (age and size), with formant frequencies and absolute fundamental frequency values having the most informative power. This first comprehensive study on male elephant vocalizations gives important indications on their potential functional relevance for male-male and male-female communication. Our results suggest that, similar to the highly social females, future research on male elephant vocal behavior will reveal a complex communication system in which social knowledge, companionship, hierarchy, reproductive competition and the need to communicate over long distances play key roles.

Social associations and vocal communication in wild and captive male savannah elephants Loxodonta africana

“…The formant frequencies and the absolute fundamental frequency were the most informative of the acoustic parameters analysed. Stoeger and Baotic () also found that larger and older males (shoulder height above 3 m and over 25 years old) had an estimated mean vocal tract length of 3.21 m ± 0.51 SD, and that vocal tract length was positively correlated with formant frequency. For younger and smaller males (aged under 25 and less than 3 m tall) the mean vocal tract length estimated from the formants was 2.36 m ± 0.26 SD, similar to that of adult females (Stoeger & Baotic ).…”

“…, Stoeger et al. , Stoeger & Baotic , Baotic & Stoeger ). Such behavioural contexts included: when individual males were spatially separated from other elephants, when males were approaching another elephant or being approached, during general group locomotion, and physical contact.…”

“…Male musth rumbles have been commonly described as highly distinctive, pulsated calls, with a fundamental frequency ranging from 11 to 17 Hz and a long duration averaging 4.4 s (Poole ). In addition to having a clear behavioural context, basic acoustic analysis has found these rumbles to be individually distinct (Poole , also see Poole for a spectrogram of a musth rumble and Stoeger & Baotic for a spectrogram of a non‐musth rumble). We also found evidence for communication of male sexual state to females.…”

“…Information pertaining to the specific function and/or acoustic structure of these signals was generally lacking in comparison to musth rumbles. However, Stoeger and Baotic () showed that male elephant non‐musth rumbles contain information about individual identity, just as they do in females (McComb et al. , Soltis et al.…”

“…() recorded males rumbling during periods of spatial separation and bonding. All of the rumbles analysed by Stoeger and Baotic () and Baotic and Stoeger () were preselected for low‐affect social contexts, while Leong et al. () recorded 17 loud rumbles and 16 rumbles (neither associated with musth) from a young male in captivity.…”