How many rumbles are there? Acoustic variation and individual identity in the rumble vocalizations of African elephants (Loxodonta africana)

Abstract

The most commonly occurring elephant vocalization is the rumble, a frequency-modulated call with infrasonic components. Upwards of ten distinct rumble subtypes have been proposed, but little quantitative work on the acoustic properties of rumbles has been conducted. Rumble vocalizations (N=269) from six females housed at Disney’s Animal Kingdom were analyzed. Vocalizations were recorded from microphones in collars around subject necks, and rumbles were digitized and measured using SIGNAL software. Sixteen acoustic variables were measured for each call, extracting both source and filter features. Multidimensional scaling analysis indicates that there are no acoustically distinct rumble subtypes, but that there is quantitative variation across rumbles. Discriminant function analysis showed that the acoustic characteristics of rumbles differ across females. A classification success rate of 65% was achieved when assigning unselected rumbles to one of the six females (test set =64 calls) according to the functions derived from the originally selected calls (training set =205 calls). The rumble is best viewed as a single call type with graded variation, but information regarding individual identity is encoded in female rumbles.