Who is calling: proof of concept for the use of laser Doppler vibrometry in identifying individual callers within African elephant vocalization bouts

We propose a novel use of laser Doppler vibrometry (LDV) for elephant field bioacoustics and behavioral ecology, allowing investigators to determine who initiates and responds within a vocalization bout. LDV has been used in a variety of applications including engineering, biomedical research, and animal communication. We present LDV data collected from one captive African elephant in a group of three elephants within an open field during vocal exchanges. While rumble vocalizations were emitted within the group, we were able to identify the caller, as well as record the call structure parameters of that specific individual's vocalization. The rumble vocalization had a duration of 5 s, with a fundamental frequency between 17 and 20 Hz and two harmonics at approximately 40 and 60 Hz. This LDV technique could be used to identify and record individual callers within a group where it is difficult to identify the caller, and where it is not feasible to use voice-activated collars. Specifically, LDV technology would make it possible to unobtrusively record an entire "let's go" rumble sequence while being able to identify the ordering of callers within the sequence. LDV technology may also facilitate vocal communication studies in species where identifying the caller within a group is challenging. Finally, LDV technology could inform studies focused on the physics of signal propagation of coordinated signals within group-living animals.