The Daily Cycle of Low-Frequency Elephant Calls and Near-Surface Atmospheric Conditions

Abstract

Elephant low-frequency calls and atmospheric conditions that influence the transmission and detection of these calls were recorded at a fixed location over a period of about 3 weeks at the end of the dry season in the Etosha National Park, Namibia. A bimodal distribution in elephant call detections is observed, with the primary maximum (42% of all calls) occurring in a 3-h period following sunset and a secondary maximum (29% of all calls) in a 2-h period following sunrise. This distribution in calls detected is shown to depend upon marked and regular changes over 24 h in near-surface atmospheric stability and velocity, which determine propagation ranges. The observed bimodal distribution of calls detected depends upon these changes in atmospheric conditions as well as the location of the caller and the rate of calling. The findings are supported by results from an atmospheric acoustic model but are at variance with observations of the number of calls made from collared elephants in markedly different habitat. Detection of calls heard at a location remote from the caller represents a valuable and noninvasive research and applied tool that, however, must take note of atmospheric conditions that govern the propagation and reception of such signals.