Exploring the potential use of seismic waves as a communication channel by elephants and other large mammals

Exploring the Potential Use of Seismic Waves as a Communication Channel by Elephants and Other Large Mammals1

SYNOPSIS. Bioseismic studies have previously documented the use of seismic stimuli as a method of communication in arthropods and small mammals. Seismic signals are used to communicate intraspecifically in many capacities such as mate finding, spacing, warning, resource assessing, and in group cohesion. Seismic signals are also used in interspecific mutualism and as a deterrent to predators. Although bioseismics is a significant mode of communication that is well documented for relatively small vertebrates, the potential for seismic communication has been all but ignored in large mammals. In this paper, we describe two modes of producing seismic waves with the potential for long distance transmission: 1) locomotion by animals causing percussion on the ground and 2) acoustic, seismicevoking sounds that couple with the ground. We present recordings of several mammals, including lions, rhinoceroses, and elephants, showing that they generate similar acoustic and seismic vibrations. These large animals that produce high amplitude vocalizations are the most likely to produce seismic vibrations that propagate long distances. The elephant seems to be the most likely candidate to engage in long distance seismic communication due to its size and its high amplitude, low frequency, relatively monotonic vocalizations that propagate in the ground and have the potential to travel long distances. We review particular anatomical features of the elephant that would facilitate the detection of seismic waves. We also assess low frequency sounds in the environment such as thunder and the likelihood of seismic transmission. In addition, we present the potential role of seismic stimuli in human communication as well as the impact of modern anthropogenic effects on the seismic environment.

INTRODUCTION

Bioseismic cues are known to be important for many arthropods (Cocroft et al., 2000), fish, reptiles, amphibians and small mammals in intraspecific and heterospecific communication, prey detection and predator avoidance and navigation (see O'ConnellRodwell et al., 2000 for review). Two primary methods of initiating bioseismic cues are: 1) percussion that causes an impact with the earth and produces waves in response to direct contact and 2) vocalizations which produce wave movements that are then coupled with the earth to cause vibrations of the earth substrate (Ewing, 1989).

Vibration signal energy depends mostly on the mass and available muscular power of the signal producer (Markl, 1983). The source signal intensity and attenuation during transmission, together with the sensitivity and depth of receptors in the receiver, and the threshold at which the receptor will be stimulated relative to the frequency and strength of the stimulus define the spatial extent of vibration signals. The Weber-- Fechner law states that the magnitude of an observer's psychological response is directly related to the logarithm of the intensity of the stimulus (Landing et al., 1998). Signal detection theory (SDT) further stipulates that detection also depends on the expectation, motivation, in situ conditions, sensitivity, decision making and finally, noise level (Tanner and Swets, 1954).

It appears there is a "sweet zone" for seismic signal transmission ranging from 10 Hz to 40 Hz, where there is a maximum efficiency of transmission of seismic energy (O'Connell-Rodwell et al., 2000). Ambient seismic noise on land from ocean waves creates peaks at about 0.14 Hz and about 0.07 Hz (White, 1965). With increasing frequency, these low frequency and storm microseisms sharply decline to negligible levels by 10 Hz. Although noise due to microseisms decreases to trivial levels above 10 Hz, the attenuation of seismic pulses increases with frequency (Frantii et al., 1962). Pre-historically, the range around 20 Hz was a quiet seismic region, carrying only vibrations associated with thunder and earth tremors making it available to elephants and other large mammals.

Both acoustic and seismic waves are subject to interference and alteration due to environmental factors. Wind shear and temperature gradients influence the acoustic propagation of sound, whereas the soil type and heterogeneity are among the factors influencing the propagation of a seismic signal (O'Connell-Rodwell et al., 2000). Airborne sound waves spread spherically rather than cylindrically, attenuating more rapidly than ground surface waves such as Rayleigh waves, losing 6 dB for every doubling of distance as opposed to 3 dB.

There is also an outer limit to airborne transmission (Uman, 1984) which is not the case for surface seismic waves. In this paper, we address the production of both acoustic and seismic waves by large terrestrial mammals, especially Asian and African elephants, that are known to produce high amplitude acoustic vocalizations.

The primary aim of this paper is to present a conceptual framework for examining bioseismic cues produced by large vertebrates through percussion and the coupling of low frequency vocalizations, especially where such signals might be used for long distance communication. We review what is known about biological sense organs that could potentially be used to detect seismic signals and discuss the data that support the possibility of the elephant being capable of detecting the seismic signals produced by conspecifics. We address the possibility that humans may also have used seismic cues at one time as a means of long distance communication and prey detection.