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The well-known trumpeting call of an elephant indicates anger or alarm. The more common but less familiar rumbles are a range of low-frequency sounds that elephants use to communicate different types of messages to one another.
Scientists have found that rumbles travel through both the air and the ground. These low-frequency calls create seismic waves—vibrations occurring underground and along the earth’s surface—which, depending on the soil type, can travel farther than the counterpart waves we hear moving through the air.
A new study has found different elephant behaviors created distinct seismic signals and that both the local geological structure and low-frequency human-generated noise may affect transmission of these signals.
Ground versus Airborne Sound Waves
When an elephant, rabbit, or person walks, we create seismic waves, the heavier and faster the walk, the greater the strength of the wave. Seismic wave transmission is also affected by physical factors, such as the underlying soil type and other noises in the same frequency range.
In areas with minimal human seismic noise, the frequencies around and below 20 Hz characteristic of elephant vocalizations are relatively noise-free. Noises at this frequency, also known as infrasound, are generally too low for us to hear.
Research by Caitlin O’Connell-Rodwell of Stanford University has shown that elephants in Namibia can both detect seismic signals and distinguish alarm calls made by neighboring groups from unfamiliar ones made by groups in Kenya.
Use of Seismic Signals to Classify Behavior
The new study, published this month in Current Biology, builds on earlier research by applying seismological modeling software that incorporates the local geological information and computer algorithms to produce more accurate estimates of the propagation of seismic waves, specifically those produced by elephants.
Researchers from the University of Oxford’s Department of Zoology and Earth Sciences and the Kenyan NGO Save The Elephants examined whether they could classify elephant behaviors by monitoring the vibrations that their movements send through the ground.
To do this, they first recorded vibrations generated by wild elephants in Kenya during different behaviors, mainly walking and calling. They filmed the elephants during recordings and later synchronized the video with the recordings to allow them to visually confirm that the vibrations originated from elephants during various behaviors.
They positioned a geophone—a device that records ground movement (velocity) and converts it into voltage—near a focal elephant. The deviation of the voltage measured by the geophone from the natural background voltage is called a seismic response and is typically used for analyzing the earth’s structure.
In this case, the researchers analyzed the seismic response from elephant behaviors using a source function—“the force strength and pattern generated by the elephant ‘at the source’”—generated by the focal animal during each behavior type.
“We applied a technique to extract the source signature of the elephants from our recordings,” co-author Tarje Nissen-Meyer, a geophysicist at the University of Oxford, UK, told Mongabay-Wildtech.
“This source function is then independent of geology and propagation effects and a direct indicator of both the size of the force and temporal evolution, i.e. for running, rumbling, regular walk, individuals versus herds. This is the crucial step towards building a classification for different behaviors.”
The researchers combined the source functions, the type of geological substrate—hard gneiss rock or three types of sand—found at the site, the amount of other noise, and seismological modelling software to estimate two factors needed to classify different behaviors.
The models: (1) estimated how far these seismic signals can travel and (2) extracted the seismic signal actually produced by the elephants, independent of geology and other seismic noise.
How Seismic Waves Travel
The researchers found that walking and vocalizing generated distinct seismic signatures, with larger animals producing greater downward force that travels farther. They also found that other noise and soil type affected their ability to distinguish the patterns over long distances. Vibrations travel farther through sand than through hard rock and, not surprisingly, when little other noise is present to interfere.
In their paper, the authors state, “Differences in elephant behavior caused detectable changes in source function properties, which remained distinguishable during modelled seismic wave propagation up to 1000 metres regardless of the noise level and terrain type.”
O’Connell-Rodwell, who was not part of this study but has studied elephant seismic communication for nearly 15 years, said the results served as an important validation of what she and colleagues have previously published in this field.
Her team’s seismic census studies suggest that elephants have a signature walk that is more similar to humans and distinct from lions, rhinos and ungulates and that all species have a signature seismic signal while walking.
“The level of detail that can be gained from recording footfalls using geophones would even pick out a musth bull from other male elephants simply because of their exaggerated gait,” O’Connell-Rodwell said.
“This has huge potential for monitoring remote waterpoints, particularly those that do not have heavy animal traffic, as the math would be simpler to sort out and quantify species, number of animals visiting and time of day.”
In the current study, the researchers were surprised to find that, for their set of source functions, the elephants’ vocalizations produced greater seismic force and traveled farther than those from walking.
“We found that the forces generated through elephant calls were comparable to the forces generated by a fast elephant walk,” lead author Beth Mortimer of the Universities of Oxford and Bristol, UK said in a statement, “This means that elephant calls can travel significant distances through the ground and, in favorable conditions, further than the distance that calls travel through the air.”
Nevertheless, the authors note, the capacity of car engines, heavy machinery, oil exploration, and other human noise in the 20–25 Hz frequency range to interfere with the transmission of seismic waves could increasingly impede animals’ seismic communication.
The authors suggest that seismic detection of rapid running by an elephant could be used to help assess an immediate poaching threat in remote areas with low levels of seismic noise. They acknowledge that deploying multiple geophones and more data and testing are needed to develop such near-real-time monitoring of elephants.
For example, a threatened herd may run but may instead bunch together to protect their young.
Further research and development are also needed to determine where to position the geophones for a given area’s elephant population and how to distinguish an elephant running when chasing another in a test of dominance, approaching water, or in play, from one that is threatened.
The research team’s modeling and signal-processing techniques and the software they used are entirely open-source (they developed the seismic modeling approach), Nissen-Meyer said, and they will remain open-source for anyone to use.
He emphasized that their results were made possible by a cross-disciplinary approach to research and added, “I am convinced that this is increasingly crucial for breaking into new territory and demands an open mindset not only across disciplines, but also for novel developments in methods and techniques.”
Mortimer, B., Rees, W. L., Koelemeijer, P., & Nissen-Meyer, T. (2018). Classifying elephant behaviour through seismic vibrations.
O’Connell-Rodwell, C. E. (2007). Keeping an “ear” to the ground: seismic communication in elephants. Physiology, 22(4), 287-294.
O’Connell-Rodwell, C. E., Wood, J. D., Kinzley, C., Rodwell, T. C., Poole, J. H., & Puria, S. (2007). Wild African elephants (Loxodonta africana) discriminate between familiar and unfamiliar conspecific seismic alarm calls. The Journal of the Acoustical Society of America, 122(2), 823-830.