Sound Underground

Forest elephants tend to flap their ears when listening for deep rumbles — how does this change when listening for seismic rumbles? © Elephant Listening Project

Can you imagine hearing the world through your feet? There’s growing research about the possibility of elephants being able to do just that: detect vibrations in the ground with sensory nerves in their feet and trunk. The idea gained traction after the 2004 tsunami in Thailand, when locals reported that nearby elephants fled to higher ground before the wave hit. Scientists theorized that the elephants responded to seismic tsunami clues but needed to prove how elephants detected those signals. Growing research suggests that seismic detection, and even communication, plays a big role in elephant life.

Caitlin O’Connell-Rodwell has led the scientific charge into seismic rumbles, and she has demonstrated that elephants both detect and respond to vibrations through several experiments. In 2004, O’Connell-Rodwell and her colleagues tested if elephants in Etosha National Park, Namibia, could detect seismic signals by playing back seismic copies of acoustic alarm calls. If elephants could detect the alarm calls, they should exhibit defensive behaviors in response. And researchers did in fact observe a pattern of defensive behaviors in response to the playback¹. Elephants that received the seismic alarm calls also left the area far sooner than elephants who had not.

Elephants seem built for picking up vibrations in the earth. Their feet are densely packed with Pacinian corpuscles (PCs), essentially pressure receptors on the ends of sensory nerves — which are present in human hands as well (although in lesser quantities than elephant feet)². Enlarged ear bones and fat deposits in their feet also bear remarkable similarities to the seismic detection systems of other species. Golden moles, for example, rely on their enlarged inner ear bones to detect vibrations as they tunnel under the sand in Southern Africa, while “acoustic fat” in dolphins, mole rats, and manatees help with detecting low frequency acoustic and seismic signals³. It’s likely that the fatty deposits in elephant feet help absorb and facilitate vibrations that travel up to the ear, an idea supported by “listening postures”² that maximize PC contact with the ground. Further field studies in Etosha National Park observed elephants freezing, shifting weight to their forefeet, lifting one foot off the ground (to promote triangulation, ie source detection), and rocking back on the heels of their hind feet when seismic signals were present².

Placida I stomping around Dzanga bai (perhaps exhibiting a listening posture) © Elephant Listening Project

But how discerning are elephant seismic detection methods? O’Connell-Rodwell’s second experiment in Etosha set out to test whether or not elephants could discriminate between types of seismic signals⁴. This experiment used seismic constructions of alarm calls from familiar and unfamiliar elephants: some elephant groups in the study received no alarm call, others received alarm calls made by a familiar elephant family, and a third group received calls from elephants they’d never heard before (stranger elephants). And while elephants responded differently based on the type of call received, the reason behind those reactions are unclear. The researchers suggested that alarm signals from unfamiliar groups could have been deemed unreliable, or the different frequencies/intensities of the alarm calls caused different responses in the observed elephant groups⁴. It’s clear more research is needed to understand the full extent of seismic detection and communication.

That’s another piece of the seismic puzzle: whether or not elephants communicate via underground signals. Many animals use percussive strikes as a threat, like kangaroos stomping their feet and elephant seals slamming their bodies against the ground³. Elephants often mock-charge and bang their trunks on the ground³. What if these percussive strikes were more than a display of strength, and could also travel long distances in the ground to other elephants? The distance these signals could travel has not yet been measured, but a person jumping creates a seismic “disturbance”³ that can be felt a kilometer away, meaning there is a lot of potential for elephantine long-distance seismic communication.

Elephant rumbles can also be loud and low enough to produce a seismic vibration, a process called “coupling”³. This characteristic of elephant calls was drawn upon by O’Connel-Rodwell’s experiments in Etosha and provide another possible method for seismic communication between elephants. O’Connel-Rodwell and others suggest that seismic communication would be especially beneficial for Forest and Asian elephants, because the dense foliage of their habitats obstructs acoustic calls³. Despite habitat similarities, these elephants are not created equal. Asian elephants have a more monotone, flat call that is better suited for coupling than the arched harmonic rumbles typical of Forest elephants. That being said, the capacity for seismic communication remains and deserves further study. There’s a whole world of noise underneath our feet!

David Larom, an environmental scientist, best expressed the exciting nature of this research:

“Like all creatures, humans perceive with limited spectra and sensory modes. We bias research with the unconcious and incorrect assumption that other animals ‘see’ the world as we do. Every time we discover otherwise, it expands the horizons of scientific knowledge and sends us the humbling and vital message that we are not, after all, ‘the measure of all things.’”⁵

References:

1. O’Connell-Rodwell, C. E., Wood, J. D., Rodwell, T. C., Puria, S., Partan, S. R., Keefe, R., … & Hart, L. A. (2006). Wild elephant (Loxodonta africana) breeding herds respond to artificially transmitted seismic stimuli. Behavioral Ecology and Sociobiology, 59(6), 842–850.
2. Bouley, D. M., Alarcon, C. N., Hildebrandt, T., & O’Connell‐Rodwell, C. E. (2007). The distribution, density and three‐dimensional histomorphology of Pacinian corpuscles in the foot of the Asian elephant (Elephas maximus) and their potential role in seismic communication. Journal of anatomy, 211(4), 428–435.
3. O’Connell-Rodwell, C. E. (2007). Keeping an “ear” to the ground: seismic communication in elephants. Physiology, 22(4), 287–294.
4. 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.
5. Shwartz, Mark. “Looking for earth-shaking clues to elephant communication.” Stanford Report. June 1, 2005. https://news.stanford.edu/news/2005/june1/elephant-052505.html

Find out more about the Elephant Listening Project
To conserve the tropical forests of Africa through acoustic monitoring, sound science, and education, focusing on forest elephants.