Elephants Talk Below the Range of Human Hearing, and Listen With Their Feet

In 1984 the biologist Katy Payne spent a week at the elephant enclosure of the Washington Park Zoo in Portland and kept noticing something she could not account for. Standing near the elephants, she felt a throbbing in the air. Not a sound exactly. A pressure, the sort of feeling you get standing near a large organ pipe.

She had spent years working on humpback whale song and recognised the possibility. She went back with recording equipment, taped the elephants, then played the tape back at several times normal speed to shift everything upward into human range.

The recordings were full of calls nobody in the building had ever heard.

Below the Floor

Human hearing bottoms out around 20 Hz. Below that, we stop perceiving pitch and start feeling vibration, if we register it at all.

Elephant rumbles run from roughly 100 Hz down into the low teens, with the fundamental frequency of many calls sitting under the human threshold. What a person standing nearby actually hears is the upper harmonics, which is why elephants have always sounded like they make a low grumbling noise rather than sounding silent. The bulk of the signal was simply below the floor.

The reason this matters is physics rather than biology. Low frequency sound is much better at travelling. Long wavelengths bend around obstacles instead of reflecting off them, and they lose energy to the air more slowly than high frequencies do. A high pitched call dies in dense bush within a few hundred metres. A 15 Hz call carries.

In practice, elephant rumbles have been documented carrying across several kilometres of savanna. Conditions matter a great deal. In the early evening, temperature inversion layers form as the ground cools faster than the air above it, and these can channel low frequency sound rather than letting it disperse upward. Under those conditions the range extends considerably, which is why calling activity tends to peak around dusk.

Then There Is the Ground

The second half is stranger. A loud rumble does not only move air, it moves earth. Some of the acoustic energy couples into the ground and travels onward as a surface wave.

Seismic waves and airborne sound do not travel at the same speed, so the two components of the same call arrive separately, with the ground-borne signal arriving first over distance. Caitlin O'Connell-Rodwell has spent much of her career on the question of whether elephants use that channel, and the evidence says they do.

Playback experiments using ground-transmitted signals with no airborne component produced clear responses. Elephants freeze, orient toward the source, lean forward onto their front feet, and sometimes lift and reposition a foot, all behaviours associated with attending to something rather than casually standing about.

The anatomy supports it. Elephant feet contain dense concentrations of Pacinian corpuscles, the vibration-sensitive receptors mammals use for detecting mechanical oscillation, and the fat pad in the foot appears well suited to transmitting vibration inward. There is also a bone conduction route: vibration passing up the leg bones through the skeleton to the middle ear, which is essentially how a stethoscope works.

An elephant standing still in what looks like idleness may be doing something closer to listening carefully.

What They Are Actually Saying

The calls are not undifferentiated noise. Long-term work at Amboseli by Joyce Poole, Cynthia Moss and colleagues has separated out a substantial repertoire.

There are contact calls used by separated group members and contact answers in reply. There are greeting rumbles during reunions, which in elephants are elaborate and loud affairs. There are the calls of females in oestrus, and the distinctive low frequency rumbling of males in musth, a period of elevated testosterone and heightened aggression. There are alarm and coordination calls that move a group without any visible signal passing between them.

Playback work indicates elephants distinguish between individuals by voice, and can tell family from non-family, which for an animal with the social range of an elephant is a considerable amount of bookkeeping done by ear.

Why It Went Unnoticed So Long

People have lived alongside elephants for as long as there have been people, and observers had reported for years that herds separated by distance seemed to coordinate movements without any apparent signal. The explanations offered ran to the mystical, largely because the obvious channel was one nobody could perceive.

It took someone who had already worked on whale song to walk into a zoo, feel a throb in the air, and think to check below the bottom of human hearing.

Sources

• Payne, K. B., Langbauer, W. R. and Thomas, E. M. (1986). Infrasonic calls of the Asian elephant. Behavioral Ecology and Sociobiology.
• O'Connell-Rodwell, C. E. (2007). Keeping an ear to the ground: seismic communication in elephants. Physiology.
• O'Connell-Rodwell, C. E., Wood, J. D., Rodwell, T. C. et al. (2006). Wild elephant breeding herds respond to artificially transmitted seismic stimuli. Behavioral Ecology and Sociobiology.
• Poole, J. H., Payne, K., Langbauer, W. R. and Moss, C. J. (1988). The social contexts of some very low frequency calls of African elephants. Behavioral Ecology and Sociobiology.
• Garstang, M. (2004). Long-distance, low-frequency elephant communication. Journal of Comparative Physiology A.