Elephant Sound

Infrasound – why it matters

Range of Human Hearing

• Two elephant rumbles (stacks of parallel lines).
• The white line is the upper boundary of ‘infrasound’ – humans can not hear frequencies below this.
• The bracketed areas show speaking frequencies for men (70-200 Hz) and women (140-400 Hz).
• The stacks of lines above the white line are harmonics of the fundamental frequency. In these calls the fundamental frequency is infrasonic.

LISTEN: low-frequency, pulsing male rumble:

Infrasound is sound that is below the threshold of human hearing. The figure to the right shows the frequency (or tone) for average humans when they talk, compared to a forest elephant rumble. Humans with very good hearing can just barely detect sounds at  20 Hz, but part of a typical elephant rumble is below 20 Hz, so humans can’t hear that.

Infrasound matters because low-frequency sound generally travels farther through closed habitats than higher-frequency sound. Forest elephants, who spend the majority of their lives hidden within the closed-canopy forests of Central Africa, exploit this characteristic when they talk to one another using very low frequency calls known as ‘rumbles’.  Whether their family groups are separated by kilometers of forest, or males are searching for females in estrous, rumbles built on a fundamental frequency in the infrasonic range can be heard over long distances, and we can eavesdrop on these conversations using recorders. The simple animation below shows the basics of this relationship.

How did we find out that elephants use infrasound?

The discovery of infrasonic communication among elephants came from a hunch Katy Payne had while visiting the Washington Park Zoo in Portland, Oregon. As she observed the Asian elephants, Katy sensed a thrumming vibration in the air and surmised she was feeling, rather than hearing, the elephants communicating.

Further work at the zoo with William Langbauer Jr. and Elizabeth Thomas revealed that elephants were indeed making infrasonic calls (1). This was later confirmed with playback experiments on wild African elephants in collaboration with Russel Charif, Lisa Rapaport, and Ferrel Osborn (2). It was concluded that elephants use their powerful, deep calls in long distance communication to coordinate group movements and to find individuals in reproductive condition. Fascinating observations of elephants in the extremely dry region of Namibia, by Michael Garstang and colleagues, suggests that they might use the infrasound produced by distant thunderstorms to find water during drought periods (3).

Forest Elephants and Infrasound

Playback experiments on the savannahs of East Africa demonstrated that savannah elephants responded to each other’s vocalizations over distances of 2 km and, because it is hard to reproduce elephant calls as loudly as the elephants themselves can call, Langbauer, Payne and colleagues estimated the actual detection range to be 4 km (2). This means that an elephant rumble could reach family members anywhere in a 50 sq.km area around the caller!

The timing, frequency, and power (loudness or amplitude) of elephant vocalizations turns out to be important as well. The propagation of very low frequency sound can vary with atmospheric conditions, wind speed and direction, and characteristics of the ground on which the elephants are standing. In addition, detection of calls by a receiver is affected by background sounds in the environment. On a typical dry season evening in the savannah a temperature inversion forms that essentially acts like a ceiling and bounces sound waves back down toward the ground (and receiver), potentially increasing the listening area of elephants as much as ten-fold — from 30 sq. km. at midday to 300 sq. km. in the same evening (4). In light of this fact it is interesting that savannah elephants make most of their loud low-frequency calls during the hours of best sound propagation (4). We do not know whether this is an innate or opportunistic response to fluctuations in the size of their communication area, but in either case it is clear that as the area shrinks and expands, so does the network of potential associates and mates that could be reached acoustically.

In the forest there are even greater challenges! In the environment roamed by forest elephants there might not be as much of a problem with the disruptions of wind on sound propagation, but the density and diversity of other acoustically active species makes for a very loud background. Still, as shown in the cartoon below, the low frequencies in elephant rumbles do still travel a long way through the forest, even though the trees can be so dense that the elephants are impossible to see.

ELP recently measured how well forest elephant rumbles travel through the rainforest in Central Africa. We used the recordings from an acoustic array, spread out around a forest clearing in Gabon. The array allowed us to pinpoint spatially where each recorded call was produced, and from this we could calculate the distance that the call had to travel to each of the recording units. Recorders were different distances from each call and so by measuring the amplitude of the call at each distance we could estimate how much energy was lost as the sound traveled through the forest.

We estimated much smaller detection distances for the average rumble in this rainforest environment compared to the savannah – only about 800 meters (not 4 kilometers). But this was almost entirely due to the relatively high level of the background noise against which an elephant would need to recognize the rumble. When the forest was at its most quiet, a rumble might be detected at distances of more than 3 km. This has large implications for how forest elephants coordinate interactions among subgroups of the family and between potential mates (5).

How different is this finding from what has been found with savannah elephants? Possibly not as different as it would first appear. Experiments in the savannah were mostly conducted under optimal environmental conditions with little or no wind. And unlike the savannah studies, which used playback experiments to determine behaviorally whether the elephants discriminated the call, we had to make assumptions about the hearing sensitivity of the elephants, something that has never been precisely measured. There are some theoretical reasons to think that animals are very good at extracting acoustic signals from the background, so forest elephants might do much better than we assumed.

Learning what elephants are saying

Elephants are very vocal creatures, yet humans remained unaware of this until 1984, when Katy Payne discovered that often their communication happens below the threshold of human hearing. At the Dzanga Bai in Central African Republic, ELP researchers estimated that we may only hear somewhere around 40% of all the calls elephants produce. Mostly this is because the higher, human audible, frequencies rapidly attenuate in the warm humid air. By the time a call has traveled 50-100 meters, only the lowest, inaudible frequencies are left.

An Elephant “Dictionary”

Although our research is still just scratching the surface of forest elephant communication, the goal of a new research project at Dzanga bai, Central African Republic,  is to match certain types of calls with behaviors to build a sort of “dictionary” of elephant vocalizations.

Elephants are capable of making extremely low frequency and powerful calls — sometimes as loud as construction tools (90 to 117 dB Sound Pressure Level). Under the best ambient conditions, these low sounds carry over distances of several kilometers and might enable elephants to stay in contact despite separation in the dense rain forest. The trick to learning what information these calls contain is to pair visual observation of their behaviors with knowledge about who is producing which calls. From there we can begin to tease apart how the sounds differ from one another. But because sometimes the calls are not audible, and elephants can produce rumbles and other sounds both through their mouths and through their trunks, we need to use an “acoustic array” to locate who is making the calls.

The video below shows how we can put these various sources of information together to better understand the complex interactions that we observe in forest clearings. This elephant family heard something startling in the forest and panicked a little bit, running to the edge of the clearing and huddling together. From 50 meters away we could only hear the very first rumble given by the matriarch. But using an acoustic array we could match elephants to the rumbles we recorded. As each rumble appears on the spectrogram at the bottom right edge of the video, we have circled the elephant producing that call.

Below are more examples of the many types of calls that forest elephants use to communicate with one another. Notice that sometimes you can see an elephant open its mouth to vocalize, but often it is very difficult to tell who is calling and who is listening!

Rumbles, trumpets, and screams

In the clip below, a sitatunga (antelope) jumps out of a hollow tree off-camera and startles the Pia and Noor families. As they huddle together, they rumble, trumpet, and scream. Pia is about 40 years old here and with her fourth off-spring, 2-year-old Pia V. Noor is about 38 and with her fourth off-spring as well, 2-year-old Noor V.

Calling for mom

This unknown infant appears lost! He rumbles and screams as he searches. He also smells the air and listens for something.

Battle Cry

A young adult male advances towards Gabin, an older male (golden yellow). You will see the younger male flap his ears as he gives a soft rumble, then he runs at Gabin in challenge. The two males clash and probably it is the younger male who roars as he gets shoved away by Gabin. Then Gabin gives two rumbles, each time accompanied by flaps of his ears.

To Trunk or not to Trunk

It turns out that elephants can actually speak with two voices! They can either talk through their mouths or from the end of their trunks, and the resulting rumbles are different and might be used for different types of communication. Although elephants produce sounds in much the same way as we do, using their vocal folds to generate the source (called “fundamental”) frequency and then modifying the sound’s structure by filtering it through the mouth and nose, the nasal passage is something else when it comes to an elephant! The trunk gives elephants an extra six feet of filter to use if they want to. The filter lets an animal concentrate sound energy in different parts of the call structure, and the longer the filter, the lower in frequency (tone) those areas of concentration can be. Scientists speculate that nasal rumbles may be particularly important for long distance communication.

Call and Response

The repertoire of elephant calls appears to be similar in all three species, but has been best studied in savannah elephants. Based on behavioral context, there are dozens of call types: of these, the majority are made by females and function in group coordination or reproduction. The “fission-fusion” social system of elephants is characterized by the splitting and coming together of variously related groups of individuals organized around adult females, meaning these matriarchal groups are constantly meeting up and then separating from each other. Powerful, low-frequency calls enable females to identify one another acoustically and thus coordinate with subgroups foraging separately — even when miles apart. Playback experiments demonstrated that free-ranging savannah elephants respond to one another’s calls over at least two and probably four kilometers during daylight hours. This type of acoustic coordination might be particularly critical in forest elephants as subgroups move through the dense rain forest, out of sight of one another.

It will be difficult indeed, if not impossible, to perform similar playback experiments on forest elephants. Generating low frequency sound requires a really big sub-woofer speaker, a big amplifier to drive the mechanism, and mega-batteries to run the amplifier. Together these components are not only physically large, but really heavy.  But the real difficulty in the rainforest is knowing where the subject is relative to the speaker, and being able to observe the subject’s response to the playback. We have estimated the potential communication distance by using an acoustic array to locate where the elephant was calling from and then measuring how fast the sounds attenuate with distance. In the forest, under conditions of average background noise, elephants can probably communicate only up to about 800 meters away from each other! But when the forest is quiet, we estimated distances of close to 5 km. Do our elephants choose their window of opportunity to call to one another?