Acoustic (that is, sound) signals are omni directional (i.e. they travel in all directions) and can be broadcast to a large audience including intended and unintended listeners, and those in view and hidden from view. Being short-lived and deliberate, acoustic signals are useful for giving information about an immediate situation, rather than about a constant state. Through reflection, refraction and absorption, acoustic signals are degraded by the environment in ways that are often very much greater for high frequency sounds than for low frequency sounds. Elephants are specialists in the production of low frequency sound and in the use of long-distance communication.
The range of sounds elephants produce
Elephants produce a broad range of sounds from very low frequency rumbles to higher frequency trumpets, snorts, barks, roars, cries and other idiosyncratic calls. Asian elephants also produce chirps. The most frequently used category of calls, at least for African elephants, is the very low frequency rumble. You can search for, listen to and read about numerous sounds through ElephantVoices calls databases - Call Types and Contexts.
To get a sense of the range of frequencies used by elephants it may be useful to compare them with the range used by people. A typical human male's voice in speech fluctuates around 110 Hertz (Hz, or cycles per second), a female's voice around 220 Hz and a child's around 300 Hz. Among elephants, a typical male rumble fluctuates around an average minimum of 12 Hz (more than 3 octaves below a man's voice), a female's rumble around 13 Hz and a calf's around 22 Hz.
In normal human speech, the vibration rate may vary over a 2:1 ratio, in other words over one octave, while a singers voice may have a range of over two octaves. By contrast, the fundamental frequency within a single elephant call may vary over 4 octaves, starting with a rumble at 27 Hz and grading into a roar at 470 Hz! Including the harmonics elephant calls may contain frequencies ranging over more than 10 octaves, from a low of 5 Hz to a high of over 10,000 Hz. Imagine a musical composition with some operatic elephants!
Elephants can produce very gentle, soft sounds as well as extremely powerful sounds. You can listen to a couple of examples below. Some of the calls produced by elephants may be as powerful as 112 decibels (dB) recorded at 1 meter from the source. Decibels are measured on a logarithmic scale and to give you some idea of how loud some elephant sounds are we have copied information from a table in The Science of Sound by T.D. Rossing which gives some examples of typical sound levels you might encounter.
How do elephants produce such a range of sounds?
Sound is produced as air expelled from the lungs is passed over the vocal chords or larynx, a structure in elephants some 7.5 cm long. The moving air causes the vocal chords to vibrate at a particular frequency depending upon the type of sound the elephant is making. By lengthening or shortening the vocal chords an elephant can produce a wide range of frequencies. The column of air vibrates in the elephant's extended vocal tract or resonating chamber and, depending upon how the elephant holds the various components of this chamber (trunk, mouth, tongue, pharyngeal pouch, larynx) it is able to modify and amplify different components of the sound.
Certain calls by elephants are associated with particular postures of the head and ears. It is our belief that, by holding its head in a certain posture and by flapping its ears in a particular rhythm and angle an elephant is able to affect the musculature around the larynx, thus modifying a particular call to achieve the desired sound. Listen here to a musth-rumble, which is associated with special pulsating ear flapping known as ear-waving.
Quite different results may be achieved with the same basic rumble at source (duration and frequency) depending upon whether the elephant holds its mouth wide open or closed, its head held high or low, the ears steady, flapping slowly or rapidly, or perhaps raised and folded. And depending upon the positioning of the trunk and the speed and duration of air moving through it, elephants are able to produce a wonderful mixture of higher frequency trumpeting sounds.
Elephants are able to produce very low frequency sounds for several reasons. First and foremost they are able to produce low sounds because they are large bodied and, just as in musical instruments, the longer and looser the vibrating string (or vocal chords) and the larger the resonating chamber, the lower the frequency produced. In addition to being large bodied animals, elephants have several adaptations that allow them to make their resonating chamber even larger and their vocal chords even longer and thus produce even lower sounds than we might expect.
First of these is the elephant's trunk which in an adult male may add as much as 2 meters on to the length of the resonating chamber.
Second, the structures of the hyoid apparatus (a series of bones at the base of the tongue) and the musculature that support the tongue and the larynx in elephants are different from other mammals. The hyoid apparatus of elephants has five rather than nine bones, and these are attached to the skull by muscles, tendons and ligaments, rather than by bones as in most other mammals. This rather loose arrangement allows for a greater movement and flexibility of the larynx and is, therefore, thought to facilitate the production and resonance of low frequency sounds.
Third, in most mammals the hyoid apparatus provides support for the tongue and for the larynx. The looser arrangement in elephants also houses a pharyngeal pouch, a structure unique to elephants located at the base of the tongue, which in addition to providing an emergency source of water, appears to function in the production of low frequency calls.
In humans, and by inference also in elephants, the muscles of the larynx help to contract and relax the vocal cords. The greater the flexibility of the larynx, the greater the ability of these muscles to stretch and relax, which in turn affects the contraction and relaxation of the vocal cords and consequently the pitch or frequency of the sound that is produced. So, the modification in elephants of the hyoid apparatus to house the pharyngeal pouch also permits an enlargement of the resonating chamber by lowering the loosely attached larynx. Consequently, elephants are able to produce very low frequency sounds.
The pharyngeal pouch
During extremely hot weather elephants may be seen to insert their trunks into their mouths and withdraw water from their throats. It turns out that elephants are able to store several liters of water in a pharyngeal (meaning in the region of the pharynx) pouch, a structure unique to elephants located at the base of the tongue. Elephants can withdraw water stored there by inserting the trunk up to the pharynx, constricting the muscles at the periphery of the pharynx to form a tight seal around the tip of the trunk and then constricting the muscles of the pharyngeal pouch so as to squeeze water upward, enabling the elephant to fill her trunk.
The most frequently produced sounds made by elephants fall in the category referred to as rumbles. These very low frequency sounds were so-named because people once thought that some of them originated in the elephant's digestive tract and so gave them the name stomach-rumbles! These very low frequency sounds have attracted a lot of interest and research for two reasons. First, the lowest components of these elephant calls are between one and two octaves below the lower limit of human hearing. And second, because lower frequency sound travels farther than higher frequency sound, elephants use the more powerful of these calls to communicate over long distances.
Sound traveling through air attenuates by the reverse square law at 6 decibels (dB) for every doubling of the distance from the source. Thus, for example, a sound measuring 100 dB at one meter from the source will be reduced to 94 dB at 2 meters, 88 at 4 meters, 82 dB at 8 meters, and so on. Sound also attenuates through "excess attenuation" as it travels through the environment. The degree of excess attenuation is affected by the frequency of a sound and the type of habitat it is passing through. But very low frequency sound, such as the very low frequencies produced by rumbling elephants, suffer from little if any excess attenuation. In grassy savannas and woodlands elephants communicating over distances of more than 100m should be able to perceive low frequency calls better than higher frequency calls. Elephant groups are frequently over 100m in diameter and sub-groups of related elephants are often separated by several kilometers. Powerful rumbling sounds are the means by which these individuals stay in touch with one another.
Some of the calls made by elephants are exceedingly powerful and may reach up to 112 dB at 1 meter from the source. These calls fall in the sound level range of "intolerable" in the table above. How far could a sound like this carry? Well, using the reverse square law we can estimate that a call of 112 dB at 1 m would be around 46 dB at 2,048 m from the source. Through playback experiments, Karen McComb has shown that during the day elephants are able to both detect these calls and to recognize the voices of particular individuals up to 1-1.5 km and occasionally up to 2.5 km from the source!
Something interesting happens to the transmission of sound at different times of the day. Out on the savanna, it has been shown that environmental conditions follow a pretty regular diurnal cycle. Around evening a strong temperature inversion usually forms and doesn't dissipate until dawn. The greatest calling areas are achieved during the formation and dissolution of these nightly inversions, especially with cloudless and relatively undisturbed weather. Under such conditions it is possible for an elephant to have a calling range of 300 km2 - an area almost the size of the entire Amboseli National Park! In other words an elephant may be able to detect the calls of another elephant almost 10 km away. During the day, without the help of an inversion and with factors such as heavy sun and wind often coming into play, calling area size is drastically reduced, ranging from a couple dozen to 150 square km.
Not only are the elephants' low frequency rumbles well suited for long distance communication, but being sounds with a rich harmonic structure they also allow a listening elephants to calculate the distance of the calling elephant. This is because at close range the full harmonic structure will be intact while with increasing distance the upper frequencies will become relatively weaker eventually leaving only the lower and mid range frequencies to persist.
The measured upper limit of hearing of air born sound in mammals varies from 12 kHz (elephants) to 114 kHz (little brown bat), and the lower limit varies from less than 0.016 kHz (elephants) to 10.3 kHz (little brown bat), a range of more than nine octaves.
Mammals with small heads and narrow spaced ears are better able to hear high frequency sounds than mammals with large heads and wide-set ears. Large mammals are generally specialised in lower frequency hearing because larger skulls can encompass longer ear canals (meatuses), wider tympanic membranes (the membrane that closes the middle ear off from the exterior) and spacious middle ears. How do these three factors favour higher sensitivity at low frequencies?
In normal air-conducted hearing sound waves set the tympanic membrane and the middle ear bones (or ossicles) in vibration, thus producing movements on the oval window and changing pressure gradient in the cochlear fluid.
One difficulty with low frequency sound is the signal to noise ratio. In the lower frequencies there tends to be a higher level of background noise, and so animals that specialize in low frequency hearing must have a way of distinguishing signal from noise. The amount of sound energy collected by the tympanic membrane increases with increasing membrane area, thus enhancing the signal to noise ratio at the level of the inner ear. So, the larger the tympanic membrane the better an animal is able hear at low frequencies. The tiny middle ear bones, or ossicles (the malleus, incus and stapes), have to be able to withstand the greater forces produced by the vibrations of a larger tympanic membrane, and so animals with large tympanic membranes also have massive (relatively!) middle ear ossicles. An incus of an adult female African elephant (collected by Joyce from the skull of an elephant named Emily who died in September, 1989 when she was 39 years old) weighed 237 mg. The malleus and stapes of this elephant were estimated by Nummela and colleagues to be 278 mg and 22.6 mg, respectively and the tympanic membrane area 855 square mm.
Large tympanic membranes do, however, present a problem: Mammalian tympanic membranes are extremely thin and the risk of scratching and damaging them may have prevented the tympanic membranes of most large mammals from evolving too large. The enormous skull of the elephant, however, has allowed the evolution of an outer ear canal of about 20 cm in length, providing adequate protection for its very large tympanic membrane. Since the large elephant middle ear bones do not impede the transmission of low frequencies and the large tympanic membrane allows high signal to noise ratios, the elephant middle ear reflects a special adaptation to low frequency hearing.
Finally, one more structure of the elephant's ear, the cochlea, may facilitate low frequency hearing. Together with their relatives the Sirenia (the dugongs and manatees), elephants are unique among modern mammals in having reverted to a reptilian-like cochlear structure that may facilitate greater sensitivity to lower frequencies. Since the cochlear structure of reptiles facilitates a keen sensitivity to vibrations it has been suggested that the similar structure in elephants may allow them to detect vibrational signals, too.
So with all of these special adaptations, just how low can elephants hear? The only study of elephant hearing sensitivity was carried out on an Asian elephant. Unfortunately, the study was completed a couple of years before it was known that elephants produce very low frequency sounds and extremely low frequency sounds were not tested. But we do know from this study that elephants have very good hearing into the infrasonic (below human hearing) range. This particular elephant, a juvenile Asian female, was able to hear down to 16 Hz at 65 dB. Since 65 dB can be described as a moderate to noisy sound, presumably elephants can hear significantly lower than this. Joyce has recordings of elephant calls as low as 8 Hz and other people have reported calls as low as 5 Hz, so it is likely that elephants have a way of detecting these extremely low frequencies otherwise why would they produce them? Recent studies have shown that elephant rumbles are also transmitted through the ground, or seismically. If we some day learn that elephants are incapable of hearing down to 5 Hz then we may find that instead they pick up these sound with the help of their sensitive feet (you can read more about this under seismic communication).
At the other end of the scale, elephants are unable to hear above 12 kHz making them the animal with the lowest high frequency hearing limit of any mammal tested.
Localization of sound
Elephants are very good at localizing sounds. It has been suggested that the larger the space between an animal's ears (the inter-aural distance) the better the ability at localizing sound because the difference in the time and intensity of a sound reaching each ear can be used as cues in localizing sound. Elephants extend their ears perpendicularly to their heads in order to better localize sounds.
One juvenile Asian elephant whose hearing was tested was able to localise clicks and noise bursts to within 1 degree. She was less good at distinguishing tones, but was better able to distinguish lower frequency tones than higher frequency tones; below approximately 300 Hz she was able to localize tone within 10 degrees with 75% accuracy, 20 degrees with about 80% accuracy and 30 degrees with 90% accuracy.
|Last Updated on Wednesday, 09 January 2013 12:10|