Most visitors to nature reserves where elephants occur are familiar with the deep rumbling sounds made by these giants of the bush. As a matter of fact, elephants have a considerable repertoire of sounds used in communicating over large distances. These sounds can vary from sub-sonic rumbles to very powerful screams and trumpeting reaching 112 decibels, almost as much as a jetliner taking off from 65 m away. Some of the intriguing questions that arise are how do elephants produce this wide array of sounds and how can they hear sounds inaudible to the human ear?
Just like humans, elephants produce vocal sounds when air expelled from the lungs is passed over the vocal cords of the larynx or voice box.
In elephants the larynx is about 8 cm long. The vocal cords vibrate when the air moves over them producing a particular sound. In elephants various frequencies of sound is produced by shortening or lengthening the vocal cords. Elephants also possess a resonating chamber consisting of the trunk, mouth, tongue, larynx, nasal cavities and pharyngeal pouch that enables the animal to modify and amplify the sound produced by the vocal cords. How the elephant holds its head, and flaps its ears presumably also affect the musculature around the larynx, which in turn modifies a call to achieve the desired sound.
A mixture of higher frequency sounds can be produced depending on whether the elephant’s mouth is open or closed, whether the head is held high or low, the ears flapping slowly or rapidly, the position of the trunk and the speed and duration of air moving through the trunk.
Those low and deep rumbles that are often heard and referred to as stomach-rumbles are not the result of an upset stomach or knot in the digestive track, but very low frequency sounds that the elephants produce as part of their communication repertoire. Several physical-anatomical adaptations allow elephants to produce these low frequency rumbles.
First of all the large body of the elephant allows for a bigger resonating chamber and longer and looser vocal chords that contribute significantly to the production of these sounds. The bigger the resonating chamber and longer the vocal chords the deeper the sound.
The larger resonating chamber and longer vocal chords in elephants are attributed to the following features:
- The trunk in an adult male may add as much as three meters to the length of the resonating chamber.
- The loose arrangement of the musculature and cartilages that supports the tongue and larynx allows for greater movement and flexibility of the larynx.
- The loose arrangement of the tongue and larynx houses a structure unique to elephants called the pharyngeal (around the pharynx) pouch, situated at the base of the tongue, which further contributes to the production of low frequency calls.
The Pharyngeal Pouch
As mentioned before the pharyngeal pouch situated at the base of the tongue, is unique to elephants. Apart from the role it plays in producing low frequency sounds, it also provides elephants with an emergency source of water as elephants can store several liters of water in the pharyngeal pouch. On hot days or when tracking long distances over dry areas elephants are sometimes seen inserting their trunks down their throats and withdrawing water from their “stomachs”. In actual fact they withdraw water from the pharyngeal pouch. This water is then either drank or sprayed over the body as a cooling mechanism.
These low frequency rumbles of which the lowest components are below the lower limit of human hearing, travels further than higher frequency sound. It is therefore thought that the more powerful of these calls serve to communicate over long distances and thereby staying in touch with other elephants and family groups. The harmonic structure of these low frequency calls also allows elephants to determine the distance of the calling elephant.
At night an elephant is able to detect the call of another elephant almost 10 km away. During the day this distance drops dramatically to about 2 km because of the warmer surrounding air and wind.
Elephants are able to detect these low frequency sounds for several reasons. The large skull allows for longer ear canals, wider tympanic membranes and spacious middle ears. The amount of sound energy collected by the tympanic membrane (ear drum) increases with increasing membrane area, which in essence implies that the larger the tympanic membrane the better an animal is able to hear at low frequencies.