A Secret Language: Infrasonic Communication in Elephants
Elephants are highly intelligent, complex, and social animals, with intricate herd dynamics. They have evolved advanced communication abilities, and use a vast array of techniques to convey messages to other elephants, not only in their immediate vicinity, but sometimes across great distances. We have previously discussed the general sensory communication methods of elephants, including sight, sound, touch, and smell. In this post, we will introduce a sophisticated and highly specialised elephant communication ability – the production and detection of infrasound.
The frequency of sound is measured in Hertz (Hz), denoting the number of cycles per second, and the range of human hearing spans from approximately 20Hz (low) to 20,000Hz (high). Infrasound (“infra” from the Latin, meaning “below”) can be commonly defined as sounds comprised of sound waves with frequencies below 20Hz, which are thus inaudible to humans. Sound waves with frequencies higher than 20,000Hz (20kHz) are known as ultrasound (“ultra” from the Latin, meaning “beyond”). Frequencies between 1Hz and 20Hz are often referred to as near-infrasound, while frequencies below 1Hz are termed far-infrasound. When humans are exposed to sufficiently powerful near-infrasound, the inaudible stimulus may be felt in the body.
The use of infrasonic communication by elephant species was first discovered in the 1980s, when a zoologist named Katy Payne noticed an unusual vibration in the air while observing Asian elephants at a zoo in the USA. Further research by Payne and colleagues confirmed this suspicion by clearly linking low-frequency calls to otherwise inexplicable elephant behaviours. Although elephants utilise a vast array of vocal and auditory cues in a range of frequencies for communication, the ability to produce low-frequency and infrasonic sounds provides them with a number of advantages when communicating over long distances. These include – in addition to communication for routine social interaction – the avoidance of rival herds and predators, coordination of herd movements to improve resource availability, and the facilitation of mate finding for individuals in reproductive condition.
Elephants are not unique in their production of infrasound. Given the anthropocentric nature of the above definition, it should be unsurprising that numerous species are known communicate using sounds with frequencies outside of the limited audible range of humans. Bats use ultrasound for echolocation, while alligators, giraffes, hippopotamuses, peacocks, rhinoceroses, and whales are all known to utilise infrasonic communication. Incredibly, infrasonic calls produced by blue whales have been recorded crossing the entire Atlantic Ocean, from South America to Africa. It has been suggested that infrasound is used in nature for more than just communication – homing pigeons and migratory bird species may use naturally occurring infrasound as a navigational tool.
It has been speculated that the majority of sound-based elephant communication occurs in the infrasonic range. Scientists speculate that infrasound production originates in the vocal folds of the elephant larynx. Air flows through these folds and causes them to vibrate, producing low-frequency sound. The unique anatomical relationship between the length, mass, and elasticity of elephant vocal folds indicates that elephants have evolved the capacity to produce lower-frequency sound than any other terrestrial animal. Elephants have been shown to have the ability to produce and detect sound over the widest range of frequencies of all nonhuman mammals. In one experiment, a female elephant was shown to respond to frequencies as low as 17Hz and as high as 10,500Hz. Other studies have demonstrated elephants detecting infrasound below 10Hz.
The outer and inner ear anatomy of elephants is comprised of large, specialised components. The large sizes of these auditory structures likely plays a role in improving the elephants’ ability to detect low-frequency sound at long distances. In addition, the size and position of the ears likely offers elephants an advantage when determining the location (direction and range) of sounds, including infrasound. It is thought that elephants are able to localise sounds with an accuracy equal to that of humans (<1°).
Elephants evolved in primarily forested environments. Although modern elephants have adapted well to a variety of different environments, such as the grasslands and dry savannas that replaced prehistorical African forests around 10 million years ago, the morphological characteristics of their initial sensory development period remain largely unchanged. In dense forests, herd members may be required to separate from the main group, in order to pass through narrow passages, for instance. In such situations, the limited potential for visual communication made it imperative that elephants could communicate with each other in other ways. This likely led to enhanced senses of smell and hearing, though olfactory ability can also be somewhat limited in jungle and heavily forested areas due to the lack of wind. As a result, early elephants were likely reliant on hearing and vocal communication to direct their herds, locate each other, and warn other elephants of predators and other dangers.
High frequency sounds behave similarly to a ray of light. While the ability of an elephant to produce very loud sounds could allow them to communicate using high frequencies over long distances in unobstructed environments, in forests, the thick vegetation would block the sound, muffling it quickly. The longer wavelengths of low-frequency sound, however, allow it to avoid the trees and other vegetation that absorb and dissipate higher-frequency sound. Thus, elephants able to communicate using lower frequencies were more successful, and over millennia, they evolved the ability to produce and perceive infrasound over vast distances.
Infrasound production is not limited to animal communication and navigation. Human processes such as wind turbines, certain engines, and explosions can cause infrasonic sound. Furthermore, some abiotic natural processes can result in infrasound, including avalanches, earthquakes, volcanic activity, meteors, and the calving of icebergs. During the 2004 Indian Ocean earthquake and tsunami, elephants in Thailand, Sri Lanka, and India were reported to have displayed unsettled behaviour and moved to higher ground before the tsunami reached the coasts of those countries. It has been theorised that these elephants were able to react to infrasonic sounds from the earthquake or tsunami long before humans were aware of it. However, most of the evidence is anecdotal, and there are many possible explanations for the elephants’ behaviour which do not involve infrasound.
A nascent theory for which evidence exists is that of weather prediction by elephants. In both India and Kenya, traditional lore has long associated the beginning of a period of rain with the sudden appearance or sighting of elephants. Scientifically, both Asian and African elephants have recently been documented predicting thunderstorms before they occur, at distances of up to 150km. The movements within cumulonimbus clouds have been shown to produce strong infrasonic signals, and it is thought that elephants may be able to hear these and locate the storm. This ability may allow elephant herds to locate water sources during times of drought, as well as areas where there is likely to be food when resources are scarce. It may also account for strange, otherwise inexplicable, direction changes and patterns which have been observed in migratory African elephant herds.
Though there are other theories which may explain the ability of elephants to accurately predict the time and location of storms, such as detection of seismic activity in their foot pads, infrasound has undoubtedly played an important role in the evolutionary success of the elephant. Our recent discovery of elephants’ infrasonic communication abilities, combined with our technological ability to detect and map infrasound, will allow us to gain a more comprehensive understanding of elephant behaviour, herd movement, and social interaction. Hopefully, we can use this understanding to protect elephants from harm, and look forward to a peaceful future of cohabitation with these wonderful animals.
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