Wednesday, June 18, 2014

Low Blows: How Elephants Produce Low Frequency Vocalizations

By Sophie Wasserman


Last month, we introduced you to the wide variety of elephant vocalizations (link)--from very low frequency rumbles to higher frequency trumpets and chirps. But at least one question still remains: how do elephants produce this incredible range of sounds?
            To get a better understanding of how elephants vocalize, let’s first examine the basics of sound production in mammals. Remember from high school physics class that sound is a travelling mechanical wave, displacing the molecules of different media like air or water and causing them to vibrate. We can perceive sound because our ears contain membranes that are designed to translate this vibrational energy into signals our brain can recognize-- but that’s a whole different blog post! The important thing to note is that in order to make a sound, you need something for the sound to travel through, like air. Hence, vocalization production starts with the lungs. Elephants have an incredible lung capacity, taking in an average of 8-10 gallons of air per breath, or about 60-80 times greater than that of the average human.

Boonsri is amazed at how much air her lungs hold!

            Of course, what goes in must also come out, and so in the next step of vocalizing, the air is expelled from the lungs by contractions of the surrounding muscles and exhaled through the trachea to the larynx. The larynx, also called the voice box, is mostly made up of cartilage and muscle. It is found in mammals, reptiles, and even amphibians. The key components to vocalizations are two small membranes at the top of larynx called the vocal folds, or vocal cords. In humans, the pressure of the breath builds up and forces the vocal folds to separate until enough pressure has been released that they can recoil and come together again. This cycle keeps repeating naturally and without direct muscle control, causing the vocal cords to vibrate and thus create sound waves. We modulate our voice by loosening or tightening these folds, creating lower or higher sounds, respectively.
Elephant's vocal cords and larynx
Via Shoshani (1998)

Cats, on the other hand, have to actively contract the muscles of their larynx to produce their characteristic purr. Until recently, it was unclear whether an elephant’s rumble was produced in the same way as a human’s hum or a cat’s purr, but a study published in 2012 seems to indicate that they fall in line with humans. To test their theory, the researchers took the larynx of a zoo elephant who had died of natural causes and hooked it up to an aluminum tube. When they passed air through the vocal cords, the larynx produced sounds similar to that of a typical elephant rumble, demonstrating that the active muscle control of purring is not necessary for elephants to produce sound.

Cats use active muscle control of their larynx to purr
via Wikimedia Commons

The final step in the process of producing vocalizations is fine-tuning and modulating sound with the mouth. For humans, the positioning of our tongue and teeth can differentiate between consonants and vowels. In the elephant’s case, the way in which it holds its head, mouth, tongue, and trunk can modify the type and frequency of sound that it produces. For example, an Asian elephant will open her mouth when rumbling but will close off the tip of her trunk when chirping and squeaking. Most trumpets are produced with trunk raised, though this could be a behavioral cue instead of, or as well as, a way to modify the vocalization produced.

Elephants modulate sounds with trunk and mouth positioning

Still the question remains, how can elephants go so low? In other words, how do their rumbles contain infrasonic components as low as 10-12 Hz, or sound waves at a frequency below the human hearing range and about 3 octaves below the typical frequency of a human male’s voice? The first reason is that they’re huge animals. For instance, compare a violin to a cello: the longer the string and the larger the resonating chamber, the lower pitched the sound. Elephants, by nature of their size, have naturally longer vocal cords of about 7.5-10 cm in length compared to the measly 12-24 mm of humans. They also have a built-in resonance chamber in their two-meter long trunk, increasing the volume of air the sound passes through and lowering the pitch of the sound produced. Finally, the elephant’s hyoid bone, which provides support for the tongue and larynx, is comprised of five bones rather than the typical nine, and it is not attached to the skull directly, but atypically connected by ligaments and tendons. This enables greater flexibility of the larynx, which in turn allows their vocal cords to stretch even beyond what you would expect and produce even lower sounds.

Elephant vs human larynx
via Herbst (2013)

All of these adaptations, which enable the ultra-low frequency rumbles, allow for the communication of elephants across vast expanses, a necessity born of the structure of elephant society. Female elephants, who live within a familial herd, often use rumbles to gather together disparate herd members while male elephants will emit distinct rumbles during musth (their peak of sexual activity) to communicate with potential mates from far away, since they tend to live on their own or with loosely bonded male compatriots. In this way, the physiology of elephant vocalizations reflects the challenges and limits of their physical and social environments. But the production of the sound is only half the story; next month we’ll look at how the elephants on the other end of the communication perceive and localize the calls of their kin. 


References

Herbst, C. T., Stoeger, A. S., Frey, R., Lohscheller, J., Titze, I. R., Gumpenberger, M., & Fitch, W. T. (2012). How low can you go? Physical production mechanism of elephant infrasonic vocalizations. Science,337(6094), 595-599.

http://www.elephantvoices.org/elephant-communication/acoustic-communication.html

http://www.npr.org/templates/story/story.php?storyId=129083762
http://www.elephantsforever.co.za/elephants-respiratory-system.html#.U6E6N-Dam_w
http://onwisconsin.uwalumni.com/features/an-elephant-never-begets/
http://news.psu.edu/story/141213/2009/10/19/research/probing-question-why-do-men-have-deep-voices



Photos:
Shoshani, J. 1998. Understanding proboscidean evolution: a formidable task. Trends in Ecology & Evolution 13, 480-487.
Cat by Sameer.S via Wikimedia Commons
Herbst, C. T., ┼ávec, J. G., Lohscheller, J., Frey, R., Gumpenberger, M., Stoeger, A. S., & Fitch, W. T. (2013). Complex vibratory patterns in an elephant larynx. The Journal of experimental biology216(21), 4054-4064.

No comments:

Post a Comment