AUDIE CORNISH, HOST:
Finally, this hour, a trick question: What sound does an elephant make? If you have a toddler nearby, you're no doubt getting an imitation of this:
(SOUNDBITE OF ELEPHANT TRUMPET NOISE)
CORNISH: But what most kids, and their parents, don't know - because most of us have never been within hugging distance of an elephant - is that their most common form of communication is this:
(SOUNDBITE OF ELEPHANT RUMBLING NOISE)
CORNISH: Elephants can feel this kind of low rumbling from miles away, and researchers have long wondered how, exactly, they do it. Well, a new study published in the journal "Science," offers some answers. Tecumseh Fitch is a senior author of the study, and an evolutionary biologist at the University of Vienna. He says there are two, competing theories about how an elephant rumbles.
TECUMSEH FITCH: One is that elephants were using principles very similar to those of human beings, to make these low-frequency sounds; and the other is that they were doing something more similar to what a cat does when it purrs, which involves active muscular contractions. And we found that the latter hypothesis isn't necessary, to explain these infrasounds.
CORNISH: Well, give us a quick lesson, then, in how the human vocalization works - the human voice works. And then explain what you found with the elephant.
FITCH: OK. Well, basically, what you do when you make a sound like "aaah" is, you let the air coming out of your lungs pass between your vocal folds, also called your vocal cords; and that, in a kind of passive way, sets the vocal folds into vibrations. So they start slapping together. And the rate at which the vocal folds slap together, determines the pitch of the sound. So when I make a sound like "aaah," that's about 100 hertz, which means 100 cycles per second. That says that my vocal folds are moving back and forth, and slapping together, a hundred times per second.
So with elephants, they make these incredibly low-frequency sounds between 10 and 20 hertz, which are called infrasounds because they're below the range of human hearing.
(SOUNDBITE OF ELEPHANT RUMBLING NOISE)
CORNISH: So we just said, actually, that these are - these sounds are too low for the human ears, and yet we've played a clip. So explain; what is it that we were just hearing?
FITCH: Well, when you listen to music from, say, a low instrument - like a cello, or a bass - on little laptop speakers or in a small transistor radio, you can hear something. But what you can't hear is the sound that actually determines the pitch. So the lowest energy is actually something that's too low for those small laptop speakers to play back. So this is very similar. What we're hearing are - what are called the higher harmonics of the sound. But the fundamental frequency, which is what determines the pitch, isn't something that we can hear.
CORNISH: What, exactly, do elephants use this particular vocalization for? I mean, what role does it play in their language?
FITCH: Well, rumbles are actually a whole class of vocalization. So there are many different rumbles that elephants make, and they use them for quite a variety of things. So, for example, the kids make them, to make contact with their mother; and the mother will answer back with rumbles. Or males who enter musth - which is the reproductive stage, where they get kind of ornery and aggressive - will produce particular rumbles that advertise that. So it's actually a whole class of vocalizations. It's a whole set of different words, if you like, that can communicate different kinds of things.
CORNISH: Tecumseh Fitch - he's an evolutionary biologist at the University of Vienna. He explained to us the biology of low-frequency elephant calls. Thank you so much.
FITCH: Thank you.
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