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Listening to Infrasound

Elephants at sunset

Sunrise, December 26, 2004.

Wit Aniwat, whose family runs an elephant camp for tourists in Thailand, was awakened by an unusual alarm clock: the trumpeting and wailing of elephants.

It was a bit out of the ordinary, but Wit thought nothing of it as the sun rose. There was work to do.

Five minutes later, another oddity. “The elephants became very agitated,” Wit recalls in NATURE’s Can Animals Predict Disaster? Astonished, he watched as the huge animals broke their chains and stampeded up a nearby hill. He and another trainer gave chase. But they hadn’t gotten very far when a terrifying sound overtook them: the sound of a towering wave of water crashing ashore and overwhelming everything in its path.

Luckily, Wit survived the tsunami. More than 200,000 other people around the Indian Ocean weren’t so lucky.

The elephants? They were fine too. Wit and many others believe it’s because they knew the wave was coming. And scientists say there’s a possibility that Wit is right. That’s because elephants are among a handful of animals known to be able to hear “infrasound,” the extremely low-frequency rumbles that are produced by natural phenomena from earthquakes and volcanoes to heavy winds and avalanches.

In fact, studies have shown that they use infrasound — which can travel vast distances through the ground, air, and water — to carry on long-distance conversations. Researchers have homed in on these “invisible” communications in just the last few decades, as sophisticated microphones and recording equipment allowed them to listen in.

Now, infrasound researchers wonder whether some animals can hear danger approaching. For instance, big storms such as hurricanes produce their own distinctive infrasonic signature. Similarly, earthquakes can produce several distinct infrasound pulses that can travel thousands of miles and much faster than water. Thus, tsunamis, also triggered by earthquakes, hit the shores only after infrasound.

In Thailand, it’s possible that Wit’s elephants picked up these signals before the wave hit, prompting them to trumpet their fear and then flee. But we may never know for sure. Elephants carrying radio tags in the region may have offered important insight. Unfortunately, researchers lost contact with the tagged elephants eight hours prior to the tsunami. When they regained transmission an hour after the tsunami hit, they found the elephants in the same vicinity as they were prior to the tsunami — just a few hundred feet from the shore. It is impossible to know exactly what happened while the transmitters were down.

Studies in zoos show that even animals known to hear infrasound don’t necessarily become agitated when they hear the signals. But researchers also note that the animals in zoos are so frequently subjected to infrasound, from their urban setting, that they may be desensitized. Scientists say carrying out experiments in the wild that might settle the matter once and for all would be very difficult and expensive.

But researchers are learning more about infrasound through other kinds of studies. Alligator researchers, for instance, are cracking the code that these huge reptiles use to signal their mates. Among other things, they’ve learned that alligators can produce an array of infrasonic signals by vibrating air inside special sound-producing sacs in their chins.

Other researchers are studying the idea that infrasonic sound can produce emotions in people. To test this, they asked people at a concert to rate their emotional responses to several pieces of music, some of which had been secretly “spiced” with infrasonic noises. More than a quarter of the listeners reported that the infrasonic melodies produced “ghostly” feelings of anxiety, uneasiness, sorrow, fear, and chills down the spine. Infrasonic sound can also make people nauseous and sick.

Those physical and emotional reactions may explain why horror movies used to feature scary, low-pitched organ music, the researchers say. And perhaps why animals too get scared when they hear a mysterious, infrasonic pulse.



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