A tiger's intimidating roar has the power to paralyze the animal that hears it and that even includes experienced human trainers. Elizabeth von Muggenthaler, a bioacoustician from the Fauna Communications Research Institute in North Carolina, presented her research at the Acoustical Society of America meeting in Newport Beach, California on December 7. Bioacoustics is the study of the frequency or pitch, loudness, and duration of animal sounds to learn about an animal's behavior. At the meeting, von Muggenthaler discussed her work analyzing the frequency of tiger sounds to better understand the part of a tiger's roar that we can feel, but can't hear.

Why study something that we can't hear?

"Humans can only hear some of the sounds that tigers use to communicate," says von Muggenthaler. "Humans can hear frequencies from 20 hertz to 20,000 hertz, but whales, elephants, rhinos, and tigers can produce sounds below 20 hertz." This low-pitched sound, called "infrasound," can travel long distances permeating buildings, cutting through dense forests, and even passing through mountains. The lower the frequency, the farther the distance the sound can travel. Scientists believe that infrasound is the missing link in studying tiger communication.

In the first study of its kind, von Muggenthaler and her colleagues recorded every growl, hiss, chuff, and roar of twenty-four tigers at the Carnivore Preservation Trust in Pittsboro, North Carolina, and the Riverbanks Zoological Park in Columbia, South Carolina. Bioacousticians found that tigers can create sounds at about 18 hertz and when tigers roar they can create frequencies significantly below this. "When a tiger roars-the sound will rattle and paralyze you," says von Muggenthaler. "Although untested, we suspect that this is caused by the low frequencies and loudness of the sound."

When the researchers played back a tape of recorded tiger sounds including audible and infrasounds, the tigers appeared to react to these sounds. Sometimes they would roar and leap towards the speakers and sometimes sneak away. The next step for von Muggenthaler is to take the recorded infrasounds to scientists who can determine whether or not tigers can hear the infrasounds. Von Muggenthaler hopes to learn more about tigers, protect them from extinction, and understand the unheard, paralyzing power in their roar.