Tigers appear to rely on booming low-frequency sounds – much of it inaudible to humans – to drive rivals away from their territory and to attract mates. The discovery may explain how the animals maintain large hunting territories, and may also help conservationists to protect the endangered animals.
Tigers produce a wide variety of sounds, from deep roars and growls to the raspberry-like “chuffing” they use to greet each other. A roar followed by a growl is probably designed to intimidate rivals.
Tiger watchers have long suspected that the animals’ vocal repertoire helps them maintain their hunting grounds. Now Ed Walsh and his colleagues at the Boys Town National Research Hospital in Omaha, Nebraska, have found that a common feature of all the tiger calls is the large amount of acoustic energy at low frequencies.
Low-frequency sound carries better than high frequencies. “It is less likely to be affected by climatic conditions such as humidity,” says Walsh. It is also less affected by ground cover, which is important for forest-dwelling tigers.
“An effective propagation distance of five miles [eight kilometres] is frequently quoted,” says Walsh, “but I don’t know of an actual study associated with the claim and doubt that anyone really knows the answer.” Walsh hopes to extend his study to provide one.
When the researchers analysed the frequency spectrum of the roar of a Siberian tiger, they found that the peak power was around 300 hertz and that it included components that stretched down into the infrasound range, below 20 hertz. “All of the utterances tend to contain acoustic energy in the low-frequency bands,” says Walsh.
The researchers confirmed the importance of low-frequency sounds to tigers by examining the auditory response of the brain of anaesthetised Siberian, Sumatran and Bengal tigers at Omaha’s Henry Doorly Zoo.
The responses of the three subspecies were similar, and show that the animals’ peak response to sound is around 500 hertz. Human hearing is most sensitive to frequencies around 1000 hertz. The shape of tigers’ inner ear confirms that they can almost certainly hear infrasound frequencies, Walsh says.
The long-term aim of the project is to discover whether the sounds made by individual tigers are distinctive enough to act as an “acoustic fingerprint”. If so, conservationists would be able to carry out censuses of the animals by placing microphones in the forest.
The problem is very complex, says Walsh. But he hopes that the loudness and duration of the sounds, coupled with their frequency distribution, will be distinctive enough to distinguish one individual from another.
“Certainly tigers can recognise each other. Females with cubs have to make sure that this is the guy they want to have around,” says Walsh. “Keepers at the zoo can recognise different tigers from the sounds they make.”
The new research was presented at the Acoustical Society of America’s annual conference in Nashville, Tennessee.