Tigers use infrasound warn off rivals

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Tigers use infrasound warn off rivals

 

Exclusive from New Scientist Print Edition

 

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.

 

Peak power

 

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.