New York, August 15 : Cat's purr is still under zoological mystery and it is still unknown why cats purr.
The most typical explanation of purring is that it is an expression of happiness and contentment. But cats also purr for many other reasons.
"Cats often purr while under duress, such as during a visit to the veterinarian or when recovering from injury. Thus, not all purring cats appear to be content or pleased with their current circumstances," io9 quoted UC Davis professor of veterinary medicine Leslie Lyons as writing.
Famous animal behaviourist Paul Leyhausen, who studied cats for several decades, suggested that purring is one way cats communicate with each other, signalling that they don't wish to fight.
A recent study found that cats emit a special kind of purr when they want humans to feed them.
What we do know about purring is how it sounds - and, to a certain extent, how it works.
Swedish linguist Robert Eklund is fascinated by the kinds of sounds that animals (including humans) make when they inhale. These sounds are called "ingressive", and Eklund and his colleagues have proposed that we define purring as "continuous sound production must alternate between pulmonic egressive and ingressive airstream (and usually go on for minutes)."
In other words, to count as a purr, the cat must make the sound as it exhales and inhales.
Eklund has recorded countless cat purrs, and found that domestic cats purr at between 20.94 and 27.21 Hz while exhaling and between 23.0 and 26.09 Hz while inhaling. Purring also produces strong harmonics.
Frustratingly, nobody is really quite sure what causes the purring noise in cats. There is no "purring organ", or specialized part of the cat throat that's responsible for this irresistible noise.
Some veterinary researchers have suggested that purring is created by the muscles of the larynx, which could be dilating and constricting the cat's vocal chords. But nobody knows for sure.
Elizabeth von Muggenthaler, a bioacoustics researcher at Fauna Communications, has suggested that cats may purr in order to heal themselves.
"Frequencies between 20-140 Hz are therapeutic for bone growth/fracture healing, pain relief/swelling reduction, wound healing, muscle growth and repair/tendon repair, mobility of joints and the relief of dyspnea. Fauna Communications has recorded many cats' purrs, at a non-profit facility and the Cincinnati Zoo, including the cheetah, puma, serval, ocelot and the domestic house cat," she wrote.
"After analysis of the data, we discovered that cat purrs create frequencies that fall directly in the range that is anabolic for bone growth," she added.