It's easy to assume that cats purr because they're happy. After all, when your kitty contentedly curls up in your lap for some well-deserved scratches and rubs, she's obviously one happy feline.

However, cats also purr when they’re frightened or feel threatened, such as during a visit to the vet.

Veterinarian Kelly Morgan equates this reaction with smiling. "People will smile when they’re nervous, when they want something, and when they’re happy, so perhaps the purr can also be an appeasing gesture," Morgan told WebMD.

A cat’s purr begins in its brain. A repetitive neural oscillator sends messages to the laryngeal muscles, causing them to twitch at a rate of 25 to 150 vibrations per second. This causes the vocal cords to separate when the cat inhales and exhales, producing a purr.

But not all cats can purr. Domestic cats, some wild cats and their relatives — civets, genets and mongooses — purr, and even hyenas, raccoons and guinea pigs can purr. However, cats that purr can’t roar, and cats that roar can’t purr because the structures surrounding roaring cats’ larynxes aren’t stiff enough to allow purring.

Roaring cats evolved this way for good reason. These cats move around a lot to catch prey, so they developed their roar to protect their prides and their territory. Purring cats, on the other hand, are smaller and more likely to be loners that don’t have to compete with each other for prey. They use scent to mark territory and don’t need a far-reaching way to communicate.

Communicating and healing

Researchers have found that purring can help a cat heal. rolehcab/Shutterstock

However, your cat may also purr to communicate with you. According to researchers at the University of Sussex, domestic cats can hide a plaintive cry within their purrs that irritates their humans while appealing to their nurturing instincts.

The team examined the sound spectrum of 10 cats’ purrs and found an unusual peak in the 220- to 520-hertz frequency range embedded in the lower frequencies of the usual purr. Babies' cries have a similar frequency range at 300 to 600 hertz.

Karen McComb, who headed the study, said cats may be exploiting "innate tendencies in humans to respond to cry-like sounds in the context of nurturing offspring."

Why would your feline do this? "Cats apparently learn to do this to get people to feed them sooner," said veterinarian Benjamin L. Hart.

Cats’ purrs are more than simply a way to communicate though. Scientists like Elizabeth von Muggenthaler, a bioacoustics researcher, believe that cats also purr to heal themselves.

She says that frequencies between 24-140 vibrations per minute are therapeutic for bone growth, pain relief and wound healing. She recorded a variety of cat purrs, including those of domestic cats, ocelots, cheetahs and pumas, and discovered that the animals’ purrs all fit into the range for bone regeneration.

In addition to repairing bones, there's also evidence that the series of vibrations caused by purring can repair muscles and tendons, ease breathing, and reduce pain and swelling.

Purring isn’t just good for cats though — it’s also healthy for cat owners. Studies show that cats do a better job of relieving stress and lowering blood pressure than other pets. In fact, a 10-year study at the University of Minnesota Stroke Center found that cat owners were 40% less likely to have heart attacks than non-cat owners — and purring might play a role in that.

"Purring is an auditory stimulus that people attribute to peacefulness and calmness," Dr. Rebecca Johnson, director of the Research Center for Human Animal Interaction, told WebMD. "That gives us positive reinforcement for what we’re doing and can contribute to the whole relaxation effect when we interact with our cats."

Ever wondered why your cat meows, chirrups, hisses and growls? We have the answers.

Be sure to check out the videos below to hear some special purrs:

Smokey earned a place in the record books with a purr that measured 67.7 decibels, but he's been recorded on previous occasions with a 92.7-decibel purr, which is equivalent to the noise of a lawnmower or a hairdryer.

Linguist Dr. Robert Eklund records Caine the cheetah as part of his research on purring.