There are few things in life as comforting as the music of a cat's purr. The combo of the rhythmic sound and the soft vibration is hypnotizing — plus, purring equals total kitty happiness, so what's more blissful than that? Well, we hate to rain on your parade, but it turns out a purr does not necessarily a happy kitten make.

But if cats don't purr just because they're happy, then why do they?

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The truth is there isn't one good reason. According to Leslie Lyons, associate professor at University of California-Davis school of veterinary medicine, “I would think maybe it's very similar to a human when we hum. We can hum when we're very happy, but also we hum when we're a little nervous and frightened to calm ourselves down. So maybe that's how a cat uses their purring.”

Kittens begin to purr when they’re just two days old and they can purr while they’re nursing. Behaviorists believe purring helps them communicate to their mother that all is well. That’s the type of purring you and I are most accustomed to — the little motor that runs when you’re playing or cuddling and petting a cat.

But purring can take on different meanings as a cat ages. Cats that are very frightened may purr. Female cats purr when they are delivering their kittens. Cats can also purr when they’re very ill or close to death. Many think it’s how cats help comfort themselves.

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It’s not only your standard pet cat that purrs. Dr. Benjamin L. Hart, distinguished professor emeritus at UC Davis vet school, notes that cats that purr cannot roar, and cats that roar cannot purr. Large cats that don’t roar, like pumas, cheetahs and mountain lions, also purr!

The truth is there are probably a lot of reasons that cats purr and it doesn't have to have a defined purpose. So just enjoy the comforting hum.

Originally posted September 2015. Updated October 2017.