When your cat climbs into your lap, tucks in his paws and begins to purr, all is right with the world. This is one thing we love about our cats: that feeling of contentment they share with us. When they become soft purring bundles of warm reassuring fur, we feel calmer and more peaceful. We may not always hear it – a soft vibrating rumble – but we can feel it. But why do cats purr? And what produces this characteristic sound?

According to veterinarian Bruce Fogle, author of The Cat's Mind, the original function of purring was to enable a kitten to communicate with his mother that all is well. A kitten is able to purr by the second day of life, and although he can't meow and nurse at the same time, he can purr and nurse. And the mother cat often purrs back, probably to reassure the kitty.

There are many theories to explain how the purr is generated. Some feel that it is a vibration of the soft palate at the back of the mouth due to increased blood flow. One study determined that purring results from nerve activation within the voice box. Veterinarian Neils C. Pederson, author of Feline Husbandry says purring originates from within the central nervous system and is a voluntary act. In other words, cats purr only when they want to.

Purring is an integral part of the feline communication system and occurs for a variety of reasons. It is classified within the "murmur vocalization" group, which involves sounds produced by a cat while the mouth is closed. In addition to purring, this group includes grunting, calling and acknowledgment murmurs. Domestic cats and some wild cats like pumas and mountain lions (almost any big cat that cannot roar) are all able to purr.

As the cat matures the meaning of the purr changes. Some cats purr to indicate contentment or pleasure, but badly frightened cats purr, severely ill cats purr, and female cats purr while delivering their kittens. It is not uncommon for cats to purr around the time prior to death. This final purring may indicate a state of anxiety or possibly euphoria, which has been described in terminally ill people.

Animal behaviorists believe that when cats purr under stressful circumstances they are reassuring or comforting themselves, much like humans sing to themselves. Frightened cats may purr to communicate submissiveness or non-aggressive intentions. A feral cat may purr to signal that he will not attack and other cats need not feel threatened. Older cats may purr when they play or approach other cats, signaling that they are friendly and want to come closer.