We associate the rhythmic chug of a cat’s purr with contentment—and in many cases it is a sign of a happy cat. However, cats may also purr when they’re anxious, giving birth, or injured.
Kittens learn to purr at about one week old (before their eyes open), and biologists believe that it developed as a way for kittens to communicate to the mother that she needs to be still while they are feeding. The mother may join in, too, to reassure her young.
Cats also produce an urgent, “solicitous” purr, which they use when wanting to be fed by their owners. This sound is a mixture of the low-pitched rumble of a regular purr and a higher-frequency meow.
Analysis of the meow element shows that its frequency is similar to that of the cry of a human baby, which may help explain our willingness to feed an insistent cat. Among older cats, purring may also communicate nonaggression, vulnerability, or a request from one cat grooming another for it to stay still.