In domestic cats, many signals that occur when interacting with humans seem to originate from when the animal was dependent on the mother. Cats have been observed to purr for most of their lifespan, starting from when they were young and suckling from their mother. Purring may have developed as an evolutionary advantage as a signaling mechanism of reassurance between mother cats and nursing kittens. Post-nursing cats often purr as a sign of contentment: when being petted, becoming relaxed, or eating. Some purring may be a signal to another animal that the purring cat does not pose a threat.
Why do cats purr?
Cats purr for a variety of reasons. Purring can be a sign of contentment or comfort, for example when being pet, while relaxing or eating. It is thought that purring may have developed as an evolutionary trait signaling reassurance between the mother cat and its nursing kittens.