If you have found your cat's purring to be a bit annoying in the morning when he wants you to get up but not when you are petting him, it is because the two are different! Cats learned to add a higher pitch purr to the lower 25 Hz pitch that is more of a cry-meow. This insistent purr is intended to elicit a faster reaction from humans. Researchers theorize that cats may have learned to tap into a mammalian response for nurturing offspring by embedding a cry within a call that is normally associated with contentment. The baby who wants to be fed cries, hence cats learn to add the high pitch to their purr.
Cats also purr when they are giving birth, nursing or wounded. Researchers have shown that purring may have an evolutionary healing advantage. Many experts theorize that the range of 25 Hz might be a sort of built-in physical therapy. This frequency is used in humans to accelerate wound healing and improve bone density. Purring may be a form of pain management and self-healing. Because cats have adapted to conserve energy via long periods of rest and sleep, it is possible that purring is a low energy mechanism that stimulates muscles and bones without a lot of energy, too. It may contribute to the lower occurrence of osteoporosis or bone dysplasias in cats than dogs.
Purring may also have contributed to the fact that there are more companion cats than dogs these days. We regularly pet our cats for their sake but also for the sense of peace and relaxation that comes from listening to a cat purr. It calms us down, lowers our blood pressure, and reduces the risk of heart attack.