A cat’s purr is unique, and we
are still not exactly sure how it works. It is believed that breathing in and
out alternately tenses and relaxes the muscles of the larynx and diaphragm,
creating pressure changes that result in turbulent airflow through the trachea.
These cyclic and rapid pressure changes are superimposed on normal breathing
and create the characteristic vibrations of purring. Other theories
suggest that purring is a rapid contraction of muscles in the larynx and
diaphragm in a constant rhythm.
Purring is instinctive. Kittens purr as early as 2 days of age. Large cats
such as lions do not purr well, but cheetahs can purr.
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A common misconception about purring is that it always indicates a state of
pleasure. Sometimes it does, but cats also purr when they are hungry, stressed,
or in pain. Cats have been known to purr just before dying. Some behaviorists
believe purring is a signal to other cats and animals that they are not a
Cat purrs are in the frequency range of 25 to 150 Hz. This frequency range
is also considered to be beneficial to healing. So, perhaps cats are attempting
to heal themselves at the cellular level.
WebMD Veterinary Reference from "Cat Owner's Home Veterinary Handbook"