Cats purr in many situations: a mother nursing her kittens, being petted or fed, visiting the vet, or recovering from an injury. Cats purr when they are happy, even while they eat, but a cat can also purr when it is hurt, hungry or nervous, so a purr does not always mean a happy cat.

For about half a century, scientists thought a purr needed the muscles of the voice box to tighten and relax in a steady rhythm, with the brain giving constant orders. A 2003 Scientific American answer said cats make the purr through on-and-off signals to the laryngeal and diaphragmatic muscles. It said a purr runs through both breathing in and breathing out at 25 to 150 hertz.

In 2023 a study in Current Biology reported that cat voice boxes taken from cats that had died could make purr-range sounds when air was blown through them. There were no brain signals and no muscle action. A Vienna team found a special pad inside a cat's vocal folds that may explain how such a small animal can make sounds as low as 20 to 30 hertz.

Why do cats purr at all? The 2003 Scientific American answer argued that it is more likely a purr is a way to communicate, and perhaps a way for the cat to help itself heal, than a sign that the cat is simply happy. One idea says purring helps a cat heal. The answer reported that sound frequencies in the purr range can improve bone density and promote healing, though this does not prove why cats purr.

Cats can make a special urgent purr when they want food. A cry or meow is tucked inside an otherwise soft, low purr.

Roaring and true purring need different throat structures, which is part of why lions roar and house cats purr. In small cats such as house cats, bobcats, lynxes, cheetahs and cougars, the hyoid bone is fully hardened. When the voice box vibrates it can make this bone resonate in a low rumble. Cheetahs have a fully hardened hyoid bone that lets them purr when they breathe in and when they breathe out.

Snow leopards cannot roar like lions or tigers, and they make softer sounds such as chuffs, meows and purr-like sounds. Lions use roaring to coordinate how their pride behaves and to defend the area they share.

Cat communication can be sorted into four kinds: visual signals, touch, sound and smell. A tail held out and moving slowly from side to side may show frustration. A crouching cat thumping its tail on the ground is in a classic defensive posture. Cats' ear flaps, called pinnae, can rotate independently forward, backward and sideways to find where a sound comes from.

In a University of Sussex study, cats were more likely to slow blink at their owners after their owners slow blinked at them than when the owners did not interact. Relaxed cats often blink slowly and half close their eyes.

Words to know

• purr: soft cat hum
• hyoid: throat bone
• pinnae: ear flaps
• resonate: ring with sound
• frustration: feeling blocked