A cat's purr vibrates at a steady low frequency that is easy to feel while petting.

The longer version

Purring is produced by rapid contractions of the muscles in the larynx, which open and close the airway about twenty-five times a second. Each cycle modulates the airflow from the lungs, and the resulting pulses give the sound its characteristic rumbling quality.

The vibration falls in a low band that transmits well through contact, so it is felt more than heard when an animal is resting against a person. Cats purr in many situations, including nursing, greeting and recovery from injury, which suggests the behaviour serves general communication and perhaps self-soothing rather than only contentment. The low frequency may also help conserve energy, since producing it requires little effort over long periods.

Where this entry sits

This collection gathers small surprises about mammals, reptiles, amphibians, birds, fish, insects, and the pets living alongside people. Expect odd details about senses, anatomy, and everyday behaviour that many people never notice in the animals around them. The full list sits on the Animals page, where every entry is listed in order.