Why Do Cats Purr?
Cats purr primarily to communicate contentment, but also when stressed, injured, or seeking comfort. The sound is produced by rapid movement of the laryngeal muscles, causing the vocal cords to separate during breathing. Interestingly, purring may have healing properties - the vibration frequency (25-150 Hz) can promote bone density and tissue healing.
Key Takeaways
- The purring mechanism involves the brain sending rhythmic signals to the laryngeal muscles, which open and close the glottis (space between vocal cords) about 25-150 times per second.
- Not all cats can purr - big cats that roar (lions, tigers) cannot purr continuously, only during exhales.
- Kittens can purr at just a few days old, using it to communicate with their mother.
Explanation
The purring mechanism involves the brain sending rhythmic signals to the laryngeal muscles, which open and close the glottis (space between vocal cords) about 25-150 times per second. As air passes through during both inhaling and exhaling, it creates the continuous purring sound unique to cats.
Cats purr in many situations: when content and relaxed, when nursing or being nursed, when greeting humans or other cats, and when stressed or injured. The latter suggests purring is not only about happiness but may be a self-soothing behavior and potentially a healing mechanism.
Research suggests that frequencies in the 25-150 Hz range can promote bone healing, reduce pain and swelling, and help heal wounds. This may explain why cats purr when injured - they may be self-healing. Exposure to purring may also reduce stress and lower blood pressure in humans, which is why cats can be therapeutic companions.
Domestic cats have developed a specialized 'solicitation purr' that they use specifically with humans when seeking food. This purr embeds a higher-frequency cry (similar to a baby's cry at around 380 Hz) within the normal low-frequency purr. A 2009 study at the University of Sussex found that humans rated this mixed purr as more urgent and less pleasant than a regular contented purr, even people who had never owned cats. This suggests cats have evolved a targeted vocal strategy to manipulate human caregivers.
The purring trait appears to be linked to the structure of the hyoid bone in a cat's throat. Small cats (domestic cats, bobcats, cheetahs) have a completely ossified (rigid) hyoid bone that allows continuous purring during both inhalation and exhalation. Big cats that roar (lions, tigers, leopards, jaguars) have a partially cartilaginous hyoid bone and a specially adapted larynx that enables roaring but prevents true continuous purring. The cheetah, despite being large, has the rigid hyoid structure and purrs rather than roars.
Things to Know
- Not all cats can purr - big cats that roar (lions, tigers) cannot purr continuously, only during exhales.
- Kittens can purr at just a few days old, using it to communicate with their mother.
- Some cats purr very quietly while others are quite loud - it varies by individual.
- A cat's purr volume typically ranges from 25 to 50 decibels, about the level of a quiet conversation, though the Guinness record holder reached 67.8 decibels.