A cat's purr sits at the exact frequency range that medicine uses to help bones heal

Cats don't just purr when they're happy — they also do it when stressed, injured, or even dying, which has long puzzled animal behavior researchers.

Most domestic cats purr somewhere between roughly 25 and 150 hertz — a range that some studies on vibrational and ultrasound therapy have linked to stimulating bone density and healing soft tissue.

That's led to a popular hypothesis that purring may have evolved partly as a self-therapy mechanism — a way to speed up recovery at minimal energy cost, which would be especially useful for a predator that needs to conserve strength between hunts.

It remains a hypothesis, not a confirmed mechanism — it's hard to prove a real cause-and-effect link in cats themselves, rather than just a coincidence of frequencies, so scientists treat the theory with caution.