The fact that cats purr is not a surprise, but there may be a surprising reason why they do it. Hint: It's not just because they are happy! Studies have shown that when cats purr, it can have a remarkable healing power, from tendon repair to bone healing. It all has to do with the frequency of that little cat purr motor.
How and why cats purr
Domestic house cats as well as cat species in the wild also purr; bobcats, cheetahs, Eurasian lynx, and puma purr although notably, lions, leopards, jaguars, and tigers do not. Believe it or not, scientists are not completely sure how and why cats purr. They do agree that the larynx (voice box), laryngeal muscles, and a neural oscillator are involved in making that sound that we interpret as contentment.
The Library of Congress explains that kittens begin to purr when they are as young as two days old. Veterinarians believe that this is an instinctual way for momma cat and her kittens to communicate. The purring says, "I am here, and I'm OK." Once a kitten learns how to purr, they don't stop. Cats purr from pleasure and contentment, but cats purr also when they are injured or in pain. In that case, it could be a self-soothing behavior, but it could also provide a source of natural healing due to the low frequency vibrations of the purr.
Purring and bone density
Scientists now believe that a cats purr is about more than communication. Studies have shown that purring may be linked to the strengthening and repairing of bones, relief of pain, and wound healing. An October 2001 study published in the Journal of the Acoustical Society of America studied 44 cats of several species. The cats' purr was recorded and analyzed in various ways.
Each feline species studied purred strongly in the range of frequencies between 25 and 150 Hz. These purr frequencies correspond to frequencies used in the treatment for bone healing, pain, edema, muscle growth and strain, tendon repair, dyspnea, and wounds. Furthermore, domestic cats, servals, ocelots, and pumas produce dominant, strong frequencies at exactly 25 Hz and 50 Hz.
These same four species have a strong harmonic frequency at exactly at or extremely near 100 Hz. The BBC explains that frequencies between 25 and 100 Hz correspond with established healing frequencies in therapeutic medicine for humans — bone healing occurs at frequencies between 25 and 50 Hz and skin and soft tissues respond to frequencies of around 100 Hz. The study's researchers concluded that the cats' purr was an internal mechanism that provided healing power.
The healing power of purring
Scientific research and literature is rare on this subject, but Wired reports that observations have been made of cats lying next to each other and purring when one is injured. There's also a theory that since both domestic and wild cats spend so much time doing nothing but lying around or sleeping, the frequency of their purr is an important mechanism for increasing bone density. Without regular use, bones can become weak over time, and this purring may stimulate their bone density and keep it at a healthy level.
Over the years, various devices that vibrate within the range of a cats purr have been proposed, for purposes of strengthening women's bones to protect them from osteoporosis to increasing the bone density of astronauts in low gravity orbits. A study published in the Menopause Review in March 2015 studied bone demineralization from postmenopausal osteoporosis, and concluded that vibrations can help increase the level of growth hormone. Whole body vibration is believed to be a stimulant of the process of bone formation.
Cats purr for more reasons than just happiness — purring may also help with healing their bones and tendons!
The specific frequency of a cats' purr, which is primarily at frequencies of 25 and 50 Hz with a primary harmonic at 100 Hz, corresponds to vibrations shown to have healing power in humans. Bone density and tendon repair happens at these frequencies in humans. Scientists theorize that cats may use their purring as an internal healing mechanism. The frequencies may also stimulate bone density to keep cats' bones strong, since they spend so much time lying around and sleeping.