Cat – Magnus Rosendahl, public-domain-photos.com

Purring would not have evolved unless it provided some sort of survival advantage. It has been commonly assumed that cats purr to express contentment, but this doesn’t explain why cats also purr when giving birth, frightened, or severely injured.

Although there has been little research conducted into the extraordinary self-healing ability of cats, evidence for the benefits of purring is mounting. There are many indications that the vibrational frequency of a cat’s purr could provide healing and perhaps even health protection benefits not only for cats but for humans as well. Specifically, researchers have found that consistent vibrational sound frequencies of 25-150 Hz, which is the range of a cat’s purr, aid in the healing of bones, tendons, ligaments, and muscles, as well as providing pain relief (Von Muggenthaler, 2001).

Purring Increases Bone Strength, Speeds Healing

Cats’ bones heal faster and more easily after fractures than those of dogs. This is evident in the fact that 90% of cats that plummet from extraordinary heights survive despite serious injuries (Whitney & Mehlhaff, 1987). There is also evidence that cats are less likely to suffer postoperative complications after surgery than dogs, and this rapid healing ability may be attributable to purring (Von Muggenthaler, 2001).

Cats suffer far less often from diseases of the bones than dogs, and given the effect that the purr frequency has on bones, it’s likely that purring plays a role in this. There are a number of osteo diseases that are rare in cats but common in dogs, including scapulohumeral joint luxations and hip dysplasia. Cats are also less likely to suffer from osteosarcoma, osteoarthritis, and myeloma (a tumour of the bone marrow’s plasma cells)(Von Muggenthaler, 2001).

Dr. Clinton Rubin and his colleagues have discovered that sound frequencies of 20-50 Hz can increase bone density, and an amusing study in which the researchers placed chickens on a vibrating plate for 20 minutes each day found that the chickens grew stronger bones as a result (Von Muggenthaler, 2001). This finding was replicated in a study of rabbits, in which bone strength increased by 20% after exposure to the 20-50Hz sound frequency. The study also found that the healing of broken bones and the speed of bone regeneration were accelerated by this vibrational frequency (Chen, Han, & Yang, 1994).

The effect of the purr frequency on bones has significant implications, given the large number of people who suffer from osteoporosis (bone loss) as they age. Dr. Rubin and his colleagues have continued to conduct research in this field that may yield treatments for osteoporosis and other bone-related problems in humans (Rubin et al., 2001; Rubin et al., 2004; Rubin, Judex, & Qin, 2006).

Additional Benefits of Purring

Research indicates that purring can help decrease dyspnea, or shortness of breath, a symptom of chronic obstructive pulmonary disease. One study found that among dogs and cats suffering from myocardial necrosis, all of the dogs had dyspnea, but none of the cats suffered from this condition (Kidd, Stepien, & Amrheiw, 2000). The incidence of primary lung tumours is also three times higher in dogs than in cats (Miles, 1988). Additionally, researchers have found that in humans, a vibration of 100Hz, which is in the purr range, can decrease dyspnea (Cristiano & Schwartzstein, 1997; Nakayama et al., 1998; Sibuya et al., 1994).

Cats also suffer from diseases afflicting the muscles and ligaments far less often than dogs, and low-decibel frequencies similar to that of a cat’s purr have proven beneficial for the healing of muscles and tendons in humans (Von Muggenthaler, 2001). One study found that following sports injuries, low-frequency biomechanical stimulation can prevent decreases in muscle strength and mass (Lake, 1992). Another found that the purr-frequency vibration can speed tendon healing in the ankle, increasing upper ankle joint mobility by up to 19% after injury (Klysczt et al., 1997). In addition, exposure to a sound frequency between 50 and 150 Hz has been found to provide relief for 82% of those suffering chronic and acute pain (Lundeberg, 1983). Thus, it is unsurprising that cats often purr when they are injured or giving birth.

Further Reading

For more information on cats, see Cat Heroes, Unusual Cat Breeds, and Cat Communication.

References:

• Chen, L.P.; Han, Z.B.; & Yang, X.Z. (1994). “The Effects of Frequency of Mechanical Vibration on Experimental Fracture Healing.” Zhonghua Wai Ke Za Zhi (Chinese Journal of Surgery), 32(4), pp. 217-219.
• Cristiano, L.M., & Schwartzstein, R.M. (1997). “Effect of Chest Wall Vibration on Dyspnea During Hypercapnia and Exercise in Chronic Obstructive Pulmonary Disease.” American Journal of Respitory Critical Care Medicine, 155(5), pp. 1552-1559.
• Kidd, L.; Stepien, R.L.; & Amrheiw, D.P. (2000). “Clinical Findings and Coronary Artery Disease in Dogs and Cats with Acute and Subacute Myocardial Necrosis: 28 Cases.” Journal of the American Animal Hospital Association, 36(3), pp. 199-208.
• Klyscz, T.; Ritter-Schempp, C.; Junger, M.; & Rassner, G. (1997). “Biomechanical Stimulation Therapy as Physical Treatment of Arthrogenic Venous Insufficiency.” Hautarzt, 48(5), pp. 318-322.
• Lake, D.A. (1992). “Neuromuscular Electrical Stimulation. An Overview and Its Application in the Treatment of Sports Injuries.” Sports Medicine, 13(5), pp. 320-336.
• Lundeberg, T.C. (1983). “Vibratory Stimulation for the Alleviation of Chronic Pain.” ACTA Physiologica Scandinavica, 523(Suppl.), 1-51.
• Lyons, L. (3 April 2006). “Why Do Cats Purr.” Scientific American, ScientificAmerican.com.
• Miles, K.G. (1988). “A Review of Primary Lung Tumors in the Dog and Cat.” Veterinary Radiology, 29(3), pp. 122–128.
• Nakayama, H.; Shibuya, M.; Yamada, M.; Suzuki, H.; Arakawa, M.; & Homma, I. (1998). “In-Phase Chest Wall Vibration Decreases Dyspnea During Arm Elevation in Chronic Obstructive Pulmonary Disease Patients.” Internal Medicine, 37(10), pp. 831-835.
• Rubin, C.T.; Judex, S.; & Qin, Y.X. (2006). “Low-Level Mechanical Signals and Their Potential as a Non-Pharmacologic Intervention for Osteoporosis.” Age and Ageing, 35(Suppl.), pp. 32-36.
• Rubin, C.T.; Recker, R.; Cullen, D.; Ryaby, J.; McCabe, J.; & McLeod, K.J. (2004) “Prevention of Post-Menopausal Bone Loss by a Low Magnitude, High Frequency Mechanical Stimuli: A Clinical Trial Assessing Compliance, Efficacy and Safety.” Journal of Bone & Mineral Research, 19(3), pp. 343-351.
• Rubin, C.T.; Turner, A.S.; Bain, S.; & Mallinckrodt, M. (2001). “Anabolism. Low Mechanical Signals Strengthen Long Bones.” Nature, 412(6847), pp. 603-604.
• Sibuya, M.; Yamada, M.; Kanamaru, A.; Tanaka, K.; Suzuki, H.; Noguchi, E.; Altose, M.D.; & Homma, I. (1994). “Effect of Chest Wall Vibration on Dyspnea with Chronic Respiratory Disease.” American Journal of Respitory and Critical Care Medicine, 149(5), pp. 1235-1240.
• Von Muggenthaler E., Fauna Communications Research Institute. (2001). “The Felid Purr: A Bio-Mechanical Healing Mechanism.” Presented and published in the proceedings from the 12th International Conference on Low Frequency Noise and Vibration and its Control held in Bristol, UK, 18th to 20th September, 2006. Also Presented at the 2001, 142nd annual Acoustical Society of America, American Institute of Physics, International Conference.
• Whitney, W.O, & Mehlhaff, C.J. (1987). “High-Rise Syndrome in Cats.” Journal of the American Veterinary Medical Association, 191(11), pp. 1399-1403.