The Purring Cat
The cat’s purr has been fascinating to us for many years. It wasn’t until recently that we have come to understand its origin and perhaps its meaning too.
Historically, the purr has created much speculation. It was thought that the sound originated from fremitus, which is caused by disrupted blood flow in one of the major blood vessels (the aorta or caudal vena cava). Theoretically, that vessel formed a sharp bend when the cat arched its back while being pet. Science proves this theory wrong. The vessels don’t develop a sharp bend; they maintain their lumen size by arching. The sound really comes from the larynx (voice box) as do the other sounds a cat makes. To produce the purring sound, the size of the airflow opening narrows, creating turbulence.1 This explains why the purr can be loud if there is a great deal of constriction or soft if the opening is only slightly constricted. It also explains why some cats purr during inhalation and some during exhalation.
The other big question that comes up is why cats purr. What is its meaning? Obviously, we can never know because we can’t ask them; however, we can look at the situations in which cats purr and make educated guesses.2, 3 Queens purr while their kittens nurse. Cats use a “greeting” purr when they encounter friendly cats or people and when they are being pet. These are times we tend to think are pleasurable for the cat. But not all purr bouts seem to be associated with potentially pleasurable events. Some cats purr at times they seem to want something, usually food. This has been called a “request” purr. Perhaps this purr is the feline equivalent of a human’s smile.4 We usually smile when things are pleasurable, but we can also put on that fake smile when we want something from someone too.
1 Remmers, J.E. and Gautier, H. (1972): Neural and mechanical mechanisms of feline purring. Respir. Physiol. 16: 351-361.
2 Moelk, M (1944): Vocalizing in the house cat: a phonetic and functional study. Am J. Psychol. 57: 184-205.
3 Moelk, M. (1979): The development of friendly approach behavior in the cat: A study of kitten-mother relations and the cognitive development of the kitten from birth to eight weeks. Adv. Study Behav. 10: 163-224.
4 Beaver, B.V. (2003): Feline Behavior: A Guide for Veterinarians, 2nd ed. Saunders: St. Louis, p. 101-102.
Bonnie Beaver, DVM, DACVB, is graduate of the University of Minnesota College of Veterinary Medicine who lives in College Station, Texas. Dr. Beaver is a professor in the Department of Small Animal Clinical Sciences at Texas A&M University.