There may be more to a cat's purr than meets the ear. A new study reports that our feline friends modify their signature sound when seeking food, adding a higher-frequency element that exploits our sensitivity to infant wails--and thus making it harder to ignore.
Although guinea pigs and even elephants can purr, felines get most of the credit for the mysterious sound. The low rumble--at 27 Hz, it's comparable to the lowest note on a piano--serves as a kind of smile, often indicating contentment. It also sometimes crops up when a cat is sick or injured, perhaps to reassure themselves, ask for help, or aid in their own healing.
Behavioral ecologist Karen McComb of the University of Sussex in Brighton, U.K., became acquainted with another function for the sound when her cat Pepo began waking her for his early-morning breakfast with an insistent purr. She lamented her loss of sleep to a few cat-owning friends and learned that they too were "purred" into feeding their pets. As an animal communication expert, McComb set out to discover what made these particular purrs so coercive.
She recruited 10 cat owners to record their pets' purrs when the cats were clearly seeking food and when they were resting or being petted. Next, McComb and colleagues asked 50 volunteers with varying levels of cat experience to listen to the recorded purrs and rank them according to urgency. Seventy-five percent of the volunteers--including some who had never owned cats--consistently identified the food-demanding, solicitous purrs as more urgent and more unpleasant than nonsolicitous purrs from the same cat. (See if you can hear the difference: normal purr and solicitous purr.)
Acoustic analysis revealed why: Those pushier purrs harbored a hidden higher-pitched meow, with a frequency similar to that of a human infant's cry. The louder that element was, the more urgent listeners rated the overall sound to be. When McComb edited out the embedded sound and left the remaining purr intact, the same listeners judged it less urgent, she and colleagues will report tomorrow in Current Biology.
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McComb thinks that the meow is present in a normal purr but is very quiet, and some cats learn to exaggerate the sound as a subtle means of commanding attention. "It's quite annoying," she says, "but it won't get them thrown out of the bedroom."
David Rice, a biomedical engineer at Tulane University in New Orleans, Louisiana, who has studied how cats purr, says that cats may have other motives for exaggerating their cries. Higher frequencies help the sound carry across greater distances, for example.
Either way, says Rice, the work supports the idea that cats train their owners, not the other way around. "Cats are fairly smart, and they'll quickly learn what works and doesn't," he says. Veterinarian Bonnie Beaver of Texas A&M University's College of Veterinary Medicine and Biomedical Sciences in College Station agrees. "It's a classic example of trial-and-error learning," she says.
Trial and error has certainly benefited Pepo. Participants found his solicitous purr the most annoying of the bunch, and they judged his regular purr the most pleasing--leading McComb to conclude she's the most "highly trained" cat owner around.