Cats' Sweet Tooth Long Gone
Monday, July 25, 2005
Curiosity about the cat has finally paid off with a scientific explanation for felines' enigmatic indifference to sweets.
Researchers, pet owners and cat chow manufacturers have long recognized that cats, in stark contrast to their canine counterparts, show no particular attraction to sugar. Having sampled two dishes of water, one spiked with sugar and the other not, a cat is as likely to lap from one as the other.
But why? Until now, scientists have not known whether cats simply lack the lingual apparatus to detect sugar; or have functional sugar detectors on their tongues but faulty wiring from their taste buds to the brain; or -- as some might presume -- are simply too snooty to admit to such a common craving.
Now researchers studying the DNA of house cats, tigers and cheetahs have settled the question: Cats both large and small harbor a genetic mutation that renders the sugar detectors on their taste buds inoperative.
"We have found a simple but elegant explanation for their behavior," said Joseph G. Brand of the Monell Chemical Senses Center in Philadelphia, who with co-worker Xia Li led the study, published yesterday in the journal PLoS Genetics.
The work fills a gap in scientists' understanding of the evolution of cats, whose indifference to sugar -- a rare trait among mammals -- complements their complete dietary dependence on meat. In humans and other animals that depend on starches and ripe fruits for a sizable part of their nutrition, the ability to detect sugars is crucial.
"This discovery highlights how a species can manage to do without an entire taste modality if it has an ecological niche that will support that lifestyle," said Charles Zuker, a Howard Hughes Medical Institute investigator at the University of California at San Diego who co-discovered the receptor for sweetness in 2001.
The research also offers a glimpse into how animals other than humans experience the world.
After all, an organism's conception of reality is completely dependent on its senses, such as taste, vision and hearing, said Nick Ryba, a National Institutes of Health researcher who co-discovered the sweetness receptor with Zuker. Yet each kind of animal perceives but a small part of the overall sensory universe.
People cannot see infrared light, for example, a part of the visual spectrum that colors some insects' worlds, and they cannot hear the high-frequency sounds to which dogs respond. Dogs, on the other hand, are red-green colorblind. And some animals, such as blue crabs, can taste nutrients called purines that other creatures have no way of sensing.
Such differences go a long way toward explaining why various creatures behave and live as they do.
"If you think about it, what we actually perceive is not in fact reality at all," Ryba said. "It's really something that our brain constructs out of the information that it receives through our sense organs."