Cats' Sweet Tooth Long Gone

Brand, who has two cats, said he could not say for sure how a cat's overall nature might be affected by never experiencing sweetness. But it may not be a coincidence, he said half-jokingly, that the dominant behavioral characteristics of a cat are that "it sleeps a lot and it's cranky."

Human taste buds contain cells with five kinds of "receptors," each specifically responsive to one of five tastes: salty, bitter, sweet, sour and umami (a taste imparted by amino acids, including flavor enhancer monosodium glutamate).

The sweetness receptor consists of two different proteins attached to each other on the surface of a cell. The two are manufactured under the direction of two genes, then embrace to make a single receptor that fires a nerve signal to the brain when sugar is present.

In the new study, the researchers collected DNA from six house cats, all of them pets of scientists at Monell. They analyzed the sequence of genetic "letters" in the two genes encoding the sweetness receptor's two proteins.

They found that one of those genes, called Tas1r2, is missing a stretch of 247 letters -- a deletion that prevents the gene from making a proper protein. With only one of the two crucial proteins, the cats have no way to taste sweetness.

The researchers found the same deletion in tiger and cheetah DNA, suggesting the mutation occurred in a common ancestor early in feline evolution. Follow-up studies are now underway using DNA from a civet, mongoose and hyena, all of which evolved from the same common ancestor. The goal is to pinpoint when in feline evolution the deletion occurred.

It is not clear whether the cat's loss of its sense of sweetness led it to pursue an all-meat diet or, as Brand and others suspect, cats were already fully carnivorous -- in which case the loss of an affinity for sweets would hardly have mattered to them.

Also unclear is why some cats at least dabble in sweets, licking the occasional melon ball, for example. But the experience of flavor is only partly due to taste and is largely a function of a food's smell, Brand noted, suggesting that cats are probably attracted to either the smell of some sweets or the salts or amino acids that are also present.

Because the Tas1r2 deletion is a big one, it is unlikely any cat would be born with a spontaneously corrected version of the gene. One of the cats in the Monell experiment was crazy about marshmallows, Brand said, briefly raising hopes that it might be a rare mutant. But DNA tests proved the cat was no different than the others.

"Whatever this cat finds attractive in marshmallows, it's not sugar," Brand said.

Zuker said he has been contacted by many people who suspect their sweet receptors are nonfunctional. "They say they don't really know what 'sweet' means," he said.

But as far as he knows, none has had a DNA test.

And no one knows if those people sleep a lot or are cranky.