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No Candy for Kitty

25 July 2005 (All day)
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Tastes like Evian. Thanks to a mutant sweet receptor gene, Maybelle can't tell what all the fuss is about.

Stealing candy from a cat may be even easier than stealing it from a baby, seeing as how felines have no love for sugary treats. Now scientists have figured out why: Cats lack a taste receptor protein that's essential for sensing sweet stuff.

Cats couldn't care less about confections. When given the choice between plain water and a sugar solution, they slurp similar amounts of both. But even though drinking the sugar water isn't a good idea--it can cause diarrhea and dehydration because felines have very low levels of the enzymes that break down sugars--cats will continue to do so even when they start to feel sick. This prompted researchers to believe that cats simply cannot detect the sugar.

To explain this deficit, researchers turned to genetics. Four years ago, scientists identified two genes, Tas1r2 and Tas1r3, responsible for the sweet receptor in a variety of animals. In the new study, biophysicist Joseph Brand of the Monell Chemical Senses Center in Philadelphia, Pennsylvania, and colleagues compared the sequences of the two genes in dogs, humans, mice, and rats--all of which can detect sweets. They found that a 240 base pair stretch of Tas1r2 present in all of these creatures was missing in the feline gene. As a result, cats cannot make one of the proteins needed to create a sweet taste receptor. "The huge deletion was a surprise to me," says Brand, who adds that the degree of the mutation makes it virtually impossible for cats to ever regain their sense for sweets.

This deletion was observed not only in domestic cats but also in the tiger and cheetah, the researchers report 25 July in PLoS Genetics. Brand and his team now plan to study related animals, such the hyena and mongoose, to help trace when and why sweet taste receptors became lost in the evolutionary process.

"We have known about cats' indifference to sweets for a while, but to get it down to the level of receptors is a very interesting development," says John Bradshaw, a behavioral biologist at the University of Bristol, United Kingdom. A question that has yet to be answered, he adds, is whether the lack of sweet taste receptors drove cats to become strict carnivores or if it's the other way around.

Related sites:
The study
Original discovery of sweet taste genes
Brand's home page

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