Leave a cookie in a room with a dog and it will be floor crumbs upon your return -- but with a cat, the treat will be safe.
Scientists say they have unearthed the reason for the feline's utter indifference toward sweet-tasting goodies: The gene for its sweet-taste receptor is riddled with errors and does not work.
The research, to be published today in the journal Public Library of Science Genetics, was conducted by scientists at the Monell Chemical Senses Center in Philadelphia and the Waltham Centre for Pet Nutrition in Britain.
Previous studies have shown that cats don't care for sugar.
Researchers at the Monell center offered dozens of domestic cats choices of water -- either plain or flavored with sugar. The cats drank equally from both bottles.
Later studies at a zoo showed the same for lions, leopards, tigers and jaguars.
"If it were a dog or a mouse or a human, they'd immediately pick out the sweet one and consume it in preference over the water," said Joseph G. Brand, associate director of the Monell center and a coauthor of the study.
When the mammalian receptor responsible for sweet taste was identified earlier in the decade, Brand and colleagues decided to investigate what was different about a cat's receptors.
They extracted DNA from cats and examined the gene for the sweet-taste receptor, which consists of two proteins that sit on the surface of taste bud cells. When sugar binds to the receptor, a signal is sent to the inside of the cell and ultimately to the brain.
The team found that the gene for one of those receptor proteins -- known as T1R2 -- is defective and can no longer make a receptor.
The same genetic defect was found in DNA from a lion and a cheetah.
The scientists propose that the cat's extreme carnivorous lifestyle renders the need to sense sugar irrelevant -- allowing mutations to accrue in the sweet-taste receptor during evolution of the cat lineage.
"The kinds of things cats are going to be eating -- rodents, crickets, spiders, snakes and birds small enough for them to catch -- are things rich in protein and fat, not sugar and carbohydrates," said James Richards, spokesman for the American Assn. of Feline Practitioners and a veterinarian at the Cornell University College of Veterinary Medicine.
A sweet receptor in cats, he said, "would be like me having a taste receptor for wood."
Richards said that cats' saliva and digestive tracts also are unsuited to carbohydrates, containing lower levels of enzymes involved in digesting them.
He said veterinarians were investigating whether the level of carbohydrates that domestic cats got in their diet was contributing to the surge in rates of cat diabetes and obesity.