The lure of sweets is the downfall of many a dieter. Cats, however, are indifferent to sugar, a trait that is rare in the mammal kingdom. Now scientists have figured out why. Felines apparently carry a defect in a gene that encodes part of the mammalian sweet taste receptor.

A lack of interest in sweets has been observed not only in house cats, but also in wild ones such as lions, tigers and jaguars. "One possible explanation for this behavior is that felines are unable to detect sweet-tasting compounds like sugars and high intensity sweeteners because their sweet taste receptor is defective," remarks Xia Li of the Monell Chemical Senses Center in Philadelphia. Li led a team of researchers to investigate the genetics of taste in cats. They determined that the animals lack 247 base pairs in a gene called Tas1r2, which encodes T1R2, one of two protein subunits that make up the sweet receptor in most mammals. "This type of gene is known as a pseudogene and is somewhat like a molecular fossil," Li says. "It presumably once coded a functional protein, but no longer does so."

The other half of the sweet receptor, known as T1R3, is normal in cats. "What we still don't know is, which came first: carnivorous behavior or the loss of the T1R2 protein," notes senior author Joseph G. Brand of Monell. "With regard to the gene, is this a case of 'use it or lose it'"? The scientists describe the work in the July issue of the journal PLoS Genetics.