BioEd Online

Life is a little less sweet for cats: an analysis of their genes shows that they lack functioning taste receptors for sugar.

Throw a dog a bone or a bonbon and he'll enjoy either. Humans too have a taste for sugar. That's because, on most mammalian tongues, specialized taste bud receptors pick up on sweet tastes and send a pleasant signal to the brain.

The receptors contain a pair of proteins called T1R2 and T1R3, which are hooked together. When sugar binds to these receptors, they set off a cascade of events within the cell that ultimately signal a sweet sensation.

Scientists first documented cats' behavioural distaste for sugar in the 1970s¹. But no one could explain why domestic cats (Felis silvestris catus) couldn't care less about candy.

Sweet on DNA

Joseph Brand of the Monell Chemical Senses Center in Philadelphia, Pennsylvania, and his colleagues decided to sequence the regions of the domestic cat's DNA that code for the T1R2 and T1R3 proteins.

Their analysis provides a clear explanation for cats' blindness to sweets: the gene for T1R3 lacks 247 base pairs of DNA, which means it cannot produce a working protein. The results appear this week in the journal PLoS Genetics².

This shortened feline version of T1R3 represents what is known as a pseudogene: a gene that has lost its ability to function. Genes that are not important for a species' survival tend to accumulate more mutations over time, according to Brand. He says that many olfactory genes in humans have met this fate: "There was no pressure to keep them intact." As a result, we possess a much weaker sense of smell than other animals.

As cats are strict carnivores, they may have had no use for tasting sugar, he suggests. "You've got to pity them, don't you?"

Brand admits that he has seen cats go after marshmallows, which are basically blown up sucrose. "But whatever the cat is eating it for, it's not the sweetness. Perhaps it likes the texture or it's bored." He also chalks up anecdotes of cats eating chocolate to the animals' appreciation of the fat and cocoa ingredients, not the sugars.

The research team has found the same genetic deletion in close relatives of the domestic cat, including the tiger and cheetah. They hope to find out whether more distant relatives, such as the hyena share the same flaw in their T1R3 gene.