Cat treats and dead genes

February 4, 2009 • 11:09 am

Lots of research with domestic cats has shown that they are indifferent to sweet foods, and do not show a general attraction to them (the reason your cat likes ice cream is because of its dairy attributes, not its sweetness).  I was watching a video on YouTube of a cat eating a graham cracker (with difficulty), and wondering why it was so fond of the thing given that it couldn’t perceive its sweetness.  A friend who works on olfaction and taste then speculated that maybe cats have the genes for producing the proteins involved in tasting sweetness, but that those genes have become disabled: since the normal diet of cats is meat, there is no reason for them to seek out sweet foods.  This result would be is similar to the many “dead genes” I describe in WEIT (e.g., humans’ broken gene for making vitamin C)–broken remnants that testify to our common ancestry from species in which those genes were active.

A friend of mine who works on genomics sent me a paper from 2005 that verifies the notion that cats do have “sweetness-detecting” genes, but that those genes are inactivated (i.e., they have become “pseudogenes”).  It turns out that mammals detect sweet food by forming a taste receptor from the joined protein products from two distinct genes, Tas1r2 and Tas1r3. A group of biologists from Philadelphia sequenced these genes in domestic cats.  Sure enough, they found that while Tas1r3 is active, Tas1r2 is nonfunctional because of mutations that prevent the gene from making a full protein.  Thus the two proteins necessary for sweet detection cannot combine, with the result that Fluffy can’t taste sugar.  Sequences of the same genes in tigers and cheetahs show the same thing: no ability to detect sweetness.

Dogs, however, can detect sweetness, and sequencing of other groups show that the common ancestor of cats and dogs must have been able to taste it as well.  Obviously, the gene has been inactivated by mutations in the ancestor of many cats (more DNA sequencing needs to be done in other cat species).  Sweets are not part of cats’ diet in the wild–they are carnivores–so there is no need to have a gene that, by causing pleasurable sensations when encountering sugar, leads the animal to seek out sugar-rich foods.  So don’t bother offering Fluffy a piece of chocolate–give her an anchovy!

n.b.  Don’t feed your dog chocolate to see if he likes sweets.  The theobromine present in chocolate can be lethal to dogs, as described here.

7 thoughts on “Cat treats and dead genes

  1. My mother didn’t *offer* the cat chocolate.

    It went on the table and stole it on its own at night.

    My own has forced me to lock up the white bread.

    Odd creatures.

  2. Interesting that cats don’t taste sweet but dogs do. I had a cat that a a white chocolate santa candy. It was pretty big and he ate the whole thing so he must have been pretty attracted to something about it. He got very sick from it but recovered. I have been told to be careful with anti-freeze because it is sweet and animals will drink it and be poisoned. I wonder if cats wouldn’t be interested in it?

  3. You shouldn’t offer your dog or cat chocolate anyway, because of the danger of poisoning. The above commenter was lucky.

  4. Not to descend into stupid cat stories (okay I guess I am), but my cat goes absolutely insane over strawberries and cantaloupe, no idea why.
    The sound (smell) of us cutting a cantaloupe open will have him running from upstairs to the kitchen–and this cat weighs 24 pounds, he doesn’t run a heck of a lot.

  5. My cats would go insane over strawberries and grapes. I’d come home from the grocery store and tun my back for a second to put something away and turn around and both of them would be in the bag with the strawberries. It was the funniest thing.

Leave a Reply