updated 7/25/2005 9:27:12 AM ET 2005-07-25T13:27:12

Cats are notoriously finicky eaters, as millions of pet owners can attest.  Now, there's a scientific theory explaining, at least in part, why cats have such snobby eating habits: genetics.

Researchers at the Monell Chemical Senses Center in Philadelphia and their collaborators said Sunday they found a dysfunctional feline gene that probably prevents cats from tasting sweets, a sensation nearly every other mammal on the planet experiences to varying degrees.

Researchers took saliva and blood samples from six cats, including a tiger and a cheetah and found each had a useless gene that other mammals use to create a "sweet receptor" on their tongues.  The gene in question does not produce one of the two vital proteins needed to form the receptors.

"Because cats can't taste sweets, they're cranky," joked Joseph Brand, Monell's associate director and an author of the paper being published Sunday in the inaugural issue of the Public Library of Science's journal Genetics.

The Public Library of Science aims to make such research freely available online and was launched out of frustration with rising subscription costs of prestigious research print journals, some of which cost more than $11,000 a year.  Instead of charging a subscription fee, the nonprofit organization charges authors $1,500 per paper submitted.

Brand said the "pseudogene" in cats is probably a big reason why they are carnivores that get by on a high-protein, "Atkin's-like" diet.  "Its sense of taste has driven it to become a meat eater," Brand said.  "Losing their sweet receptor has probably changed their dietary habits."

Brand said the paper is a culmination of a lingering question that nagged at him since he visited the Philadelphia Zoo with a colleague 25 years ago to watch the feeding habits of big cats.

All mammals have receptor cells on their tongues that send taste signals to the brain to process.  The receptor cells are clustered together as taste buds.  Each human taste bud is comprised of 50 to 100 receptor cells representing the five major taste sensations: salty, sour, sweet, bitter and umami, the taste of the food additive MSG and fermented soy products, among other foods.  Most mammals' sweet receptors are created by two proteins, one of which cats are missing.

The study was paid for, in part, by the research arm of the pet food giant Mars Inc., which is looking to make better-tasting cat food.  The company has the rights of first refusal to commercialize the discovery published Sunday, Brand said.

Brand said the discovery could help veterinarians treat ill cats.  "Everyone knows that cats are finicky," said Brand, who owns two cats.  "And one big issue is how to make food palatable enough for a sick cat to eat."

The research team also received funding from the National Institute of Health, the U.S. Department of Veterans Affairs and the National Science Foundation. Brand declined to say how much the project cost.

The nonprofit Monell center studies the senses of smell and taste and has produced a number of scientific breakthroughs, including a study to be published this year showing that gay men sense body odor differently than straight men.

Researchers said that beyond improving the taste of cat food, the study will help scientists better understand food-related diseases such as diabetes in humans — and how diet influences evolution.

"This may have implications for all sorts of medical conditions," said Dr. Alan Hirsch, founder of the Smell and Taste Treatment and Research Foundation in Chicago.  Hirsch, who was not affiliated with the study, said that the study suggests obesity and related diseases such as diabetes are caused by more than simply overindulging a sweet tooth.  "Even in the absence of the taste for sweets, cats still get heavy," Hirsch said.

Copyright 2005 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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