By Susan Milius
As a rough rule of tongue, animals that have lost the power to taste sweetness tend to be specialized meat eaters.
A gene crucial for detecting sweet taste carries disabling glitches in seven of 12 mammals analyzed in a new study. The sweet-blind animals are spotted hyenas, Asiatic small-clawed otters, two catlike wild hunters (fossa and banded linsang), sea lions and two kinds of seals — all predators.
A sweet detector probably wouldn’t give these carnivores much of an advantage as they hunt their dinner, speculates study coauthor Gary Beauchamp of the Monell Chemical Senses Center in Philadelphia. So mutations in that sweet detector gene, Tas1r2, could easily spread through populations, Beauchamp and his colleagues propose March 12 in the Proceedings of the National Academy of Sciences.
This loss isn’t universal among dedicated meat eaters, though. Red wolves, Canadian otters and aardwolves (hyena relatives that stalk termites) turn out not to have lost their genetic sweet spot. “Or haven’t lost it yet,” as Beauchamp puts it. Raccoons and spectacled bears, which eat broader diets, also have intact genes to taste sweetness, the researchers found.
Vegetarian animals such as the bamboo-loving giant panda also can detect sweetness in their diet, research has shown. The great panda has instead lost the ability to detect what’s called umami, the protein-related flavor of MSG.
From the opposite point of view, some animals that don’t specialize in meat nevertheless may have lost their ability to taste sweetness. For instance, chickens, which eat both plant and animal foods, don’t seem to notice sweetness in their food and appear to lack the functional sweet gene, says Peihua Jiang, also of Monell and a coauthor of the new study.
Chickens are just one reason that Huabin Zhao of Wuhan University in China isn’t convinced by the meat-eater/sweet-loss scenario. He has found sweet loss among vampire bats, which are blood feeders. Narrow diet specialization might be a better explanation, he suggests.
The Monell group had already published findings showing that domestic cats, which are highly carnivorous, are indifferent to sweetness. But to see the disabling genetic glitches in other types of carnivores “really gives you a better feeling for how evolution works,” says sensory neurobiologist Emily Liman of the University of Southern California.
People, too, have had their use-it-or-lose-it sensory evolution, she says. Humanity does have a full set of taste detectors: sweet, sour, salty, bitter and umami. But people aren’t so great when it comes to odors, and even worse at detecting the rich animal-to-animal chemical communications known as pheromones.
