Do Animals Possess Sweet Tooth Receptors?

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Does your dog Scruffy stare longingly at your half-eaten strawberry ice cream cone as you lick it? Does he scarf up cookie crumbs better than any vacuum cleaner? If so, your pet is one of many animals that like sweet things.

Some animals would even love a sip of your aspartame-sweetened diet soda, while others couldn’t even taste the sweetness in the frosting on a slice of birthday cake.

Recently, scientists at Monell Chemical Senses Center in Philadelphia discovered an animal that likes almost all the same kinds of sweets that humans do. And you might be surprised to find out who our sweet twin is.

Cat vs. Dog

If you don’t feel like sharing your dessert, try spending time with a cat. Hannah Melio, 10, would love to share cookies with her cat Mimi, but when she breaks off a piece, Mimi “won’t even look at it.” Many pet owners have noticed that their cats don’t have much of a sweet tooth, and now scientists understand why.

In mammals, two proteins, called T1R2 and T1[R.sub.3], work together to detect sweetness. Monell scientists have discovered that, in cats, the gene that provides instructions for making T1R2 doesn’t work.

The cat study’s lead author, Xia Li, calls the T1R2 gene a “molecular fossil,” something that may once have allowed cats to taste sweets, but remained a part of cats’ anatomy long after it stopped functioning.

The same may be true for the rest of the cat family–cheetahs and tigers also have a defective T1R2 gene.

Cats might not want to steal your Skittles, but we do share plenty of taste preferences with them. Just like many of us, cats tend to avoid bitter tastes, like unsweetened chocolate, and sour tastes, like spoiled milk.

Cats may also enjoy the savory umami taste found in foods such as fish and aged cheeses. Your cat might even lick that ice cream cone we mentioned earlier, but it’s probably seeking the cream rather than the sugar.

Sweetness is one of the many things that dogs and cats disagree about. Says Jack Greco, 10, “My dog [Buddy] loves chocolate. When we came back from Hershey Park, Buddy ate all three of my Hershey bars [and got sick].”

Dogs have been shown to enjoy at least four different kinds of natural sugars that humans like. Most mammals are more like dogs than cats, according to Li. They seek out sugar because it’s a great source of energy.

Buddy got sick, though, because that sugar was mixed with lots of chocolate, which is extremely toxic to dogs, because it contains theobromine. This stimulant affects the dog’s nervous system and heart muscle and increases the frequency of urination.

Does your pet like sweets? You can find out by trying this scientific test: Fill two identical bowls with the same amount of water. Set them side-by-side. Dissolve some sugar in one of the bowls.

Let your pet drink for a few hours. Then measure how much water is left in each bowl. See below to check your findings.

Artificial Sweetness

Dogs enjoy sugar, but they don’t seem to care for the ingenious sugar substitutes we humans have created–not sucralose (in Splenda), or aspartame (in Equal and diet soft drinks), or saccharin (in Sweet’n Low).

Artificial sweeteners are generally less popular among animals than natural ones. Rats and mice like a few of them. Pigs like sucralose and saccharin, but not nearly as much as humans do.

Meerkats, ferrets, and mongooses didn’t like any of the artificial sweeteners they were offered during testing.

The mammals that are most like us in their taste for artificial sweeteners are our closest relatives, the primates. The most recent primates to emerge–African and Asian monkeys and apes (along with humans)–are among the few mammals that can taste aspartame.

The University of Zurich’s Dieter Glaser argues that the same sophisticated sweet receptor that allows these animals to taste aspartame may once have helped them shift from an insect-based diet to a more efficient fruit-based one. This change could have played a role in developing the human brain.

Scientists believed that no mammal other than these primate groups could taste aspartame-until they tested the red panda. Red pandas can taste at least three artificial sweeteners, including aspartame.

Researchers know that the red panda’s sweet receptors are structured quite differently from other animals’, but why it evolved in this unusual way remains a mystery.

Our Surprising Sweet Twin

So far we have considered only mammals, but other animals also have a sweet tooth. Nobody who has been to a picnic would be surprised to hear that ants like natural sugars.

They also enjoy sugar alcohols, a natural class of sugar substitute, but they can’t seem to taste artificial sweeteners. Hummingbirds like sugar, but aren’t interested in saccharin or aspartame.

Many animals have yet to be tested, but Monell researchers have already identified one animal whose sweetener preferences very closely mirror ours: the fruit fly.

Monell’s Beth Gordesky-Gold tested 21 different substances on fruit flies, 20 of which taste sweet to humans. The fruit flies favored the same 20.

When it comes to sweeteners we are not a perfect match for fruit flies, but we have more in common with them than with most mammals, including many monkeys.

This seems surprising. Since not only do we look and act very different from fruit flies, but the genes that code our sweet receptors follow a completely different sequence.

However, our tastes may have evolved the same way as fruit flies’ because we occupy the same environmental and ecological niche: We both originated in tropical and temperate Africa, and we’re both omnivores–eaters of plants and meats–who also especially enjoy fruit.

The fact that fruit flies share our taste in sweets may come in handy for us. Many artificial sweeteners have unpleasant effects on our digestion or create risks of other health problems.

Australian biologist Anne Rae is using fruit flies to help identify new sweeteners. Fruit flies are presented with two sweeteners at a time–one dyed red, and the other blue. If the flies’ abdomens turn blue after feeding time, then perhaps we humans would like the blue one better too!

According to scientists, you can expect your dog to lap up more water from the sweetened bowl, but your cat to drink about the same amount from each. Don’t try, this too often. Sugar is no better for your pet’s teeth that it is for yours.
Randoph, New Jersey.

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