 If you've ever owned a cat, you probably noticed that they really like meat. But where you might reach for a candy bar or a bottle of Coke, cats don't care much for sweets. As a matter of fact, researcher Gary Beecham and his colleagues at the Monal Chemical Senses Center in Philadelphia recently discovered that cats are unable to taste sweet. It seems that millions of years ago, a common ancestor of all modern cats lost the ability to perceive sugars. But how do we know they lost it? What if they just never had the ability in the first place? Let's puzzle through that for a second. Mammals have taste receptors on cells on their tongues. These receptors bind to or catch molecules that carry taste information, sodium ions that taste salty, sugars that taste sweet, and toxins that taste bitter. We need taste receptors to identify different energy sources in our environment. For example, humans, unlike cats, have evolved to depend on carbohydrates and sugars, especially in societies where access to meat is low, and the taste of carbs is a signal to our bodies that the food in question has lots of stored energy. These receptors are actually composed of two distinct proteins that work together. The genes that code for those proteins are called TASS 1R2 and TASS 1R3, but their names are not so important here. Both of our sweet receptor genes are fully functional, so humans have no problem tasting sweet foods. But the story is different for all cat species, tigers, lions, house cats, you name it. One of their two sweet receptor genes is broken, and they therefore cannot taste anything sweet. But this is actually okay because their carnivorous lifestyle does not depend on eating carbs. This also answers our question. Cats once had a functional sweet receptor. The proof is there in their genes, and they lost it over the course of their evolution. They have the gene. It's just broken. But cats are not the only meat-eaters on Earth. There are many other highly carnivorous mammals, so what is the status of their sweet receptor? It turns out that some species of marine mammals, including dolphins, have also lost the functionality of their sweet receptor. Not only that, dolphins cannot taste bitter foods either. Because dolphins swallow their food whole, they probably don't need to taste much of anything. For dolphins, their limited palate likely reflects the way they eat, not just what they eat. So, many carnivorous mammals cannot taste sweet because they have a broken sweet receptor. But is the receptor broken in the same place for these species? In other words, does each species have the same mutation in their sweet receptor gene? Genetic evidence shows this is not the case. In fact, every time scientists identified a broken sweet receptor in a new species, the mutation that caused the dysfunction was unique. This points to a fascinating case of convergent evolution, when different species independently develop a similar trait. Like cats, humans have also lost a few traits over the years, like tails for balance, long toes for grabbing, and fur for insulation against the harsh elements. Our ancestors adapted to a world where they didn't need these traits anymore, while other traits, like our massive brains and our upright stance, developed in their stead. The story is much the same for cats. They can live just fine without a functional sweet receptor. But they've developed other traits, like the ability to taste the basic unit of energy in our body, adenosine triphosphate, or ATP. And what kind of food has high amounts of ATP? You got it. Meat. So, I'll send you home with an easy do-it-yourself science experiment. All you need is a cat, preferably yours, two bowls of water, and some table sugar. Dissolve about a tablespoon of the sugar in water, and place the sugar water next to an equal amount of regular water. Let your cat decide which water it finds the most enticing, and keep track of how long it spends sipping the sugar water, and how long it spends sipping the regular water. Your cat should not be able to tell a difference between the two, and should spend equal amounts of time at either bowl. Have a dog, perform the same experiment, and watch your dog go crazy for that sugar water. Yes, dogs still have a fully functional sweet receptor.