The third in the series might surprise you: it turns out that cats can't taste sweetness! Anyone who's ever tasted cat food knows it certainly isn't sweet. If you want to read details, see here: http://genetics.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pgen.0010003.
It turns out that most ordinary (that is, tame pet) cats have normal taste, but they can't taste sweetness. This is not true of humans of course, but most other mammals, including dogs, mice, horses, etc. can all tell when something is sweet. Cats can't! And scientists have wondered why for a long time.
How do you know what tastes sweet? Your tastebuds on your tongue detect sugar or sweet compounds (in honey, nutrasweet, splenda) and send a signal to your brain that says, "Hey, that's sweet!" The tastebuds have little proteins on them called receptors that actually attach to sugar molecules and then get turned on to send the message to your brain. When your tongue is burnt, you can't taste anything, because those receptors on your tongue are temporarily missing!
So why can't cats taste the sweet? It turns out their DNA for the sweetness receptor is ok. The sweetness receptor is actually a pair of proteins called "taste receptor 2" and "taste receptor 3." Taste receptor 3 is fine in cats, but taste receptor 2 isn't. Even though the DNA is there, there are changes in it (called mutations) that mess it up so that it never makes it to the cat's tongue. This kind of gene for taste receptor 2, which is actually there but not turned on, is called a pseudogene. With only half of a sweetness detector, the cats can't tell what's sweet and what isn't.
Amazingly, domestic cats aren't the only ones with the problem. Lions, cheetahs, jaguars and tigers can't taste sweet either. Somewhere along in evolution, one ancient cat lost the ability to taste sweetness, and so all its descendants did, too.
What does this mean for cats? Well, there's no point tempting them with sugar in their milk, for one thing. But also, it might explain why cats are carnivores. Since fruit and veggies don't taste good to them, they might prefer the flavor of meat. Or it could be the other way around: maybe because they started eating meat, they lost their sweet tooth.
In any case, it's a mystery, and makes taste even more interesting. In fact, even among humans, only some of us can taste certain types of bitterness (like phenylthiocarbamide, PTC, which is in cigarettes). With lots of taste receptors on our tongues, some people really do have "good taste."
rani
