This came as a surprise, said Monell biologist Gary Beauchamp, who led the project, along with Monell molecular biologist Peihua Jiang. People assumed that the ability to taste sweet things was nearly universal in mammals. It was even more surprising that dolphins couldn't detect bitter, he said, because bitter perception is important for many animals to detect poisons.
In On the Origin of Species, Darwin discussed loss of function in animals, especially loss of vision in some creatures that live in darkness. Darwin tried to stress that evolution is not some sort of progression or improvement. Living things change to fit their environments, and if a trait or ability is no longer necessary, it can be lost.
It makes sense that the animals losing their sweet sensation were those that became adapted to an all-meat diet, Beauchamp said, and don't need to distinguish nutritious plants from toxic ones. Dolphins may need very little taste sense because they eat nothing but fresh fish, and they tend to swallow them whole.
Humans retained all our taste receptors but we've had our losses too. Our DNA carries genes for a thousand different smell receptors, each giving us access to different airborne chemicals. But in any given person, less than half of them work. That's why some people can't smell cilantro and others can't smell truffles.
In the world of taste, the biology of salty and sour is not completely sorted out, but scientists have a pretty good handle on the mechanics of sweet, bitter, and umami. The ability to taste those flavors is tied to specific genes, which hold the recipes for proteins called taste receptors that are made in our tongues. Interestingly, scientists find taste receptors in other parts of the body - the intestines and the pancreas - where they may be useful for regulating insulin secretion.
Taste is the most ancient sense, said Thomas Finger, a biologist at the University of Colorado. Even some bacteria have chemical sensors to help them distinguish nutrients from poisons. Our bitter receptors and those of modern fish show enough similarities to indicate a common genetic ancestor. It's interesting, he said, that such an ancient mechanism is so readily lost in our fellow mammals.
Animals that can detect sweets seem to be hardwired to like them, as they are hardwired to dislike bitter compounds, said Colorado's Finger.
That makes it easy to test these senses. Animals that detect sweets prefer sweetened water to plain, while those blind to sweets have no preference. Interestingly, chimps and humans both perceive the taste of aspartame as sweet, but rats are indifferent to it.
Beauchamp said he didn't know why the broken taste genes become dominant in cats and the seven other mammals. Did they benefit from not tasting sweet stuff, or was it just random chance that spread these broken genes?
Cats can get sick on starchy or sweet food, so it could be to their disadvantage to detect sweets. On the other hand, the reason cats get sick may be tied to the loss of the sweet receptors in the cats' internal organs and intestines, a loss that might alter the ability to digest carbohydrates.
Dog owners won't be surprised to know that their animals can taste everything we can. That's why some dog-food manufacturers can use starchy or sweet ingredients as fillers and keep their customers coming back for more.
Could there be other taste receptors that remain unknown because we humans don't have them? Beauchamp said there's some suspicion that rats and mice taste something we don't, since they have a particularly powerful love of starchy foods. Perhaps someday we can genetically engineer ourselves to taste flavors we can't even imagine, or, if we wanted to conserve resources, we could change our receptors so that tofu was more delicious than steak.
Contact Faye Flam at 215-854-4977, firstname.lastname@example.org, on her blog at www.philly.com/evolution, or @fayeflam on Twitter.