Every day, I feed my cat small, round, hard pellets that look about as appetizing as old gravel, and she gets so excited about them. I tasted one (for you, dear readers!) and I would describe the taste as falling somewhere between the meh of cardboard and the bleh of rancid fish. Not recommended. For her part, the cat flinches if I consume an orange anywhere near her; you can tell she thinks I am disgusting for eating them. It seems pretty clear that she and I have different tastes in food. Are such differences simply matters of individual preference, or is there a biological basis for them?
It’s hard to know what something tastes like to someone else. My personal experience of peanut butter (disgusting) is likely to differ from yours (mmm, yum), despite our belonging to the same species. However, we can say with some certainty that both of us can taste peanut butter, and that it will not taste like lemons to either of us. Humans have five major types of taste receptors: sweet, umami, bitter, sour, and salty. Sugar is sweet, hamburgers and mushrooms are umami, coffee and India pale ales are bitter, lemons are sour, and salt is salty.
Taste isn’t just a nice aesthetic treat: it serves an important purpose for animals. Taste helps an animal determine whether it’s a good idea to eat something. “Sweet” tastes tell you that there is sugar in this food; “umami” tells you that there is protein. Both of these are good things to eat, hence our fondness for both flavors. “Bitter” tastes suggest that something might be toxic. Many humans have come to culturally enjoy bitter tastes like coffee, but these are learned preferences: your instinct, when first presented with something bitter, is to spit it out. Interestingly, bitter receptors are very broadly sensitive, meaning that we aren’t good at distinguishing between different bitter tastes. Presumably this is because if something is toxic, you don’t need to identify the exact type of toxin, you just need to quit eating it. “Sour” things are acidic, and generally also avoided. “Salty” things are literally salty—containing sodium—and taste good at low concentrations of salt (pretzels! V8 Juice!) but bad at high concentrations (sea water).
Sometimes taste receptors get triggered in weird ways. Some substances trigger sweet receptors at low concentrations but bitter receptors at higher concentrations, leading to the sensation of an aftertaste.
We know the genes that encode each of these types of taste receptors. This lets us look for them in other animals: if the gene for a certain receptor isn’t present in a species, then that species probably can’t taste that taste. In mammals, this approach has shown that giant pandas don’t taste umami (Zhao et al. 2010). This makes a lot of sense: while all the panda’s relatives are carnivores, the panda itself is vegetarian. When your diet is 99% bamboo, you really don’t need to be able to sense the taste of meaty protein: it just never comes up, so why bother?
Cats have lost the gene for the sweet receptor, and can’t taste sweetness. If you offer a cat sugar water and regular water, the cat won’t like the sugar water any more than the regular water. (Humans and mice, on the other hand, will prefer the sugar water.) A salted caramel would just taste like salt to your cat.
A number of other mammalian carnivores besides felines have also lost the sweet receptor, including the Asian small-clawed otter, the fur seal, and the spotted hyena. It’s thought that this loss is due to these species eating only meat: meat doesn’t tend to taste sweet, so you really don’t need that receptor.
Some carnivores have gone even further in their loss of taste: sea lions and dolphins can’t taste sweet or umami, and probably not bitter either. Sea lions and dolphins both eat fish, so you would think that they would need to be able to taste umami; the key to understanding them, though, is knowing how they eat. Both of these species swallow fish whole, meaning they really don’t need to taste anything—they just grab it and swallow it. So their unneeded taste receptors have degenerated, much like the eyes of species that live in constant darkness (Jiang et al. 2012). A similar phenomenon was found in birds that eat fruit: tanagers, which crush the fruit in their bills, are good at distinguishing different levels of sweetness, whereas manakins, which swallow fruits whole and so presumably don’t get a chance to taste them, aren’t (Levey 1987).
In fact, fruit- and nectar-eating birds are a bit of a puzzle, because it doesn’t seem like they should be able to taste sweet things. A number of bird species—as diverse as Zebra Finches, Amazon Parrots, and Mallards—have been examined for the sweet receptor gene, and none of them have it. (Their close non-avian relative the alligator does, though, so if you were planning to make an ice cream sundae for an alligator, go for it.) This is fine if you’re a bird that eats mostly bugs or rodents or other birds, none of which are likely to taste very sweet; but if you eat fruit or nectar, don’t you need to be able to taste whether you’re getting any sugar from your food?
It turns out that in hummingbirds, at least, the answer is yes, and hummingbirds have improvised to be able to taste sugar without a sweet receptor. The same receptor that detects umami taste in mammals, fish, chickens, and hummingbirds’ close relatives the swifts has been repurposed in hummingbirds to detect sugar (Baldwin et al. 2014). Many other bird species live off of nectar or sugary fruit, and it will be interesting to find out if they have solved the sweet-perception problem in the same way as hummingbirds.
Mice have taste receptors very similar to ours, with just a few differences. The umami receptors of mice are less picky about what types of protein they respond to than ours, meaning that more different kinds of protein taste umami to mice. And while we think aspartame (that thing in diet soda that lets it have zero Calories) tastes sweet, mice don’t. (Neither do hummingbirds.)
Fruit flies have evolved taste perception independently of vertebrates, but they still taste fairly similar things. Like us, they like sugar and low concentrations of salt, and dislike bitter things and high concentrations of salt. Unlike us, though, they don’t seem to taste umami or sour. Instead, they have a taste perception pathway that is sensitive to carbon dioxide, which lets them detect fermenting things like rotting fruit—which is, after all, what a fruit fly is looking for. Also unlike us, they have taste receptors not only on their mouthparts but also on their wings and legs. If you’re going to spend a lot of time walking on your food, you might as well put some taste receptors on your legs.
Because we know which genes code for each receptor, we can do strange things to taste receptors to see what happens. If you put bitter receptors in the cells that are supposed to be sensing sweet tastes, the animal will seek out bitter foods, because those foods will taste sweet to it. And “expression of a blue light receptor in sweet cells should, in principle, make blue light ‘taste’ sweet” (Yarmolinsky et al. 2009). Um, why have we not done that yet, guys?! Also, I volunteer. I would like blue light to taste like mint chocolate, please.
Baldwin MW, et al. 2014. Evolution of sweet taste perception in hummingbirds by transformation of the ancestral umami receptor. Science 345:929-933.
Jiang P, et al. 2012. Major taste loss in carnivorous mammals. PNAS 109(13):4956-4961.
Levey DJ. 1987. Sugar-tasting ability and fruit selection in tropical fruit-eating birds. The Auk 104(2):173-179.
Yarmolinsky DA, Zuker CS, Ryba NJP. 2009. Common sense about taste: from mammals to insects. Cell 139(2):234-244.
Zhao H, et al. 2010. Pseudogenization of the umami taste receptor gene Tas1r1 in the giant panda coincided with its dietary switch to bamboo. Molecular Biology and Evolution 27(12):2669-2673.
*Photos obtained from Flickr and used via Creative Commons. Many thanks to these photographers for using Creative Commons!