A lack of taste – how dolphins, cats and other meat-eaters lost their sweet tooth

By Ed Yong | March 13, 2012 9:00 am

Imagine a world without sweetness, where you couldn’t taste the sugary rapture of cakes, ice cream or candy. This is what it’s like to be a cat. Our feline friends carry broken versions of the genes that build sugar detectors on the tongue. As such, they’re completely oblivious to the taste of sweet things.

So are Asian otters. And spotted hyenas. Sea lions and dolphins too. In fact, Peihua Jiang from the University of Zurich has found that a wide variety of meat-eating animals can’t taste sugars. The genomes of these carnivores are wastelands of broken taste genes.

Two genes are largely responsible for our sweet tooth – Tas1r2 and Tas1r3. They encode two proteins that combine to form a sensor, which dots our tastebuds and detects sugar molecules. In 2005, Xia Li and Joseph Brand showed that all cats, from house tabbies to wild tigers, carry a useless version of Tas1r2. Over time, it has picked up so many mutations that it has become genetic scrap. It’s a ‘pseudogene’, incapable of making a working protein.

Now, Li and Brand have teamed up with Jiang and others to look at the same gene in a variety of other meat-eating mammals. They looked at a dozen species, and found that seven of them also have broken versions of Tas1r2. These include the Asian short-clawed otter, spotted hyena, spotted linsang (a beautiful tree-climbing civet), fossa (a Madagascan mongoose-relative) and three sea-going hunters – the sea lion, the fur seal, and the Pacific harbour seal.

All of these species are exclusive meat-eaters. Like cats, they feast on flesh, and nothing else. And all of them had disabled their copies of Tas1r2 in different ways. This suggests that the ancestor of the carnivorans had a working sweet-tooth gene, which deteriorated independently in different branches of the family tree.

Indeed, several carnivorans have working copies of Tas1r2, including the dog, ferret and giant panda. Jiang found five more: the aardwolf (a termite-eating hyena), Canadian otter, spectacled bear, raccoon, and red wolf. Unlike the members of Team Cat, these species sometimes stray from a meat-only menu.

Taste tests confirmed the stories that the genes were telling. For example, the Asian small-clawed otter (with a broken Tas1r2) will drink from a bowl of sugary water as often as it will from a bowl of plain water. Cats show the same indifference, but the spectacled bear (with its working copy) clearly prefers the sugary drink.

Jiang thinks that mammals don’t need to taste sweet things if they only eat a diet of meat. In such “hyper-carnivores”, the evolutionary pressure to keep a pristine sugar sensor dies away, and the gene in question is free to pick up crippling mutations.

Things are even worse in many sea-going mammals. Jiang found that sea lions and bottlenose dolphins have bust versions of not just Tas1r2, but two others responsible for detecting umami, a taste often described as “savouriness” or “meatiness”. The dolphin also has ruined copies of 10 genes that pick up bitter tastes. And indeed, both dolphins and sea lions have very few taste buds in their tongues. These hunters swallow their food whole, and may have little need to taste their morsels first.

Studies like these remind us how evolution is quick to jettison abilities that living things no longer need. But they also remind us that we’re only ever experiencing the world through a sensory keyhole. Other animals can see wavelengths of light or hear frequencies of sound that we cannot. We, in turn, can taste things that many predators can’t detect. What we sense is only a small fraction of all there is to sense. And that fraction has been shaped through evolution to best meet the demands of our lifestyle.

Reference: Jiang, Josue, Li, Glaser, Weihua, Brand, Margolskee, Reed & Beauchamp. 2012. Major taste loss in carnivorous mammals. PNAS http://dx.doi.org/10.1073/pnas.1118360109

image by Ducklover Bonnie

More on evolution and broken genes:


Comments (32)

  1. Georgia

    I’ve always heard that cats can’t taste sweet, but I have an anecdote I was wondering if anyone could explain.
    We had to give our old cat a pill once a day when I was growing up, I can’t remember what for exactly but I think something for his kidneys. We tried shoving it in and he’d always just spit it out later. We tried crushing it in his food or water and he’d never go near the stuff. Finally, my dad tried crushing it in warm sugar water (warm so the sugar would dissolve). Our cat would beg for it every night like it was a treat! Now, clearly this wasn’t a scientific study, but we’re pretty sure the sugar did the trick.
    My one thought is that maybe the sugar reacted with something in the pill and therefore just made it less bitter overall though that doesn’t really explain his eagerness to eat it.

  2. Some very interesting research, and, as usual, you’ve explained it in a way that is both extraordinarily clear and provides us with useful context. I especially appreciate your last paragraph, which reminds us that there is almost always much more going on in the natural world than we tend to appreciate.

    Our cat loves bananas and apples and melon. Now I wonder what about them appeals to her.

  3. E.

    I have had cats that loved doughnuts and cake, including angel food cake, spaghetti, corn (a common fondness in cats, apparently, especially fresh sweet corn raw or cooked), and knew another that liked Fudgsicles. Melon, mentioned above, is another common food for cats to love. I know cats are obligate carnivores, but they’re clearly enjoying something (to the point of dumpster-diving and countertop raids) in the foods they are not supposed, by science, to find interesting.

    As these tastes vary from cat to cat, it is probably more complicated than anyone studying it has found.

  4. It’s very interesting that the dolphin has apparently lost so many of its bitter-taste receptors: you might think these would be useful for avoiding putrid or poisonous food. So does this reveal anything about the evolution/biological function of taste? E.g. is it mainly a reward mechanism to drive an animal to seek out certain foods, or a way of avoiding undesirable/ toxic foods? If it’s largely reward-driven, we could have some interesting speculations about biological bases of the current obesity epidemic.

    On a related note, I can confirm that dogs do indeed have intact TAS1 genes. Mine put his to abundant use one Friday evening, just after all the local vet clinics had closed, to find, steal and gobble an entire bar of Kendal mint cake. Given that said mint cake was also coated in dark chocolate, I spent the entire evening frantically trying to calculate the theobromine LD50 for a greedy little Jack Russell while simultaneously panicking down the phone to the out-of-hours vet. The object of all this fuss, meanwhile, quietly stole a cushion and settled down in front of the TV to watch CSI. Wasn’t ill, wasn’t even sick. Which only goes to show :)

  5. My cat likes to lick plastic. Mostly grocery bags. I did some cursory research on the why of this a couple of years ago, and came up with an article that hypothesized that plastic-licking behavior may be indicative of a (more) mutated “sweet tooth” gene — the cat in this case thinks that the plastic tastes sweet somehow, because this bit of junk DNA (for cats anyway) might now be more prone to mutate in odd ways since it isn’t necessary for survival/repro. Does that make any sense? I’m trying to find this article again, but I can’t for the life of me remember where I dug it up in the first place. Has anyone else heard of this idea?

  6. Not sure my cats have read this article! One regularly steals donuts ( he recognises the bag) and toast with jam on it and the other chooses to ignore my coffee but drinks my hubby’s which has sugar in it. We wondered whether he just liked pinching the alpha male’s drink but even when it was placed next to me he would drink that one rather than the un-sugared one.
    we have also had the trying to get a pill into a cat issue – not the easiest thing to do – and have noticed that hidden in something sweet helps whereas when hidden in favourite meat food it gets left on the plate.

  7. ChasCPeterson

    My kid’s cat loves maple syrup.
    Just more evidence in support of the hypothesis that cats are inscrutably weird.
    Possibly on purpose.

  8. Allison

    My cat loves honey, cake, and fruit. She won’t touch salmon, tuna, or any other savory human food. I’ve heard this before, that cats don’t taste sweetness in food, but I really think my cat does.

  9. Scott

    Am I the only one here who has never heard of an aardwolf?

  10. amphiox

    The various cats people have brought up so far could easily have been reacting to/attracted to other aspects of the foods in question besides pure sugar content.

  11. Stolen Dormouse

    It’s interesting that the closely related group of bears, wolves/dogs, and racoons all kept functional sweetness receptors, while cats lost them.

    My dog provides another anecdote that Canids, unlike cats, are not obligate carnivores. He likes sweet breakfast cereals (perking up from a nap when he hears us opening the bag) But he loves green or red bell peppers and also carrots, vegetables that provide some sweetness, but no fat. The crunchy texture may be attractive, though. However, he’ll chew on a piece of lettuce, but won’t eat it–perhaps it is too bitter; he just leaves the chewed-up leave on the floor.

    My wife and I find the smell of his dry dog food repulsive, though he seems to love it. But scent is another story, that has already been studied.

  12. Conventional wisdom has it that it’s the sweet taste that draws cats to drink antifreeze containing ethylene glycol. If it’s not the sweet, what is it?

  13. Why assume that the gene was broken after the all meat diet started? Maybe in some cases the gene breaks first then the creatures goes after stuff it can taste. After all, how many of us consume quantities of tasteless mush (other than tv shows of course)?

  14. Georgia

    @looloolooweez We had a (different) cat who obsessively chewed plastic bags to the point that we had child locks on all the cabinets. Our vet told us that they dust corn starch on plastic bags so they don’t stick to each other and that cats often eat/lick bags to go after that.

  15. Kaviani

    Ditto #12 – I’ve tried repeatedly to get an answer for this and the best I can come up with is “cats are just stupid”. I refuse to accept that. WHAT compels them to not only taste it but drink it? Their gustatory functions must be more complex than we are assuming.

  16. I had the same question about cats, ethylene glycol and other things they are not supposed to eat.

    Look up the aardwolf. It’s a wolf-shaped aardvark. Honestly, where does the Universe come up with these ideas! Cool post!

  17. Cathy

    I’ve got all of you beat. My room mate had a cat that would eat anything. Anything. We played a little game called “Will Digi eat it?” in which we proffered a food that we knew was safe and let her decide if she wanted to eat it or not. Salad? All over that. (Come to think of it, she viewed houseplants as a sort of vertical salad, and they had to be kept on the highest shelf. ) Bread? All over it. Fruits? Totally there. All vegetables we ever tried, except raw carrots – too crunchy. Raw carrots were the one thing she disdained. Cook them, and she happily nommed away.

    The Digit also got herself in trouble with things that were NOT safe for cats, ranging from garnache dark chocolate frosting (for which she landed herself in the emergency vet for a weekened) to, I kid you not, a bee that had been killed with flying insect spray (which involved an hour long tearful phone call with poison control while the oblivious cat napped in the sunshine.) The sport of “Will Digi eat it?!” never grew old. Digit also had some other mutations – she was the runt of the litter, had a hormone issue so that she never quite developed a real meow or a full size frame (forever kitten!) and her sweet tooth shall live in infamy. Her favorite of all? Steamed broccoli.

    Currently The Didge is being spoiled senseless by her grandma, as her owner is living in Japan for a few years teaching English. Grandma once found that Digit had gotten into the fruit bowl, and started having to keep the bread in the fridge.

  18. Michelle M

    One of my cats like edemame and tofu. I’ve had cats that liked various melons. One cat loved persimmon. One cat I brought home loved to lick plastic, after watching it go nuts over plastic, two of the others tried it and decided they liked plastic licking too…but never to the same extent.

  19. Some carnivores evolve over time into omnivorous species – I wonder if that’s more likely for carnivores the retain functional sweet-tasting genes. That would also imply that species that have been obligate carnivores for a long time are going to stay that way, because they’ve lost the useful taste senses for plants.

    Interesting that dolphins can barely taste. I’ve always been jealous of their world of sonar, but we’ve got at least one sensation that’s mostly beyond what they can feel. Maybe the difference between our sense of taste and theirs is comparable to the difference between a dog’s sense of smell and ours.

  20. CargoUK

    I’ve always loved the German word ‘Umwelt’ in the context of the final paragraph. In Sensory Physiology lectures (many years ago) we were given the definition as ‘Sensory World’ as an aid to understanding the concept.
    As a side question, can anyone point to real scientific data on the question ‘Cats can’t metabolise/shouldn’t eat pork/ham’? I see a lot of opinion but no data on the interwebs.

  21. At times it would be good if we did expand our perceptions a little. Given that we at least have the cognition to do it.

    Oh, and aardwolves are termite eaters from sub-saharan Africa, not easy to spot.

  22. Aardwolves are basically hyenas that have gone myrmecophageous.

  23. Gerdien

    I once had a growing-up youngster tomcat that ate used teabags and boiled endive …
    He grew to be a very large tom, and chose the wild life.

  24. Jill

    I had a cat that was clearly a re-incarnated italian grandma. Loved cofee & ate cigarettes. Seriously.

  25. Sharon Bakar

    I had a cat that was crazy for papaya, and another that loved durian – the smelliest fruit in the world. Actually durian is very sweet and relies on large mammals including tigers to eat it and redistribute its seeds. I guess the other animals it appeals to must love that sweetness …

  26. ChuckB

    My mother once had a Siamese tom that absolutely loved cantalope – would start begging if she even picked one up as if she were going to cut it open. Was a very smart cat: learned to jump up and wrap it’s paws around the knob of the door to the basement stairs, twist it, and cause it to open a crack, then nose it open to get out.

  27. Terry Hutchin

    I had a cat who loved sweets. It wasn’t the case that she’d eat anything, as she was a very finicky eater and never weighed more than 6 lbs in her entire life. But she loved sweets.

    Our vet said it was just the dairy that she was attracted to; however, she also ate sweets which weren’t dairy-based.

    Since the people-food she liked had nothing in common except that they were all sweet, I assume that this is what attracted her.

  28. Folks, not that these cat anecdotes aren’t lovely, but enough now. This isn’t a cat forum.

  29. Ranger Jim Kirk

    Interesting article.

    I don’t have any particular stories to tell about cats that my ex and I have lived with, but my personal opinion is that what we are seeing is “evolution in action.” While dogs are not quite omnivores these days, they are certainly moving in that direction, IMO. And cats do like to munch on weeds and other greens occasionally (and then, of course, there’s catnip, eh? 😉 ).

    But I think that Stephan Zielinski had a serious question when he said, (#12) “Conventional wisdom has it that it’s the sweet taste that draws cats to drink antifreeze containing ethylene glycol. If it’s not the sweet, what is it?”

    And then I believe that there is a sweet that cats like – and can eat – which dogs cannot, but I can’t recall what it is (which is why I don’t give my Biet Cat any sweets).

  30. Ranger Jim Kirk

    No, Ed Yong, this isn’t a “cat forum,” however it is a discussion about “how dolphins, cats and other meat-eaters lost their sweet tooth.”

    And I dare say that since most people are familiar with the eating habits of the only “obligate carnivore” that most of us will ever live with, e.g. the domestic cat, the comments and “anecdotes” about the eating habits of cats are germane to this discussion, eh?

    “Studies like these remind us how evolution is quick to jettison abilities that living things no longer need. … What … [both animals and humans] sense is only a small fraction of all there is to sense. And that fraction has been shaped through evolution to best meet the demands of our lifestyle.”

    — ” A lack of taste — how dolphins, cats and other meat-eaters lost their sweet tooth.”, Ed Yong, (c) Discovery Magazine, 2012 [my apologies if the copyright is yours.]

  31. Pam Glatczak

    My cat likes sweet dairy based foods, but I am sure it’s the dairy. Butter, margarine, frosting, whipped cream, ice cream. Only a little; he’s not a glutton.
    But he also likes peanut butter (oil?) and he likes to lick glossy photos, and will chew and eat the styrofoam packing peanuts, both of which we have to keep put away.

    He loves any pill that is in a capsule. That resulted in a long vet day and a cat that refused to vomit until he got home and found a rug to puke on in our house of mostly tile and hardwood.

    Is it the glycerin coating on the capsules? Originally I thought it was because they were a little sweet, but now I wonder.

    I had cats that loved banana bread too.

  32. Pam Glatczak

    Oh and Mr Yong, I am loving the cat stories as well as the article. I do believe most people reading this article have a thing for cats!


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