Sugar at Suppertime

Say “cat” to your average person on the street, and images of furry, purring, string-chasing pets should come to mind.  To a biologist, that same word may provoke this  question: “Do you mean the pet or the family?”  After all, the term “cat” refers to both our pet cats and their close relatives, from big cats like panthers and tigers to smaller ocelots and wildcats (the closest living relative to the domestic variety).  Stick a house cat next to a tiger and (assuming the former doesn’t flee from the latter) you’ll notice a number of similarities in body design and behavior.  But, there’s one characteristic common to all cats (big and small) that may not be readily apparent: these creatures can’t taste sugar.

It’s been documented for decades that members of the cat family (including our domestic feline companions) lack a sweet tooth.  This quirk goes beyond avoiding birthday cake and Girl Scout cookies; all cats, from lion to lynx, are as indifferent to sugar as many Americans are to exercise.  Cats show no inclination toward consuming any sweet, sugar-laden morsel.  Instead, they crave meat: the feline diet is dramatically high in protein.  In fact, their diet is so protein-rich that they have streamlined their gastrointestinal tracts, making them notably shorter than those of mammals with a more varied diet.  It’s as if the cat family is saying, “Why waste so much time, energy, and space making a long and coiling intestine to break down and absorb nutrients from tough plant matter when we are mostly eating meat?”

I’ll skip dessert.

In 2005, a paper published in the journal PLoS Genetics explained precisely why members of the cat family show such an indifference to sweet substances.  The answer lies within the genetic makeup of cats: one of the genes necessary to taste sweet substances is “broken” and doesn’t work properly.  The scientists who made this discovery at the Monell Chemical Senses Center and the University of Pennsylvania (both in Philadelphia) reported this remarkable finding not just for our beloved domestic cat, but also in “big cat” species like the tiger and the cheetah.  The gene in question is named Tas1r2.  This gene normally works in conjunction with another gene to allow humans and many other mammals to detect and taste sweet, sugary substances.  But in the cat family, the gene Tas1r2 contains so many deleterious mutations that it is simply no longer able to do its job.  It’s nonfunctional, a relic of an ancestral past where meat wasn’t such a major part of the cat diet.  In cats, the gene Tas1r2 doesn’t seem to have a function anymore; it simply sits there, a shell of its former self.

Thus, it appears that cats are a little more quirky than we’d previously given them credit for.  However, these findings left some unanswered questions:

  1. Which came first?  Did cats first lose the ability to taste sweet substances and then switch to a high protein, all-meat diet?  Or vice versa?  Or did both dietary characteristics evolve gradually in the feline lineage?
  2. Is the inability to taste sweet substances unique to cats, or are there other groups of mammals that have lost this ability?  If there are, do these other species also have high-protein, strictly-carnivorous diets?

A paper published recently in The Proceedings of the National Academy of Sciences of the United States of America has shed some light on this subject.  It appears that cats are not alone in their inability to taste sugar, though some of the other mammals that have joined them in this anti-sugar crusade are not what you would expect.

First, some background on the sense of taste (known formally as “gustation”).  Many mammals (including humans) detect five distinct tastes using the taste buds on the tongue, palate, and epiglottis: sweet, sour, bitter, salt, and umami.  Each of these five tastes consists of different types of chemicals.  Our taste buds sense the differences between these chemicals, and transmit this information to the brain, which must initiate appropriate responses for each taste.  Bitter tastes, for example are thought to act as a warning against ingesting potentially hazardous or poisonous substances.  Sweet tastes, on the other hand, indicate the presence of energy-rich sugars.  Our taste buds must detect the different chemical signatures of these two tastes (sweetness vs. bitterness), and transmit this information accurately to the brain, so that the brain will initiate the proper responses (avoid “bitterness,” ingest “sweetness”).

Biologists believe there is a high degree of similarity between different mammalian species when it comes to how these tastes are detected.  For example, the ability to detect and taste salt is pretty similar between different mammals, and rightly so.  After all, we all use our ability to taste salt to help maintain proper salt balance in our tissues and organs.  We use bitterness as a signal to avoid potentially harmful substances, umami to detect “savory” substances like high-protein meat, sweetness to recognize energy-rich sugars, and sourness to detect acidic substances.  For all these tastes, mammals carry an assortment of genes which, within taste buds, recognize different types of chemicals that are present in these different tastes.  Sometimes, these genes work together to detect different taste chemicals, like how the Tas1r2 gene and its partner work together to detect sugars; other taste genes work alone.  As we’ve already seen for cats, sometimes these taste genes are no longer needed and simply fade away.

In fact, this new study from The Proceedings of the National Academy of Sciences of the United States of America provides evidence that the phenomenon of taste loss in mammals may not be as rare as once thought.  Scientists from the Monell Chemical Senses Center and the University of Zurich, including some who previously studied the loss of the Tas1r2 sweet-sensing gene in cats, looked at a range of species within a mammalian group called the Carnivora.  Carnivora mammals include dogs, cats, seals, bears, raccoons, otters, and many other species.  As the name suggests, these species rely on a largely carnivorous diet, though some Carnivora species (like bears) can eat more varied omnivorous diets of both meat and plant matter.

Within the Carnivora, the research team found seven species that have likely lost, or severely curtailed, the ability to taste sweet compounds.  Like cats, these species have a heavily mutated and nonfunctional Tas1r2 genes, and range from the spotted hyena to the Asian small-clawed otter.  Biologists specifically tested one of these species, the Asian small-clawed otter, for the ability to detect sweet substances.  They compared the otter to a Carnivora species with a working Tas1r2 gene, the spectacled bear.  In a behavioral test, while the bear could taste sweet substances, the otter was as indifferent to them as a cat would be.  Thus, it appears that “breaking” the Tas1r2 gene with harmful mutations may be a common mechanism by which many carnivorous species lose the ability to taste sweet substances.

This research team also reports another interesting observation: the dramatic loss of taste genes in some marine mammals.  Several lineages of mammals returned to the ocean tens of millions of years ago.  These mammals include seals and sea lions, cetaceans (whales, dolphins, and porpoises), and sirens (manatees and their relatives).  Biologists long ago noticed that many marine mammals, including dolphins and seals, have much lower numbers of taste buds in their mouths and on their tongues compared to land mammals.  Many speculated that the aquatic lifestyle had dulled the taste senses in these creatures, and that taste was less important for their feeding behaviors compared to their land-based mammalian cousins.  As if to prove the point to us, some sea lions and cetaceans swallow their prey whole, keeping food in their mouths for only a short amount of time and dramatically decreasing the time window in which taste buds could try to detect the different chemical signatures of the food being ingested.  The team from the Monell Chemical Senses Center reports that three Carnivora species they studied are aquatic mammals: two species of seal and a species of sea lion.  All three species had heavily-mutated and nonfunctional Tas1r2 genes, indicating that these species had likely lost the ability to taste sweet compounds like some of the land-based Carnivora.  The research team reported that sea lions also have a heavily-mutated and “broken” gene that is necessary to taste umami, indicating that this species has likely lost the ability to detect at least two of the five tastes.

Tastes like chicken.

The results are even more dramatic for the bottlenose dolphin.  Dolphins and other cetaceans aren’t Carnivora.  They actually evolved from ancient relatives of deer, pigs, camels, and the hippopotamus.  But, given the prior research showing a dramatic reduction of taste buds in cetaceans and other aquatic mammals, this research team chose to investigate the taste genes in the bottlenose dolphin and see if it has lost key genes in taste  sensation and detection.  Like the sea lion, the bottlenose dolphin has lost key genes for tasting sweet and umami compounds.  This team also found some evidence that dolphins have also lost at least one of the genes necessary to taste bitter compounds.  Thus, of all the species reported in this recent paper in The Proceedings of the National Academy of Sciences of the United States of America, aquatic mammals show the greatest loss of genes required for taste detection, with dolphins losing genes required to detect at least three of the five tastes.

These results present biologists with a number of new questions to study:

  • Can we test other species within Carnivora (or other mammalian lineages) to see if they have truly lost the ability to taste sweet?  After all, in this most recent study, scientists merely reported on the loss of taste genes, and only tested behavioral responses to sugar for the Asian small-clawed otter (which had lost its Tas1r2 “sweetness” gene) and the spectacled bear (which has a working Tas1r2 gene).
  • How widespread is taste gene loss in mammals?  Is it just confined to species that have very specific diets (like carnivorous cats and otters) or who live in aquatic habitats (like dolphins and sea lions)?  Or, are there other circumstances under which species could lose taste genes?
  • How does this pattern of gene loss proceed?  Did diets become specialized (strictly carnivorous) or species move into aquatic habitats before losing many of these taste genes to deleterious mutations?  Or vice versa?

Future studies should help address these questions.  But for now, we can at least begin to understand why dolphin obesity rates are nonexistent, while human waistlines continue to grow.

FURTHER READING:

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About James Urton

I went to school to become a molecular biologist.  At some point in this long education, I discovered that I love communicating science to the general public: talks, writing, at a pub, on the street corner...  Whatever venue will let me hold your attention for a few moments.  Unfortunately, I can't do this for a living, since no one will pay me.  So, I have a job as a molecular biologist at the University of Washington, where I get to work with great scientists on some really awesome projects, and I'll blog about science here at Muller's Ratchet in my spare time. Why should the general public want to know anything about science? Here's my explanation (which also explains why I chose the name Muller's Ratchet for this site). Briefly as a graduate student (before I had to devote all of my time to graduating), I blogged at Adaptive Radiation.
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One Response to Sugar at Suppertime

  1. Staffan says:

    I always thought my cat liked molasses because of the sugar. I mean there isn’t much else in it.

    I wonder if this Tas1r2 will be used for a gene therapy for obesity. Or maybe it could be done less drastically by reducing the sense of smell instead. I suspect that my undeveloped olfaction is the reason why I’m less interested in food than most people.

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