Anyone who’s watched a cat throwing up after munching on grass
knows that our feline friends aren’t natural plant eaters. So you might
be surprised to discover that these carnivorous animals share some
important genes that are more typically associated with herbivores. And
this might help explain why cats aren’t always easy to please when it
comes to food.
New research
suggests that cats possess the genes that protect vegetarian animals
from ingesting poisonous plants by giving them the ability to taste
bitter. Animals use their sense of taste to detect whether a potential
food is nutritious or harmful. A sweet taste signals the presence of
sugars, an important source of energy. A bitter taste, on the other hand, evolved as a defence mechanism against harmful toxins commonly found in plants and unripe fruits.
Evolution has repeatedly tweaked animals' taste buds to suit various
dietary needs. Changes in an animal’s diet can eliminate the need to
sense certain chemicals in food, and so receptor genes mutate, destroying their ability to make a working protein. I can haz chlorophyll.Lisa Sympson/Wikimedia Commons, CC BY-SA
One example of this comes from strictly meat-eating cats, who can no longer taste sweetness.
But if bitter detection evolved to warn of plant toxins, then it stands
to reason that cats, which (usually) eschew plants, shouldn’t be able
to taste bitter either. Humans and other vegetable-munching animals can
taste bitter because we possess bitter taste receptor genes. If cats
have lost the ability to taste bitterness, we should find that their
receptor genes are riddled with mutations.
Geneticists at the Monell Chemical Senses Center
in Philadelphia scoured the genome of cats and other carnivorous
mammals like dogs, ferrets, and polar bears to see if our carnivorous
cousins have bitter genes.
They were surprised to find that cats have 12 different genes for
bitter taste. Dogs, ferrets, and polar bears are equally well endowed.
So, if meat eating animals are unlikely to encounter any bitter morsels,
why do they boast genes for tasting bitterness?
Taste test
To find out, Peihua Jiang, a molecular biologist at Monell, put cat
taste buds to the test. He inserted the cat taste receptor gene into
human tissue cells in the lab. When combined, the cell and the gene act
as a taste receptor that responds to chemicals dropped onto it.
Jiang discovered that the cat’s taste receptors responded to bitter
chemicals found in toxic plants and to compounds that also activate
human bitter receptors. The cat bitter taste receptor, known as Tas2r2,
responded to the chemical denatonium benzoate, a bitter substance
commonly smeared on the fingernails of nail-biting children.
So why have cats retained the ability to detect bitter tastes?
Domestic cats owners know how unpredictable cats' dietary choices can
be. Some of the “presents” cats bring to their owners include frogs,
toads, and other animals that can contain bitter and toxic compounds in
their skin and bodies. Jiang’s results show that bitter receptors
empower cats to detect these potential toxins, giving them the ability
to reject noxious foods and avoid poisoning. Hair of the dog.Michal Hrabovec/Flickr, CC BY-NC-SA
But how often do meat-loving cats actually get exposed to bitter and
toxic compounds in their diet, compared with the plethora of plant
toxins that their vegetarian counterparts have to contend with? Jiang
suggests this is not enough to explain why cats have retained such an
arsenal of receptors.
Instead, cat taste receptors may have evolved for reasons other than
taste. In humans, bitter taste receptors are found not only in the
mouth, but also in the heart and in the lungs, where they are thought to
detect infections. It remains to be seen if feline bitter receptor genes also double-up as disease detectors.
The discovery of feline bitter receptors might explain why cats have
got a reputation as picky eaters. But their unfussy canine counterparts
have a similar number of bitter taste receptors – so why are cats so
finicky? One answer might lie in how the cat receptors detect
bitter-tasting compounds. Research published
earlier this year by another team of researchers showed that some of
the cat taste receptors are especially sensitive to bitter compounds,
and even more sensitive to denatonium than the same receptor in humans.
Perhaps cats are also more sensitive to bitter chemicals than dogs,
or they may detect a greater number of bitter compounds in their
everyday diet. Food that tastes bland to us or to a dog could be an
unpleasant gastronomic experience for cats. So rather than branding cats
as picky, perhaps we should think of them as discerning feline foodies.
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