（很多）食肉动物尝不出甜味# Animals - 动物园
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http://news.discovery.com/animals/carnivores-taste-sweet-120312
WHY CATS, OTHER CARNIVORES DON'T TASTE SWEETS
With no need for carbohydrates, many carnivorous animals have lost the
ability to detect sweet flavors.
Lions and Asian otters don't care for sweets but raccoons and spectacled
bears will eat almost anything. Now a new study helps explain why.
Independently and fairly recently, genetic mutations have made various
carnivores unable to taste sweet foods.
Probably because these species were already subsisting off of meat-only
diets that lacked sweet flavors when the mutations first occurred, they did
just fine after losing their sweet receptors -- giving rise to entire
species of animals that lack appreciation for cookies or fruit.
For omnivorous creatures that chew their food, on the other hand, the
ability to taste carbohydrates remains a matter survival, and their sweet
receptors remain intact.
Besides offering a window into the unique sensory worlds of other animals,
the research adds to our understanding of the complexity of taste perception
. By better understanding how the system works, this and research like it
could lead to a variety of applications, including the development of better
artificial sweeteners or sweet enhancers.
For decades, scientists have known that cats show no preference for sweets.
Then in 2005, researchers at the Monell Chemical Senses Center in
Philadelphia published research showing that domestic cats have a mutation
rendering their taste receptors unable to bind to sweet molecules. The same
was true of their wild cousins, including lions, tigers and jaguars.
Many people were unable to accept this news about their feline pets.
"When we first published the data on cats, it got a tremendous amount of
publicity and a lot of people saying, 'My cat likes sweets and you're wrong,
'" said biologist Gary Beauchamp, director of the Monell Center. "But
invariably they liked ice cream or cake, and sweetness was confounded with
fat and other things."
"In retrospect it seems obvious," he added. "But it was to my surprise when
we found out that this [loss of sweet taste] has happened repeatedly and
independently in many species."
To investigate whether other animals might share the finicky cat's lack of
appreciation for desserts, Beauchamp and colleagues analyzed taste receptor
genes of a dozen species of carnivores. All of the animals have taste
perception systems that are similar to the human system, with specific known
genes that code for receptors for each of the five tastes: sweet, salty,
sour, bitter and umami.
Using computer algorithms, the researchers could then scan each gene in each
species to see if it contained any sequences that would make it unable to
produce the proteins needed to sense each taste quality.
Of the 12 animals studied, seven had mutations that made them unable to
taste sweets, the researchers report today in the journal Proceedings of the
National Academy of Sciences. All seven of those eat meat and only meat,
and some inhale their food without even chewing. The list included
bottlenose dolphins, sea lions, spotted hyenas and fossas (a cat-like
carnivore).
Dolphins and sea lions also appear unable to taste the savory flavor umami,
and dolphins might also be missing the ability to detect bitter flavors.
On the other hand, sweet-sensing genes were still functional in aardwolfs (a
member of the hyena family), Canadian otters, red wolves, spectacled bears,
and raccoons. The last three are meat-eaters who also eat fruits and other
foods.
In a follow-up experiment that used behavior to back up the genetics, Asian
otters showed no particular preference for water laced with sugar or
artificial sweeteners, while spectacled bears almost unanimously chose the
sweetened liquid.
When the researchers looked more closely at the genes, they saw that, for
the most part, different mutations independently disabled sweet receptors in
different species -- suggesting that taste receptor mutations have popped
relatively recently in the scheme of evolution.
And an animal's diet, it appears, determines whether a mutation will
disappear or stick around.
Understanding from a genetic perspective what animals can and cannot taste
could help zookeepers and other handlers design desirable diets for
creatures in captivity, said Thomas Finger, a neurobiologist who studies
taste and smell at the University of Colorado School of Medicine in Aurora.
On a deeper level, the study offers insights into how life on Earth is
constantly evolving.
"Nature's always tossing the dice and mutating genes all over the place,"
Finger said. "This says that losing a taste gene in an environment where
nutrition doesn't depend on it doesn't matter. That loss will persist,
because there's no reason for it to be eliminated."
WHY CATS, OTHER CARNIVORES DON'T TASTE SWEETS
With no need for carbohydrates, many carnivorous animals have lost the
ability to detect sweet flavors.
Lions and Asian otters don't care for sweets but raccoons and spectacled
bears will eat almost anything. Now a new study helps explain why.
Independently and fairly recently, genetic mutations have made various
carnivores unable to taste sweet foods.
Probably because these species were already subsisting off of meat-only
diets that lacked sweet flavors when the mutations first occurred, they did
just fine after losing their sweet receptors -- giving rise to entire
species of animals that lack appreciation for cookies or fruit.
For omnivorous creatures that chew their food, on the other hand, the
ability to taste carbohydrates remains a matter survival, and their sweet
receptors remain intact.
Besides offering a window into the unique sensory worlds of other animals,
the research adds to our understanding of the complexity of taste perception
. By better understanding how the system works, this and research like it
could lead to a variety of applications, including the development of better
artificial sweeteners or sweet enhancers.
For decades, scientists have known that cats show no preference for sweets.
Then in 2005, researchers at the Monell Chemical Senses Center in
Philadelphia published research showing that domestic cats have a mutation
rendering their taste receptors unable to bind to sweet molecules. The same
was true of their wild cousins, including lions, tigers and jaguars.
Many people were unable to accept this news about their feline pets.
"When we first published the data on cats, it got a tremendous amount of
publicity and a lot of people saying, 'My cat likes sweets and you're wrong,
'" said biologist Gary Beauchamp, director of the Monell Center. "But
invariably they liked ice cream or cake, and sweetness was confounded with
fat and other things."
"In retrospect it seems obvious," he added. "But it was to my surprise when
we found out that this [loss of sweet taste] has happened repeatedly and
independently in many species."
To investigate whether other animals might share the finicky cat's lack of
appreciation for desserts, Beauchamp and colleagues analyzed taste receptor
genes of a dozen species of carnivores. All of the animals have taste
perception systems that are similar to the human system, with specific known
genes that code for receptors for each of the five tastes: sweet, salty,
sour, bitter and umami.
Using computer algorithms, the researchers could then scan each gene in each
species to see if it contained any sequences that would make it unable to
produce the proteins needed to sense each taste quality.
Of the 12 animals studied, seven had mutations that made them unable to
taste sweets, the researchers report today in the journal Proceedings of the
National Academy of Sciences. All seven of those eat meat and only meat,
and some inhale their food without even chewing. The list included
bottlenose dolphins, sea lions, spotted hyenas and fossas (a cat-like
carnivore).
Dolphins and sea lions also appear unable to taste the savory flavor umami,
and dolphins might also be missing the ability to detect bitter flavors.
On the other hand, sweet-sensing genes were still functional in aardwolfs (a
member of the hyena family), Canadian otters, red wolves, spectacled bears,
and raccoons. The last three are meat-eaters who also eat fruits and other
foods.
In a follow-up experiment that used behavior to back up the genetics, Asian
otters showed no particular preference for water laced with sugar or
artificial sweeteners, while spectacled bears almost unanimously chose the
sweetened liquid.
When the researchers looked more closely at the genes, they saw that, for
the most part, different mutations independently disabled sweet receptors in
different species -- suggesting that taste receptor mutations have popped
relatively recently in the scheme of evolution.
And an animal's diet, it appears, determines whether a mutation will
disappear or stick around.
Understanding from a genetic perspective what animals can and cannot taste
could help zookeepers and other handlers design desirable diets for
creatures in captivity, said Thomas Finger, a neurobiologist who studies
taste and smell at the University of Colorado School of Medicine in Aurora.
On a deeper level, the study offers insights into how life on Earth is
constantly evolving.
"Nature's always tossing the dice and mutating genes all over the place,"
Finger said. "This says that losing a taste gene in an environment where
nutrition doesn't depend on it doesn't matter. That loss will persist,
because there's no reason for it to be eliminated."
