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."