Scientists from the Monell Center report that seven of 12 related mammalian species have lost the sense of sweet taste. As each of the sweet-blind species eats only meat, the findings demonstrate that a liking for sweets is frequently lost during the evolution of diet specialization.
Previous research from the Monell team had revealed the remarkable finding that both domestic and wild cats are unable to taste sweet compounds due to defects in a gene that controls structure of the sweet taste receptor.
Cats are obligate carnivores, meaning that they subsist only on meat. In the current study, published online in Proceedings of the National Academy of Sciences Early Edition, the Monell scientists next asked whether other strict carnivores have also lost the sweet taste receptor.
To do this, they examined sweet taste receptor genes from 12 related mammalian species with varying dietary habits. They once again found taste loss and to their surprise, it was widespread in the meat-eating species.
Senior author Gary Beauchamp, Ph.D., a behavioral biologist at Monell, comments, "Sweet taste was thought to be nearly a universal trait in animals. That evolution has independently led to its loss in so many different species was quite unexpected."
The integrity of the sweet taste receptor gene was closely related to the animals' diets. Sea lion, fur seal, Pacific harbor seal, Asian otter, spotted hyena, fossa, and banded lingsang, species that are exclusive meat eaters, all had defective sweet receptor genes.
In contrast, intact sweet receptor genes were found in aardwolf, Canadian otter, spectacled bear, raccoon, and red wolf. These species include both exclusive meat-eaters and those that also eat other foods in addition to meat.
Further examination revealed that the defective portion of the sweet receptor gene varies among the seven species with non-intact receptors. Together, the findings suggest that diet-related taste loss has happened repeatedly and independently throughout evolution, demonstrating the importance of dietary niche in the structure and function of an animal's sensory system.
To further explore the relationship between feeding behavior and taste function, the researchers next examined sweet and umami (savory) taste receptor genes in two mammals that have "returned" to the sea, the sea lion and bottlenose dolphin. These animals were selected because they swallow their food whole, suggesting that taste may not play an important role in their dietary selection.
As predicted, taste loss was extensive in these animals. Sweet and umami receptor genes were nonfunctional in both, and the dolphin also had nonfunctional bitter taste receptor genes.
"Different animals live in different sensory worlds and this particularly applies to their worlds of food. Our findings provide further evidence that what animals like to eat - and this includes humans - is dependent to a significant degree on their basic taste receptor biology," said Beauchamp.
The current research raises questions beyond diet choice, as taste receptors have recently been identified in many organs throughout the body, including intestine, pancreas, nose, and lungs. These extra-oral taste receptors are believed to serve many different functions.
"Our findings clearly show that the extra-oral taste receptors are not needed for survival in certain species," notes lead author Peihua Jiang, Ph.D., a molecular biologist at Monell.
"The animals we examined did not have functional sweet, umami, or bitter taste receptors, so it will be important to identify how their functions were replaced throughout the body."
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