Abstract
A growing body of behavioral and genetic information indicates that taste perception and food sources are highly coordinated across many animal species. For example, sweet taste perception is thought to serve to detect and motivate consumption of simple sugars in plants that provide calories. Supporting this is the observation that most plant-eating mammals examined exhibit functional sweet perception, whereas many obligate carnivores have independently lost function of their sweet taste receptors and exhibit no avidity for simple sugars that humans describe as tasting sweet. As part of a larger effort to compare taste structure/function among species, we examined both the behavioral and the molecular nature of sweet taste in a plant-eating animal that does not consume plants with abundant simple sugars, the giant panda (Ailuropoda melanoleuca). We evaluated two competing hypotheses: as plant-eating mammals, they should have a well-developed sweet taste system; however, as animals that do not normally consume plants with simple sugars, they may have lost sweet taste function, as has occurred in strict carnivores. In behavioral tests, giant pandas avidly consumed most natural sugars and some but not all artificial sweeteners. Cell-based assays revealed similar patterns of sweet receptor responses toward many of the sweeteners. Using mixed pairs of human and giant panda sweet taste receptor units (hT1R2+gpT1R3 and gpT1R2+hT1R3) we identified regions of the sweet receptor that may account for behavioral differences in giant pandas versus humans toward various sugars and artificial sweeteners. Thus, despite the fact that the giant panda's main food, bamboo, is very low in simple sugars, the species has a marked preference for several compounds that taste sweet to humans. We consider possible explanations for retained sweet perception in this species, including the potential extra-oral functions of sweet taste receptors that may be required for animals that consume plants.
Highlights
We and others have argued that taste function and diet are intimately connected through coordinated evolutionary processes that fit one to the other [1,2,3,4]
We approached this study with two competing hypotheses: (1) the plant-based diet of the giant panda would lead to the prediction that they, like all other plant-eating mammals tested, have fully functioning sweet taste perception, whereas (2) their almost total reliance on a plant diet lacking in simple sugars would lead to the prediction that they have lost sweet taste function, as have obligate carnivores
This latter hypothesis was reinforced because the giant panda has lost the function of a similar receptor for amino acids and savory tastes that have been suggested to be associated with meat [7,26]
Summary
We and others have argued that taste function and diet are intimately connected through coordinated evolutionary processes that fit one to the other [1,2,3,4]. Giant pandas exclusively consume plants, and one might predict that they should have a functional sweet taste system. This hypothesis is consistent with genetic studies that predict that genes that encode the sweet taste receptor (T1R2 + T1R3) are intact and functional [7]. Their diet, 99% of which consists of bamboo, a single plant species that has a low sugar content [8], might lead one to predict that they, like strict carnivores [1,5,6,9,10], have lost sweet taste function
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