What do spotted hyenas, Asian small clawed otters, and dolphins have in common?

According to a recent paper published in the Proceedings of the National Academy of Sciences, these species have all independently lost the ability to detect sweet foods.

No more lollipops for these critters (Photo credit: Stuart Miles)

Because of mutations in a specific taste receptor gene (Ts1r2), domestic cats do not show the preference for sweet foods common to many other animals.  Jiang and colleagues speculated that other meat eaters may have also lost their sweet sensation. They explored this possibility by examining the Ts1r2 genes from 12 species in the order Carnivora.  [Despite its name, Carnivora includes both strictly carnivorous species (like cats) as well as omnivores (like bears) and herbivores (like the giant panda).] Five species—the aardwolf, Canadian otter, spectacled bear, red wolf, and racoon—have intact Ts1r2 genes. But the other species—Asian small clawed otter, Pacific Harbor seal, Fur seal, sea lion, spotted hyena, fossa, and banded linsang—all have mutations in their Ts1r2 genes that should make them nonfunctional. All of the species with the mutated Ts1r2 genes are strict meat or fish eaters and presumably have very little need to detect sweetness. Interestingly, different mutations were found in different species, suggesting independent evolution. 

Asian Short-clawed Otter

Asian small/short clawed otter (Image credit: Dave Gunn)

How does having a mutated Ts1r2 affect animal behavior?  To explore this, Jiang and colleagues tested two Asian small clawed otters (with mutated Ts1r2 genes) and four spectacled bears (with intact Ts1r2 genes) with something called the two bowl test. The animals were presented with (you guessed it) two bowls: one containing water and one containing a naturally or artificially sweetened liquid. Normally animals that can detect sweetness will preferentially drink from the bowl containing sweetened liquid over the bowl containing plain water. And this was indeed the case for the spectacled bears—they strongly preferred to drink the sweetened liquid. The small clawed otters, however, showed no preference for sugar water over plain water and actually preferred water over one of the artificial sweeteners. These results support a “don’t use it? you lose it!” theory of sweetness detection.

What happens when you don’t even chew your food? (photo credit: Bill Longshaw)

What about animals like dolphins and sea lions who swallow their food whole? Jiang and colleagues discovered that not one, not two, but three sweet receptor genes (Ts1r1, Ts1r2, and Ts1r3) were mutated in these species! This makes sense given that behavioral studies by other groups have found that dolphins have an impaired ability to taste sweetness. 

These results show that as species limit their diet to certain foods, the ability to taste other foods can be lost. This is not unique to sweet foods: some macaque monkeys (and humans) have mutations in another taste receptor gene that prevents them from detecting bitterness in some foods. 

ReferencePeihua Jiang, Jesusa Josue, Xia Li, Dieter Glaser, Weihua Li, Joseph G. Brand, Robert F. Margolskee, Danielle R. Reed, and Gary K. Beauchamp. Major taste loss in carnivorous mammalsProceedings of the National Academy of Sciences, March 12, 2012 DOI:10.1073/pnas.1118360109

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