Vampire bats showed indifference to sweet flavors, so we now know they don't have a sweet tooth. These bats also had trouble detecting bitter, salty and sour tastes. That's significant, the researchers believe.
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"Mammals typically have five primary taste modalities dedicated to the evaluation of diets, of which the bitter taste serves as an important natural defense against the ingestion of poisonous foods and is thus believed to be indispensable in animals," they wrote.
So much for that theory about being indispensable, since vampire bats have lost much of their ability to taste bitter flavors.
Hong and Zhao say that vampire bats find their food, not by taste, but by using a combination of smell, echolocation and heat detection. All of these allow them to "find their prey and locate the skin with rich capillaries."
It remains a mystery as to when and how vampire bats first got on their unusual all-blood diet. If the ancestors of the bats ate other things, then these animals might have once possessed a better ability to taste multiple flavors. That ability then could have been lost over time as the bats sipped more blood.