In gaining their taste for blood, vampire bats may have nearly lost their ability to taste bitter.
A new study in Proceedings of the Royal Academy B reveals that vampire bats, unlike nearly all other mammals, have a significantly reduced ability to taste bitter compounds.
Bitter tastes can be unpleasant, but—because many toxic compounds taste bitter—they can also be lifesaving. So animals lacking bitter taste receptors would seemingly be at a significant disadvantage.
“We thought they were essential to [every] animal species,” said Huabin Zhao, a biologist at Wuhan University in China and one of the authors of the genetic study, conducted over the past year in Zhao’s lab.
But vampire bats have a very specialized diet. The three species that comprise this group—the common vampire bat, Desmodus rotundus; the white-winged vampire bat, Diaemus youngi; and the hairy-legged vampire bat, Diphylla ecaudata—are the only mammals that subsist entirely on blood. They find their prey both by detecting its odor and by using special infrared sensors that tell them where the warm, blood-rich patches of skin are. This means that vampire bats are very good at feeding only on blood; they tend not to accidentally consume other items that could be toxic. (Watch a video of vampire bats feeding.)
Bitter taste receptors seem to have a function opposite that of the taste receptors for sweet, salty, sour, and umami. While studying taste receptors in vampire bats, Zhao found that as the species evolved to feed only on blood, they lost the ability to taste sweet and umami. Zhao wondered whether the same evolutionary pressures might have also reduced their sensitivity to bitter.
He searched through the three vampire bat genomes for signs of bitter taste receptors, then compared these receptors to those found in 11 non–vampire bat species. Each of the vampire bats had nine bitter taste receptors, but many of them were nonfunctional. In contrast, most bitter taste receptors worked fine in other bat species.
Zhao sees the specialized diet of the vampire bats as the culprit. ”In evolution,” he said, “it’s use it or lose it.”
That means that vampire bats have probably been slowly losing their bitter taste perception since they first started diverging from other bats about 65 million years ago. In time, Zhao says, all of their bitter receptors may eventually be rendered nonfunctional. Or, he adds, their bitter taste receptors may have another important, as-yet-undiscovered, non-oral function.
“The most exciting functions of bitter receptors probably remain to be discovered,” said Danielle Reed, a scientist at the Monell Chemical Senses Center in Philadelphia, who was not involved with the study. “We know that bitter receptors are used in nasal tissue to sense the presence of bacteria and help sense chemicals in sperm, and are essential for male reproductive health and [the ability to] sense plant toxins in the gut and digestion. Cells need to know about chemicals in their local environment, and bitter receptors help cells surveil their surroundings.”