It isn't just their creepy diet that sets vampire bats apart. A new study shows that the blood-sipping bats don't learn to avoid tastes that make them sick--unlike almost every other animal ever tested.
Most animals learn to recognize a toxic food very quickly. If a snail or a quail tastes something new and then falls ill, it avoids the flavor in the future. Since vampire bats only eat blood, they never meet new flavors and they're practically guaranteed never to get a toxic meal. (If your blood is toxic, you'll probably die before a vampire bat has a chance to get you.) That suggests there was no evolutionary reason for bats to keep the ability to recognize bad food, says Bennett Galef, one of the study's co-authors, who studies animal behavior at McMaster University in Hamilton, Ontario.
To test that hypothesis, Galef's former student John Ratcliffe, now a graduate student in zoology at York University in Toronto, tested for taste aversion in four species of bat: the common vampire bat, Desmodus rotundus, and three close relatives that eat insects and fruit. He fed each bat a meal spiced with cinnamon or citric acid, unfamiliar flavors. Right after dinner, he injected them with lithium chloride to make them vomit. For the next several days, the bats got to choose between normal and new-flavored food. The insect- and fruit-eaters avoided the food they had tasted before getting sick, but vampire bats were almost equally happy with both flavored and unflavored blood. The work appears in the February issue of the journal Animal Behaviour.
Galef says that vampire bats probably lost the ability to learn just because they didn't need it anymore. "There's an expense for maintaining neural tissue and any organization that isn't maintained breaks down over time," Galef says. But Ratcliffe also suggests that the loss of taste aversion may have helped early vampire bats drink blood, which makes many animals, including humans, vomit. "Early vampires may have been able to ingest blood in greater and greater quantities and keep that blood down," Ratcliffe says.
Laurence Nolan, a biopsychologist at Wagner College in New York, isn't swallowing Ratcliffe's vomiting explanation. "It doesn't make as much sense to me as the absence of stabilizing selection pressure," Nolan says. He says he looks forward to adding vampire bats to his lectures on conditioned taste aversion.