Vampire bats may not offer eternal life, but they do share blood with other bats. You see, if a vampire fails to feed for two nights, it starves to death. But University of Maryland zoologist Gerald S. Wilkinson says the vampire bats of Costa Rica have developed a food-sharing system that allows a starving bat to live for a few more hours. Thus the unfortunate bat gets another chance to find blood on its own. The food-sharing is a type of behavior that biolgists call "reciprocal altruism." Unlike pure altruism, where someone does something for nothing, reciprocal altruism implies that the donor will one day be repaid for his or her acts of kindness. Among vampire bats, Wilkinson found that on any given night about 7 percent of the bats from a roost fail to secure a meal, which the bats get by using their razor-sharp teeth to nick a horse, cow or even a sleeping human, and then lap quietly away at the running blood. Bats that fail to feed solicit a free meal by grooming and licking the donor, which regurgitates some blood.