What's wrong with drinking a little blood now and then? I mean, don't vampires have a bad rap? Or at least vampire bats?

Common vampire bats have short ears and small, cone-shaped muzzles with specialized heat sensors at the tip, enabling this creature to discover areas on the skin of prey where blood flows closest to the surface. Like other bats, they hunt at night and are able to navigate in complete darkness through echolocation, or sonar. This particular species, though, having evolved to feed on the blood of mammals, has developed a special sensitivity to the distinctive sounds produced by the slow, in-and-out breathing of a deeply-sleeping domestic cow or goat or other mammal, including, on rare occasions, a person. Having located a good source of blood, the bat lands, approaches cautiously by scurrying on the ground, and finds a likely spot on the prey's body for reaching the blood. If hair or fur is in the way, the bat uses her canines to shave an area clear down to the skin, and then, with some very sharp upper incisors, she penetrates the skin while injecting an anti-coagulant saliva.

A female vampire bat weighing around 40 grams can drink half her body weight of blood in about twenty minutes. It's good to drink fast, in case the source of that blood should wake up in a bad mood. However, all that extra weight could make it hard to fly away. Thus, evolution has given this winged mammal a digestive system that quickly transforms blood's non-nutritious liquids into urine, and she will start to expel the urine within a couple of minutes after the drinking has begun. By the time she's ready to fly away, the bat has actually increased her body weight only by a fifth to a third--still light enough that, with an extra crouch and a flinging leap, she casts herself into the air and swoops off to the roost, where she will spend the rest of the night with the colony: around a dozen other adult females plus their young of both sexes and one adult male.

Blood ought to be easy to find, but vampire bats under two years old fail to feed in one night out of three, while adults come back hungry in about one out of ten. Hunger represents not merely a gnawing discomfort, but the potential for a fast downward spiral proceeding into weakness and a reduced ability to forage for blood and ending, after three hungry nights, in death by starvation. It might make sense, then, to share food on occasion, which they do with mouth-to-mouth regurgitation, a transfer of partly-digested blood from the full to the hungry that superficially resembles kissing. One bat returns to the roost hungry after a night of futile blood-hunting. Another sidles over and provides the kiss of life.

University of Maryland zoologist Gerald S. Wilkinson, who has studied vampire bats in Costa Rica, reports that during 100 hours of observation inside several roosts, he saw food sharing through regurgitation on 110 occasions. Seventy-seven of those regurgitations were done by mothers feeding their dependent young, while 33 happened between two adults or adults and juveniles. Considering just those 33 regurgitations not involving mothers and their young, Wilkinson found that the partners were sometimes close relatives and sometimes unrelated but preferred roostmates.

Wilkinson also noted that the adult females spent around 5 percent of their roosting time grooming each other in a pattern that focused on the same close relatives and preferred roostmates. Unlike a lot of mammalian mutual grooming, moreover, this did not seem to be an exchange of services--removal of one another's skin parasites, for example--but rather a series of mutual examinations that focused on the partner's stomach: How's the tummy today, my dear? A vampire bat's stomach becomes notably distended after a good feed, so stomach examination could be a way of determining a bat's recent feeding history, determining who's hungry and who's full. It may also be, so Wilkinson believes, a way of keeping score and identifying cheaters: those bad bats who have received blood regurgitations when they were in dire need but who now, when they're rotund and obviously full, refuse to share.

Wilkinson believes those blood-trading vampire bats provide a good example of reciprocal cooperation partly because in a significant number of cases the blood regurgitations occur between unrelated roostmates. We can understand why relatives might share blood when one is full after a satisfying night of blood-sucking while another is weak from hunger--that's ordinary nepotism--but why non-relatives? The answer is reciprocal cooperation. The unrelated roostmates trust one another to return the favor.

Your Halloween lesson: Even vampire bats share. So . . . Trick or treat! 

About the Author

Dale Peterson, Ph.D.

Dale Peterson, Ph.D., has written nearly a dozen books about conservation, natural history, and animal science and scientists.

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