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Anight-IOng after most visual predators have stopped prowl ing-vampire bats emerge from their roosts and take to the wing, flying low across the landscape in search of warm-blooded prey. Within an hour or two, having found appro priate victims and fed on their blood, the bats return to the roost to sleep, feed their young and interact with nestmates.

Until recently little was known about either the behavior or the life history of the common vampire bat, Desmodus rotundus. For many years biologists were more interested in the animal's physiology than in its social organization, which was thought to be relatively simple. A number of recent studies, however, reveal that vampire bats are remarkably social: females cluster together during the day but at night reassort themselves, creating a fluid social organization that is main tained for many years. Moreover, it is now known that long-term associa tions among females …