Female vampire bats live in colonies made up of unrelated females and their offspring, and they feed exclusively on blood. At feeding time, these bats leave the roost, find a mammal (usually, a nice juicy cow), bite it without being detected, and lap up the blood. On a good night, they can consume almost one third of their body weight in blood before they head back to the roost — which is like a 150-pound person having a 50-pound dinner.
But if a bat can't find food (sometimes, no cows are to be had), it's in big trouble. If it goes more than two nights without eating, it starves. In fact, studies have shown that vampire bats fail to find food often enough that, left on their own, most would be dead within a year. Yet vampire bats tend to live a long time. Why? Vampire bats share the wealth.
Back at the roost, the bats who were successful share some of the blood they collected with the bats that are starving. Then, when they themselves are on death's doorstep, they get a sip of blood in return. This exchange among the bats meets the conditions necessary for altruistic reciprocity:
1 The colonies are long-term communities of the same individuals who have many opportunities to share blood as needed. In one case, the same pair of bats lived together for more than 10 years. Familiar bats — those with which the donor has a longer association — are more likely to receive a donation of blood, because they're the ones most likely to be around to reciprocate in the future.
Gerald Wilkinson removed a sample of bats from a colony and held them without food until they reached the weight at which they would solicit food from other bats. Then he compared how the bats remaining in the colony responded to their returning (hungry) compatriots and how they responded to hungry bats from a different colony. He found that the full bats were likely to feed the hungry bats with which they had previous associations and were much less likely to feed the bats that were strangers.
1 The altruistic act is more beneficial to the recipient than harmful to the giver. When a bat is close to starvation, a little bit of blood can buy a fair bit of time. But that same amount of blood isn't as big a loss to the donating bat that's flush from a successful feeding trip.
i Vampire bats seem to have mechanisms in place to make sure that other bats can't cheat:
• They're able to recognize and remember other individual bats.
• Before a hungry bat solicits food from one of its roostmates, it grooms that bat's belly. Given that full bats are noticeably more rotund than starving bats, the hungry bat probably has a good sense of who has food to share.
• The bat that's being asked for food can assess the condition of the bat that's asking. Roostmates that are close to starvation are much more likely to be fed than roostmates that are only a little hungry.
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