The most famous lenders in nature are vampire bats. These bats congregate in the thousands inside caves, and every night fly out to look for prey. When they find a sleeping bird or careless mammal, they make a small incision in its skin, and suck its blood. But not all vampire bats find a victim every night. In order to cope with the uncertainty of their life, the vampires loan blood to each other. A vampire that fails to find prey will come home and ask a more fortunate friend to regurgitate some stolen blood. Vampires remember very well to whom they loaned blood, so at a later date if the friend returns home hungry, he will approach his debtor, who will reciprocate the favour. However, unlike human bankers, vampires never charge interest.