A common vampire bat that goes two or three nights without a meal can starve, and on any night some of them come home empty. Back at the roost a fed bat will bring up blood for a hungry one, usually a relative, sometimes not. In a captive colony, Gerald Carter, Damien Farine and Gerald Wilkinson fasted a female for a day, then took her main donor, usually her mother or daughter, out of the room before she came back. The bats that had fed more non-relatives in earlier years had more donors waiting and got more food. While the mother was there, those wider favours had bought nothing extra. The kindness to strangers was not for ordinary nights. It was for the night the one who always helps is gone.