Food sharing in vampire bats: reciprocal help predicts donations more than relatedness or harassment

Common vampire bats often regurgitate food to roost-mates that fail to feed. The original explanation for this costly helping behaviour invoked both direct and indirect fitness benefits. Several authors have since suggested that food sharing is maintained solely by indirect fitness because non-kin food sharing could have resulted from kin recognition errors, indiscriminate altruism within groups, or harassment. To test these alternatives, we examined predictors of food-sharing decisions under controlled conditions of mixed relatedness and equal familiarity. Over a 2-year period, we individually fasted 20 vampire bats (Desmodus rotundus) and induced food sharing on 48 of 52 days. Surprisingly, donors initiated food sharing more often than recipients, which is inconsistent with harassment. Food received was the best predictor of food given, and 8.5 times more important than relatedness. Sixty-four percent of dyads were unrelated, approaching the 67% expected if nepotism was absent. Consis...