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

Abstract

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 days. Surprisingly, donors initiated food sharing more often than recipients, which is inconsistent with harassment. Food received was the best predictor of food given across dyads, and 8.5 times more important than relatedness. Sixty-four per cent of sharing dyads were unrelated, approaching the 67 per cent expected if nepotism was absent. Consistent with social bonding, the food-sharing network was consistent and correlated with mutual allogrooming. Together with past work, these findings support the hypothesis that food sharing in vampire bats provides mutual direct fitness benefits, and is not explained solely by kin selection or harassment.

Figure 1.

Relationships between food donated and predictor variables. z-score for log food donated was predicted by z-scores of (a) log food received (R² = 0.27, p < 0.0002), (b) allogrooming received (R² = 0.14, p < 0.0002) and (c) relatedness (R² = 0.04, p < 0.0012). A bubble plot (d) shows multivariate relationships by scaling bubble size to relatedness and bubble darkness to allogrooming received.

Figure 2.

Relative importance on food donated of several predictors. Proportion of R² is shown for four predictor variables. An interaction effect (see text) is not shown. The full model explained 38% of the variation in food donated.

Figure 3.

Allogrooming given correlates with allogrooming received. Allogrooming giving is plotted against allogrooming received for dyads that did not share food ((a) n = 214, r = 0.62, p < 0.0002) and dyads that did share food ((b) n = 98, r = 0.81, p < 0.0002). On non-trial days, dyads that shared food both gave and received more allogrooming than non-sharing dyads (F_1,310 = 32.9 and 41.0, p < 0.0002 for both).