Blood-sharing in vampire bats

Scientists working on cooperation among vampire bats (Desmondus rotundus) face the same difficulties in measuring all the essential parameters, but are in a somewhat better position. Some 8'%, of adult vampire bats fail to find food on any given night (Wilkinson 1984), but on such nights without a blood meal they are often fed by successful roostmates. This behaviour is vital for the recipients, since their energy budget leaves them 48-72 h of food deprivation before starvation (McNab 1973). Wilkinson (1984) showed that food sharing by regurgitation of blood "depends equally and independently on degree of relatedness and an index of opportunity for reciprocation". He suggested that w is probably high enough to fit the IPD due to the long lifespan of the individuals and the stable composition of groups in which the average coefficient of relatedness is comparatively low (0.08-0.11; Wilkinson 1988). Fig. 6 shows that b>c because the exponential postfeeding weightless enables a donor to lose less time to starvation by regurgitation of blood than the recipient gains. Hence, it is plausible that T>R>P>S. Furthermore, the cognitive abilities of vampire bats appear to be sufficiently well-developed to guarantee recognition of cooperators and defectors (Wilkinson 1984). While it is reported that bats do in fact deny feeding those roostmates that refused to regurgitate previously (i.e. act retaliatorily; Wilkinson 1984), it still remains to find out which strategy the bats actually use. An applicability of the IPD should be confirmed by further investigations.

Fig. 6. Predictive curve of post feeding weightlosses in vampire bats (Desmondus rotundus). A donation of 5% of pre-feeding weight when at weight D should cause a donor to lose C hours but provide the recipient at weight H with B hours. B >C for all E > F. See text for details. Redrawn from: Wilkinson, G. S. 1984. Reciprocal food sharing in the vampire bat. Nature 308: 183.