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Regurgitated food sharing in vampire bats is a cooperative behavior that has garnered scientific interest as an example of reciprocal helping among kin and non-kin. The amount of food given is estimated via the duration of mouth-licking. However, a growing body of evidence across other animal taxa, especially social insects, shows that mouth-to-mouth material transfer can serve many functions besides food sharing. In this review, we asked whether and to what extent mouth-licking in the common vampire bat (Desmodus rotundus) could be explained by functions other than regurgitated food sharing. We first review the evidence, including new analyses of published data, that food sharing occurs during mouth-licking bouts in vampire bats. We then review interpretations of mouth-licking in other mammal species and assess the likelihood that various hypothetical functions suggested in other species could occur in vampire bats. We conclude that the primary function of prolonged bouts of mouth-licking in vampire bats is sharing of ingested blood, but that microbial sharing is another likely benefit, and that short bouts of mouth-licking also function as social signals of begging or offering of food. Future work on this behavior should keep alternative explanations in mind when interpreting observations.