One of the three species of vampire bat, Desmodus rotundus (or Old-fashioned Fatty to the other two species) needs to consume half its body weight in blood each day. Since hanging onto a mammal by its teeth for twenty minutes can be a very tricky operation, many young inexpert bats fail to drink their fill. Studies on food-sharing have shown that although bats that go hungry for three days are likely to starve to death, other successful roostmates regurgitate blood both to famished related and totally unrelated recipients. Close observation has shown that this is not a willy-nilly occurrence and that individuals deny food to upstart roostmates from whom they have previously been refused nourishment.

This suggests mutual back-scratching despite the use of such behaviour to demonstrate altruism in the animal world. Wilkinson (1986) also found that food was traded for non-food benefits and bats exchanged nosh for claw massages and football cards.

When compared to the vampire bat's subtle and sophisticated levels of social awareness, the fruit-eating, nectar-sucking megabat's nocturnal cries of "me, me, me" are enough to put any self-respecting Chiroptera to shame.