A common vampire bat lives on a narrow energy margin. A meal is roughly half its body weight in blood, and the water is shed rapidly through urine while the bat is still feeding. Miss two consecutive nights and an adult can starve.
Gerald Wilkinson's fieldwork in Costa Rica in the 1980s documented donors regurgitating blood into the mouths of unfed roostmates. Recipients gained far more survival time from a donation than donors lost, which is what makes the exchange worth making. Later work by Gerald Carter and colleagues showed that sharing is predicted by a history of reciprocal grooming and past donations, not by kinship alone, and that bonds formed in captivity persist when the bats are released into a wild colony.
The usual overstatement is that this proves calculated accounting. The bats need not be counting anything; a simple rule that favours long-standing social partners produces the same pattern. What is genuinely unresolved is how much of the behaviour is driven by relationship history versus immediate begging cues.