Vampire bats feed each other, and who they feed is better predicted by who fed them than by how closely related they are. · published 5 October 2026
Twenty vampire bats were individually fasted on 48 days across two years, and the amount a bat had previously received from a given partner predicted what it gave back to that partner better than genetic relatedness did - about 8.5 times better.
These are the working notes, unedited. Everything the video
states as fact comes from here, and nothing else may be stated as fact.
The sections on limits and on what the finding does not show are
part of the brief, not an afterthought — see
the method.
Why it matters to the bat. A common vampire bat lives on blood and cannot store much of it. Miss two or three nights’ feeding and an adult is close to starving. A bat that has fed well can regurgitate part of its meal to a roostmate that has not, at modest cost to itself and very large benefit to the other - which is exactly the situation in which helping might evolve, and exactly the situation in which it is hard to tell altruism from arithmetic.
The original observation. Gerald Wilkinson, “Reciprocal food sharing in the vampire bat”, Nature, volume 308, 1984. Wilkinson documented wild vampire bats regurgitating blood to roostmates, including bats they were not closely related to, and argued the pattern fitted reciprocal exchange rather than kinship alone.
The obvious objection. Two cheaper explanations have to be ruled out before reciprocity is worth claiming. One is kin selection: a bat helping relatives is propagating shared genes, and needs no exchange to explain it. The other is harassment - a hungry bat badgering a fed one until it gives in, which looks like generosity and is closer to a mugging.
The experiment. Gerald Carter and Gerald Wilkinson, “Food sharing in vampire bats: reciprocal help predicts donations more than relatedness or harassment”, Proceedings of the Royal Society B, volume 280, issue 1753, 2013. Twenty common vampire bats were studied over two years. Rather than waiting for natural shortages, the researchers fasted individual bats to induce sharing, doing this on 48 separate days, so that every bat was at some point the one in need and at some point the one with a meal. Relatedness was known. Who had previously given to whom was recorded.
What they found. How much food a bat had received from a particular partner was the best predictor of how much it gave back to that partner. The paper reports that figure as roughly 8.5 times more important than relatedness. Bats were feeding the individuals that had fed them, more than they were feeding family.
Limits
Twenty animals, in captivity, with shortages created by the researchers rather than arising naturally - which is what makes the design clean, and also what makes it artificial. Captive bats cannot leave, cannot forage independently and have a fixed set of partners, all of which could strengthen exchange relationships beyond anything a wild roost sustains. Relatedness and shared history are also entangled in any real colony: animals that grow up together are often kin, and separating the two is the whole difficulty.
What this does not show
“Reciprocity” here is a description of a statistical pattern over time, not a claim that a bat is keeping a ledger or feels indebted. The study measured what the bats did, not what they understood. Nor does it make vampire bats unusually moral - this is a behaviour that pays, in an animal for which a missed meal is life-threatening and a shared one is cheap.