Vampire bats share blood with unrelated roostmates

Vampire bats share blood with unrelated roostmates in a pattern that may be explained by reciprocal altruism: a successful forager gives some of its surplus to a hungry companion, with the possibility that help will be returned when circumstances reverse. Wilkinson’s observations, the economic value of a donation to donor and recipient, and a recognition experiment together make this a case for examining cooperation beyond genetic kinship.

From Kinship to Reciprocal Help

The discussion distinguishes genetic relatedness from cooperation between unrelated individuals. Kin altruism can favor care for close relatives because shared genes may persist through those helped; the book notes, for example, that parent and child have a relatedness of one half, as do siblings on average. But this explanation does not account for every act of assistance. Wilkinson’s observations of blood-sharing included instances between bats that were not genetically related, so the question becomes whether repeated interaction can support helping without kinship.

The contrast matters because an isolated donation could benefit its recipient while costing the donor. Reciprocal altruism offers a different account: individuals that meet repeatedly may exchange help over time. The book therefore treats unrelated blood-sharing not as evidence that kinship is irrelevant, but as a case where familiarity and the prospect of future interaction may help explain cooperation.

A Meal Surplus and a Hungry Roostmate

Vampire bats feed at night, and success is uncertain. Some return to the roost without having found food; others that do find a victim may have obtained more blood than they need immediately. On a later night, fortunes can be reversed. This uneven alternation of hunger and surplus creates an opportunity for a bat that has fed to assist a companion that has not.

Wilkinson witnessed 110 instances of regurgitation. Seventy-seven could readily be understood as mothers feeding their young, and many of the other donations involved genetic relatives. The remaining cases included blood-sharing between unrelated bats, which the book identifies as the harder cases for a purely kin-based account. These unrelated donors and recipients tended to be frequent roostmates: they had repeated opportunities to encounter one another, the kind of continuing association that reciprocal-help models require.

The Costs and Returns of Donation

To assess whether the exchange could fit a Prisoner’s Dilemma, Wilkinson measured how quickly starved bats lost weight. From those rates, he calculated how long a bat at different levels of hunger would take to starve, translating donated blood into hours of prolonged life. The value of a given amount of blood was not constant: it added more hours for a highly starved bat than for one that was less hungry.

That difference makes the exchange potentially favorable to a recipient without making it equally costly to the donor. A donor’s blood loss could increase its own risk, but the increase was comparatively small beside the recipient’s gain in survival prospects. The book argues that the same quantity of blood is therefore more valuable to a desperate recipient than to a bat that has fed successfully. On an unlucky night, a bat could benefit greatly from receiving help; on a lucky one, it could gain slightly by keeping its surplus instead.

That temptation to withhold help gives repeated interaction its importance. Refusing to donate would matter as a strategic defection only if a bat’s future partners could respond by withholding help in turn. The book consequently links the plausibility of reciprocal blood-sharing to both unequal need and the opportunity for bats to meet again, while treating the model as an interpretation of the evidence rather than a demonstrated account of every exchange.

Recognition and Familiarity

Repeated cooperation also depends on distinguishing potential partners. Wilkinson tested whether vampire bats could recognize one another by taking one captive bat away for a night and leaving it hungry while the other bats were fed. He then returned the hungry individual to the roost and observed which bats gave it food. The procedure was repeated with different bats taking turns as the one deprived of food.

The captive population combined two groups originally taken from caves many miles apart. If bats favored familiar companions, the starved individual should receive help mainly from bats from its own original cave. In thirteen observed donations, twelve came from an old associate from the same cave; only one came from a bat from the other cave. The book notes that chance could not be ruled out, but calculates that the odds against this distribution were less than one in 500. It concludes that the bats were biased toward feeding familiar roostmates rather than strangers.

Evidence and Its Limits

The observations bring the argument together without erasing distinctions among the cases. Most of Wilkinson’s recorded donations were to offspring or other relatives; the unrelated examples are the ones that require an account beyond shared genes. Their occurrence among frequent roostmates, the sharply unequal value of blood under different levels of hunger, and the experimental preference for familiar bats collectively support reciprocal altruism as a plausible explanation.

The chapter also sets the case beside a question about human blood donors in countries such as Britain, where donors are unpaid. It resists treating those donations as straightforward disguised self-interest: regular donors do not receive preferential treatment when they need a transfusion, nor even small gold stars. That comparison underscores the difference between identifying a plausible evolutionary mechanism and claiming that every helpful act is reducible to simple reciprocity. For vampire bats, the evidence makes repeated assistance among unrelated companions intelligible as a possible exchange sustained by familiarity and future need.