One of the most interesting behaviors in which vampire bats engage is reciprocal food sharing because it is one of the few instances of altruism in the animal kingdom. A vampire bat will die if it goes two consecutive nights without feeding. After a while without food, their body weight drops too much to maintain a stable body temperature, and they will die of hypothermia. Individuals must consume up to 50 or 100% of their body weight in blood virtually every night. Even though the necessity for nightly feeding is so great, 7 to 30% of bats in a cluster fail to obtain blood on their own on a given night for various reasons, whether they are injured, giving birth, sick, or simply fail at attempting to get blood. It is because of the necessity of nightly blood and the difficulty in obtaining it, that food sharing in bats is critical to their survival. Given that 7 percent of adults fail to feed on a given night and two nights in a row without feeding will result in death, there should be an annual mortality rate of 82%. However, the actual mortality rate among adults is only 24% per year. Thus, food sharing greatly reduces the mortality rate of vampire bats (Wilkinson 1984).

Wilkinson (1990) defines altruism as a behavior where one individual “is giving up some of a resource that is critical to its survival”. Since true altruism is not evolutionarily stable, the question remains of how this behavior remains in the population. For a while, seemingly altruistic behavior in the animal kingdom was seen as a fault in Darwin’s theory of natural selection. But, it was eventually realized that all of the seemingly altruistic behaviors were not in fact completely altruistic because they actually indirectly increased the fitness of the individuals involved.

There are two types of “altruism” in animal behavior, kin altruism and reciprocal altruism. Kin altruism involves an individual sacrificing for his blood relatives. Since his blood relatives share his genes, he will in fact be increasing the spread of his genes if he helps his relatives increase their reproductive fitness even though he is sacrificing his own direct fitness. Reciprocal altruism, which occurs in vampire bats, will increase the fitness of each individual involved, as long as the benefits of reciprocating outweigh the costs over time, and cheaters can be excluded (Case 2004).

With food sharing in vampire bats, the benefit of receiving food when it is needed outweighs the cost of sharing one meal. In a single night a bat may only be on the donating end, and for that single night the cost may be greater than the benefit. But, through a lifetime of reciprocating, if she receives an equal amount of food as she donates, she will benefit more than if she did not engage in food sharing at all. Wilkinson (1990) estimates that a recipient of food sharing gains 18 hours of time until starvation, while a donor only loses 6 hours. So, over a lifetime of reciprocating between donating and receiving, each bat will gain many more hours than it loses.

Another tenet of reciprocal altruism is that it must be possible to identify and exclude cheaters from the reciprocal system. Wilkinson states that he has not been able to confirm whether cheaters actually exist, but he thinks that if they did exist, the bats would be able to recognize them since they seem to display individual recognition. Bats often engage in social grooming, and Wilkinson believes that this social grooming helps to identify roost mates. Furthermore, social grooming is often a prelude to food sharing, so it may be the case that the grooming is used to identify possible food sharing partners. Hungry bats will often groom potential donors, and it is possible that while the donor is being groomed she is able to recognize the groomer and remember if she has ever shared her food before. Since recognition of cheaters is possible, and regurgitating food is an active decision, excluding cheaters from the system would not be difficult.

Thus, food sharing in bats fits the four major criteria for reciprocal altruism. The bats are not related enough for it to be kin altruism. The resource given up is critical to survival, the benefit over time is greater than the cost, and cheaters can be identified and thus excluded.

 

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