9.7: Apparent altruism toward unrelated individuals can evolve through reciprocal altruism.

It is ironic that studies of altruism among animals reveal that natural selection has primarily produced selfish behavior. And, as we saw in the previous section, when behavior appears to be altruistic, this is frequently because individuals are helping kin and, by doing so, are promoting the reproduction of copies of the genes (i.e., the alleles) that they share with close relatives. Does any apparently-altruistic behavior toward non-relatives occur in the animal world? Yes, but there’s not much of it. Here we’ll explore the conditions that give rise to reciprocal altruism, how it may have arisen, and why it is so common among humans but so rare among most other animal species.

We start by examining one species with well-documented altruistic-appearing behavior: the vampire bat. Vampire bats live in social groups of 8–12 mostly unrelated adults, roosting primarily in caves and hollow trees. They feed by landing on large mammals such as cattle, horses, and pigs, piercing the skin with their razor-sharp teeth, and drinking the blood that flows from the wound.

Because of their small body size (about the size of your thumb) and their very high metabolic rate, vampire bats must consume almost their entire body weight in blood each night. If they go for more than about 60 hours without finding a meal, they are likely to die from starvation. Here’s where the apparent altruism comes in: a bat that has not found food and is close to death will beg food from a bat that has recently eaten. In many cases, the bat that has just eaten will regurgitate some of the blood it has consumed into the mouth of the hungry bat, saving it from starvation. This act obviously has very high benefit for the recipient of the blood, but it comes at a cost to the sharing bat, which loses some of the caloric content of a meal it has just obtained (FIGURE 9-12).

Kin selection is responsible for some of the blood sharing (females often regurgitate blood for their own offspring), but, in many cases, bats give blood to unrelated individuals. How might this behavior have arisen? To answer the question, it is important to note three other features of vampire bats. First, they are able to recognize more than a hundred distinct individual bats. Second, bats that receive blood donations from non-relatives reciprocate significantly more than average with the bats that have shared blood with them. And third, bats that are not familiar with each other (and do not have a history of helping each other) generally do not regurgitate for each other.

One method proposed to explain the evolution of this apparent altruism is that the bats giving blood to other bats in need are repaid the favor when they are in need of blood. In other words, the act only seems to be selfless, when in actuality it is selfish. With such reciprocal altruism, both individuals (at different encounters) give up something of relatively low value in exchange for getting something of great value at a later time when they need it most. In other words, they are storing goodwill in another individual, in much the same way that a person might put money in a bank for a rainy day. In both cases, individuals are protected from some of the world’s uncertainties.

Taken together, studies of bats and other mammals show that reciprocal altruism can evolve if the following three conditions are met:

• 1. Repeated interactions among individuals, with opportunities to be both the donor and the recipient of altruistic-appearing acts
• 2. Benefits to the recipient that are significantly greater than the costs to the donor
• 3. The ability to recognize and punish cheaters, individuals that are recipients of altruistic-appearing acts but do not return the favor

In the absence of these conditions, selfishness is expected to be the norm among unrelated individuals. But in the presence of all three conditions, reciprocity masquerading as altruism is likely to occur, as it does with blood sharing among the vampire bats. The three conditions required for the evolution of reciprocal altruism are not satisfied in many animal species, which may be why altruistic-appearing behavior among unrelated individuals is rare.

As rare as it is in other species, reciprocal altruism is very common among humans. Friendship, among other human relationships, is built on reciprocity and is almost universal. The opportunity for friendship may be enhanced by our long life span and our ability to recognize thousands of faces and keep track of cheaters. These features are essential in individuals engaging in reciprocal altruism, because an individual becomes very vulnerable when he or she acts in an altruistic manner toward an unrelated individual. The risk is that the altruism will not be repaid, in which case the cheater enjoys greater fitness than the altruist.

While cooperators, those who repay altruistic-appearing behavior, have evolutionary advantages over loners, this advantage disappears if the cooperators are the givers all or most of the time, never or rarely getting anything in return. The importance of keeping track of cheaters and of identifying good potential reciprocity partners (i.e., other cooperators) may be why humans are so interested in social information and seemingly trivial gossip. As with many of the complex behaviors we have explored, humans probably use some rules of thumb when making decisions about reciprocal altruism. This might include keeping track of all available social information to best identify any promising individuals with whom to engage in reciprocal altruism (FIGURE 9-13). This includes information about health, generosity, social status, and reproduction. Researchers have even hypothesized that the evolution of maintaining an interest in social information, while valuable in most contexts, may now have some maladaptive manifestations. We may find ourselves interested in and distracted by the social lives of Angelina Jolie, Brad Pitt, or other individuals whom we’ll most likely never meet.