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More on Altruism

Why do vampire bats show kindness to each other, and is it a selfless act?

More on Altruism

According to Trivers' theory, an individual will help another in anticipation that he will receive help if he needs it in the future. Rather than saying 'You scratch my back and I'll scratch yours', individuals that practice reciprocal altruism are saying, "I'll scratch your back now, and you can do the same for me when I need it". The trait has been illustrated in a number of distinct animal groups. One of the most well-known examples is from a study of vampire bats by Gerry Wilkinson, a zoologist at the University of Maryland.

...And I'll Give Blood For You

Vampire bats must have blood in order to survive. If a bat goes more than 48 hours without blood, it will begin to starve. Naturalists have observed that it is not uncommon for bats who have fed well to 'share' their food with bats who have not been so lucky on a particular night by regurgitating blood to feed those that might starve to death. Trivers used the example of vampire bats in his 1971 paper on reciprocal altruism. He theorized that certain conditions must be met in order for a bat to be fed by other bats. These included:

  1. The benefit of receiving must be greater than the cost of giving. In the case of the bats, the donor is giving up some of its food. The receiver is getting a chance to continue living.
  2. The group must remain together long enough to make reciprocation a possibility.
  3. There must be a way to exclude 'cheaters' from the reciprocity.

Wilkinson tested the third condition by introducing starving bats into the colony of bats he was observing. Some of the bats that he introduced were ones that he'd removed from the colony and isolated. Others were total strangers to the group, culled from other colonies. He found that the bats who had been part of the colony were far more likely to be fed by the other bats, suggesting that they had some way of recognizing those who had 'paid into' the system already.

It's All About Payback

Both chimpanzees and wild dog packs have shown similar altruistic behaviors. In one study, capuchin monkeys helped each other with a task even if only one of them received a reward. The test was structured in such a way that the same pair of monkeys was consistently paired to perform a task. Researchers concluded that the capuchin monkeys helped each other in anticipation that the other monkey would help them when it was their turn. Wild dogs often practice food sharing with members of the pack who have not hunted. In both cases, animals who did not reciprocate were excluded in the future.

Altruism in Humans

How does all this apply to humans? Elliot Sober suggests that natural selection does explain the evolution of true altruism. In his explanation, natural selection would favor those individuals who genuinely care about the welfare of others, ensuring their survival, while those who care ultimately only about themselves do not survive.


Considered logically, Sober's theory makes sense. A person who consistently does things to benefit others is most likely to be helped by others when he is in need. Reduced to its simplest terms, reciprocal altruism is scientific proof that if you're nice to other people, overall, they will be nice back to you.

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