Beneficial interactions are usually beneficial to all parties. But sometimes, an organism appears to sacrifice itself to help others. Recent evolutionary thinking has shown, however, that real sacrifice is extremely rare. If the behaviour is costly, it will usually be shown to benefit close relative, unless, of course, the behaviour is performed with the expectation that one day the favour will be returned. This sort of behaviour is known as reciprocal altruism.
For example, vampire bats must find a blood meal at least once every 30 hours. Bats that have not fed recently will be close to death, and a little bit of blood will go a long way. Vampire bats that have fed recently will often share blood with less lucky bats, even if they are not related. However, they are much more likely to share with bats they know ­ colony mates, for instance ­ than with bats they have never seen before. This way, they can be more certain that the favour will be returned.
