Vampire bats are example of mammals that show altruistic behaviour. They share food (in this case blood) with other bats without any benefits for themselves. True reciprocal altruism is very rare. In most cases altruism is a combination of kin selection and other forms of cooperation. Reciprocal altruism is described as when one animal acts for the benefit of another animal and in doing so decreases its own fitness. The reason for this altruism is because in the future the giving animal expects to be able to receive that same service in its own time of need – and vampire bats are a perfect example of such behaviour.
Vampires have to eat 50% to 100% of their body weight per night. If they lose more than 25%, they are unable to maintain their body temperature. It takes 60 hours to lose 25% of bat’s body weight. Bats that have 24 hours before reaching the minimal body weight are said to be in a critical state, and these are the ones that are fed first by other bats within the same roost.
The ‘feeding success’ – bats under two years old fail 33% of time, whereas adult bats only 7% of the time. That is because infant bats are still weak and learning how to give a painless bite to a prey so they are not shaken from their prey and can drink their blood.
In an experiment 110 bats were observed. 70% or regurgitations were between mother and her child, the rest were between mother and a child other than her own, or other adult females. In the experiment it turned out that bats were feeding each other within one roost and often, but not always, relatives. Most often, regurgitation occurred between to individuals that were feeding each other, reversing roles. What is more, bats indicate each other by series of calls.
Benefits: By donating, a bat indirectly benefits because it is more likely to obtain a regurgitation from bats to which it has previously donated.
By regurgitating 5 milliliters of blood, the donor bat lost 6 hours of time remaining to starvation, whereas the bat that received the blood gained additional 18 hours before starvation.
Reciprocal is more beneficial and practical, because vampires live in small groups and this way have high probabilities for giving and receiving help from the others.
