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•  Blood sharing in vampire bats as an example of the development of altruistic behaviour by natural selection

Altruism is behaviour which benefits another individual at the cost of the performer

  • Ostensibly, it is in opposition to natural selection as it reduces the potential for the altruistic individual passing on their genes
  • However, it improves the chances of the other individual passing on genes into the same gene pool (i.e. inclusive fitness)

If the individuals are closely related, altruistic genes will persist in the gene pool and be naturally selected

  • Enhancing the reproductive success of relatives who share common genes is called kin selection
  • Organisms that live in social clusters will also promote the conservation of altruistic genes via reciprocal altruism

The occurrence of altruistic behaviours will be determined by three factors (known as Hamilton’s rule: rB > C)

  • The cost to the performer (C) should be small, while the benefit to a receiver (B) and degree of relatedness (r) should be large

Hamilton’s Rule

hamiltons rule


Blood Sharing Among Vampire Bats

Vampire bats commonly regurgitate blood to share with unlucky roost mates who were unable to gain independent sustenance

  • Vampire bats cannot survive multiple successive days without food, however food can often be difficult to find
  • The small cost of sharing blood (lost time until starvation) is less than the benefit received (time gained)
  • Hence sharing blood improves the fitness of the entire brood (via reciprocal altruism), increasing the occurrence of altruism

Cost-Benefit Analysis of Blood Sharing