Helping in cooperatively breeding long-tailed tits: a test of Hamilton’s rule
Ben J. Hatchwell, Philippa R. Gullett and Mark J. Adams
Phil. Trans. R. Soc. B 19 May 2014 vol. 369 no. 1642 20130565 doi:10.1098/rstb.2013.0565
When asked if he would lay down his life for his brother, the population geneticist J. B. S. Haldane is said to have quipped “No, but I would to save two brothers
or eight cousins.” Haldane’s response was an early invocation of what has been mathematically codified as “Hamilton’s Rule”:
$rB > C$
which says that an altruistic behavior by will evolve if the benefit to the recipient of the help ($B$) times the relationship between the helper and the recipient ($r$) is greater than the cost to the helper ($C$).
How well Hamilton’s Rule describes social evolution has been the subject of some controversy among evolutionary biologists but there have been very few empirical tests of it.
We set out to do just that with our data from our long-term study of long-tailed tits
Long-tailed tits are cooperative breeders, meaning that related individuals help each other to raise their offspring. Birds who fail to breed in a particular year join the nest of one of their relatives as a helper. This behavior is altruistic because helpers pay a cost: they have lower survival to the next year when they decide to help, this imperiling their future chances of having their own offspring. However, they also get a fitness benefit. The chicks that they help are more likely to survive. Because the helper is on average related to these chicks, the benefit to the helper’s ultimate fitness (in terms of the number of copies of their genes that make it into the next generation) outweighs the costs.
Inclusive fitness theory provides the conceptual framework for our current understanding of social evolution, and empirical studies suggest that kin selection is a critical process in the evolution of animal sociality. A key prediction of inclusive fitness theory is that altruistic behaviour evolves when the costs incurred by an altruist (c) are outweighed by the benefit to the recipient (b), weighted by the relatedness of altruist to recipient (r), i.e. Hamilton’s rule rb > c. Despite its central importance in social evolution theory, there have been relatively few empirical tests of Hamilton’s rule, and hardly any among cooperatively breeding vertebrates, leading some authors to question its utility. Here, we use data from a long-term study of cooperatively breeding long-tailed tits Aegithalos caudatus to examine whether helping behaviour satisfies Hamilton’s condition for the evolution of altruism. We show that helpers are altruistic because they incur survival costs through the provision of alloparental care for offspring. However, they also accrue substantial benefits through increased survival of related breeders and offspring, and despite the low average relatedness of helpers to recipients, these benefits of helping outweigh the costs incurred. We conclude that Hamilton’s rule for the evolution of altruistic helping behaviour is satisfied in this species.