Adopting kin enhances inclusive fitness in asocial red squirrels

Orphaned animals benefit from being adopted, but it is unclear why an adopting parent should incur the costs of rearing extra young. Such altruistic parental behaviour could be favoured if it is directed towards kin and the inclusive benefits of adoption exceed the costs. Here, we report the occurrence of adoption (five occurrences among 2,230 litters over 19 years) in asocial red squirrels (Tamiasciurus hudsonicus). Adoptions were always between kin, while orphans without nearby kin were never adopted. Adoptions were confined exclusively to circumstances in which the benefits to the adopted juvenile (b), discounted by the degree of relatedness between the surrogate and the orphan (r), exceeded the fitness costs of adding an extra juvenile to her litter (c), as predicted by Hamilton's rule (rb > c) for the evolution of altruism. By focusing on adoption in an asocial species, our study provides a clear test of Hamilton's rule that explains the persistence of occasional altruism in a natural mammal population.