Red squirrels adopt orphans

When does it pay to adopt? The benefits to the adopted infant are obvious, but how does the adoptive parent benefit? Rearing offspring is costly, both in terms of time and energy. Why would a parent endure these costs if the offspring is not their own?

The short answer is - when the orphan is a closely related kin. Or as J.B.S. Haldane once quipped, I would lay down my life to save “two brothers or eight cousins”.


Figure 1. A red squirrel. (Photo by Giles Gonthier)

Adoption of orphaned young has been reported in at least 62 mostly social mammal species. However, until now no one has calculated the fitness costs and benefits of adoption. Gorrell and colleagues (2010) report the first case of adoption in the asocial red squirrel (Tamiasciurus hudsonicus) where fitness is directly quantified. They studied red squirrels in the Yukon, Canada over 19 years and had detailed ancestry information for over 2,230 litters. During that time they observed 5 cases of adoption of orphaned young. Although these adoptions involved different females and took place in different years, they always involved kin.

The researchers were able to calculate the fitness cost to adoptive parents and their young (including adopted young). They found that females that adopted orphaned young suffered reduced fitness because their own young had reduced survival. However, this cost was offset when females adopted closely related offspring by an increase in inclusive fitness (the sum of the genes contributed via an individuals own offspring and by genes contributed indirectly via helping kin) . Thus, adoption in red squirrels operates via kin selection. Exactly how the adoptive mother recognizes the orphan as kin is not known.

References

Gorrell, J.C., McAdam, A.G., Coltman, D.W., Humphries, M.M., and S. Boutin. 2010. Adopting kin enhances inclusive fitness in asocial red squirrels. Nature Communications, 1:22 doi: 10.1038/ ncomms1022