"Microchimerism and inclusive fitness"
David Haig
Harvard University

Fetal cells colonize the mother’s body during pregnancy and persist indefinitely. Maternal cells similarly colonize the fetal body. This phenomenon of ubiquitous microchimerism has not been considered from the perspective of inclusive fitness theory. Offspring cells in the mother’s body might provide postnatal benefits to the offspring by increasing lactation and by delaying the birth of a younger sib. Sibling rivalry may be played out within the mother’s body.