The evolutionary ecology of mammalian placental invasiveness

Eutherian mammals differ markedly in placental form and function. In species with invasive placentation the fetal epithelium of the placenta is bathed in flowing maternal blood, giving the fetus an opportunity to extract resources directly from the mother and to secrete substances which modify maternal blood chemistry. In species with non-invasive placentation the fetal tissues are separated from maternal blood by a barrier of maternal cell layers which limits the ability of the fetus to control resource transfer during gestation. My thesis explores the role played by placental invasiveness in the evolution of eutherian brain size, life history and reproductive isolation. I find that the relationships between brain size, body size and lifespan, and the rate at which hybrid inviability evolves, differ strikingly between species with invasive versus non-invasive placentation. I propose a number of physiological mechanisms that may account for such differences among eutherian mammals.