If you really think about it, fetuses are parasites: A host is invaded, blood flow is altered, hormones are introduced to prevent miscarriage, and the host's (a.k.a. mother's) immune system needs to be depressed to prevent rejection. The parasite wants to remain in utero as long as possible, the host wants birth to occur as early as possible. A sunny, romantic way to look at motherhood, eh?
A forthcoming paper in The American Naturalist argues that that mammalian species with invasive placentas (as we primates have, along with, for example, most rodents and lagomorphs), can successfully hybridize with species that are much more genetically distant, apparently as a byproduct of more strongly reducing their immune response during pregnancy. Indeed, as they note, "invasive placentation engenders an approximate halving of the rate at which eutherian mammal species evolve hybrid incompatibility."
The abstract reads:
A central question in evolutionary biology is why animal lineages differ strikingly in rates and patterns of the evolution of reproductive isolation. Here, we show that the maximum genetic distance at which interspecific mammalian pregnancies yield viable neonates is significantly greater in clades with invasive (hemochorial) placentation than in clades with noninvasive (epitheliochorial or endotheliochorial) placentation. Moreover, sister species with invasive placentation exhibit higher allopatry in their geographic ranges, suggesting that formerly separated populations in mammals with this placental type fuse more readily on recontact. These differences are apparently driven by the stronger downregulation of maternal immune responses under invasive placentation, where fetal antigens directly contact the maternal bloodstream. Our results suggest that placental invasiveness mediates a major component of reproductive isolation in mammals.
The paper is M.G. Elliot and B.J. Crespi (2006) "Placental invasiveness mediates the evolution of hybrid inviability in mammals," The American Naturalist DOI: 10.1086/505162
I understand stand how a fetus can be viewed as parasitic, but, seriously, is the claim have any place outside a pot-smoke filled college dorm?
Living creatures experience great costs to reproduce. But the impetus for reproduction is internal and exists without and before the presence of the parasitic spawn. And what do the parents recieve? Their genes survive from generation to generation. Certainly, there is intergenerational conflict during gestation... but does that make a parasite out of an embryo?