Mother’s Little Helper? The Placenta and Its Role in Intrauterine Maternal Investment in the Common Marmoset (Callithrix jacchus)

Abstract

Litter size variation within the Callitrichidae is the result of complex interactions among genetic and environmental factors, and occurs across many facets of the reproductive cycle, from ovulation number to intrauterine litter size reduction to neonate lactation competition. Selection appears to have acted upon the callitrichine ability to make ``decisions{''} relative to maternal nutritional state and litter size in a way that has yielded a highly sensitive and plastic system. Natural variation in marmoset litter size, birth weights and placental weights, and maternal condition create an opportunity to test hypotheses related to intrauterine growth retardation and maternal investment. We present evidence suggesting that differences in fetal/placental weight ratios in marmosets represent distinct strategies of intrauterine resource solicitation by members of litters of different sizes as a result of prenatal parent-offspring conflict. Individual triplets are associated with a smaller share of the placenta by weight than are twins, suggesting a mechanism by which triplets increase placental efficiency in the face of finite maternal resources and uterine space constraints. Twin and triplet fetuses appear to pursue different intrauterine strategies for maximizing allocation of the maternal resources via the placenta. Since complete triplet litters are almost never successfully reared to weaning, maternal limitations of energy intake and investment in offspring from conception to weaning appear to be in conflict with the triplet strategy of optimizing the intrauterine environment through placental development and function.