Placentas provide nutrients to, and remove waste from, developing offspring. Though straightforward in their purpose, they are surprisingly morphologically diverse. Two key ways that placental morphology can vary are in the amount of surface area available for exchange between mother and young (interdigitation) and the amount of tissue separating their blood supplies (invasiveness). Variation across taxa of both forms is thought to be driven by paternal, maternal and offspring conflicts over resource transfer, as resource transfer is regulated by the mother but can be manipulated by the fetus via placental hormones. These conflicts arise because while it is in the best interest of the young and their fathers for them to grow quickly and as large as possible, mothers must often limit the amount of resources that they provide to individual offspring. However, why some mammalian species have one type of placenta while others have another remains unclear. A new study in PNAS by Michael Garratt at the University of New South Wales and his colleagues suggests that placental morphological evolution is inextricably tied to life history evolution – the pace of life.
Garratt and his colleagues tested the relationships between placental morphology and life history traits across 155 mammalian species using phylogenetic generalized least squares models. They collected demographic data from the literature, including age of first reproduction, gestation time, yearly reproductive output, maximum lifespan, onset of senescence, rate of senescence and generation time.
The authors found that transitions from the ancestral, large surface area placentation to derived, medium and small area placentation are associated with transitions to traits of ‘slower’ life history strategies, such as having fewer offspring per year, a longer gestation and a longer lifespan. The authors posit that smaller placental surface areas may be associated with slower paced life history strategies because of the reduced urgency to provision offspring when environmentally driven mortality is low.
Interestingly, the story of transitions in the evolution of invasiveness of placentation shows the opposite trend. The transition from the ancestral, highly and moderately invasive placentation, with direct or relatively direct contact between the blood supplies of mothers and their young, to the less invasive form is associated with a transition to a faster paced life history. Garratt and his colleagues speculate that the wall of tissue associated with less invasive placentation could allow a wresting of control of resources from offspring to mother, and may prevent the manipulation of maternal physiology by fetal hormones, when the need to provision many young is great and the conflict between individual interests is intense.
The results show that evolutionary transitions in placental morphology and life history traits are linked, though which came first is unclear. This chicken and egg dilemma leaves room for future studies of the specific timing of correlated evolution between placentation and life history strategies. Additionally, because some combinations of the two morphological types, such as small surface area and non-invasive placentation, were always found to correspond, the physiological basis of this trend needs to be examined. It appears that the mammalian placenta is a pacemaker of sorts, a watchful little mother of parents and offspring, regulating the dynamics of growth and senescence. And, as widely studied as this mother has been, she still has secrets to reveal.