Egg laying or live birth, yolk or placenta — reptile mothers have evolved a wide range of solutions to the problem of providing nutrients to their developing offspring, each method with its own draw backs. For species that nourish their young through the mother’s placenta, rather than through the yolk, plentiful food might be a necessity.
Reptiles have evolved live birth over 100 separate times. In lieu of external eggs, offspring of live-bearing species are nourished internally with yolk sacs. However, at least six times in lizards and once in our own mammalian evolution, reptiles evolved a complex placenta capable of transporting nutrients to developing offspring.
Placentotrophy (placenta = ‘cake’ and trophē = ‘nourishment’) may have evolved to overcome the costs incurred by yolk provisioning in other live-bearing reptiles. Lizards that allocate yolk to their young do it en masse and prior to ovulation such that developing offspring have all of the nutrients they need. This is beneficial as it safeguards the embryos against resource fluctuations said Dr. James Van Dyke, lead author of the paper published in The American Naturalist. “[However,] this means that [the mother] has to carry the equivalent mass of all of her offspring, in yolk form, throughout embryonic development...[which might] reduce her ability toforage, or to escape predators,” said James.
If the young could be fed exclusively through the placenta, the female need only be burdened with their maximal weight at the end of gestation. “However,” explained James, “this requires that females be able to successfully forage throughout pregnancy, so that they can replace all of those nutrients that were once present in the yolk. [And] this is what our experiment tested.”
James’ study used Southern Grass Skinks, a placentotrophic species, to test the effects of food limitation on maternal and offspring fitness. When food was limited, the female skinks had reduced success in offspring development. This is in contrast with skinks that use yolk to nourish their young, which are unaffected by food limitation. This suggests that in order to evolve placentotrophy, resources must be constantly available during pregnancy. Placentotrophy is dynamic however, when resources do fluctuate, the mother can abort and cannibalise her embryos to regain precious nutrients.
The reproductive system of the Southern Grass Skink may be analogous to the transition our own ancestors went through around 200 million years ago. Studies such as this one help to answer the origins of our own placental arrangement.
Originally published in the University of Sydney's School of Biological Sciences newsletter, 24th May 2014