Yellow bellied three toed skinks are an interesting bunch. They’re apparently in a little bit of a transitional period with regard to the final stages of their reproductive cycle. Skinks living in the coastlands of New South Wales are currently laying eggs in order to reproduce … while their counterparts located in the mountains are giving birth to live young. This difference is shedding some of light on the evolutionary processes in play during the transition from egg laying to live births.
Evolutionary records shows that nearly a hundred reptile lineages have independently made the transition from egg-laying to live birth in the past, and today about 20 percent of all living snakes and lizards give birth to live young only.
“By studying differences among populations that are in different stages of this process, you can begin to put together what looks like the transition from one [birth style] to the other.”
The last quote was from James Stewart, a biologist at East Tennessee State University who co-authored the study on the animals. It goes into some detail about potential problems encountered during the evolutionary transition, like the method by which nutrients like calcium are efficiently passed from the mother to the developing embryo. In mammals, it’s done by way of the placenta. In egg laying animals, it’s done through the use of the yolk plus the contents of the shell (which is high in calcium). While the live-bearing skink has neither of these mechanisms, researchers looked for alternative methods of transfer:
Stewart and colleagues, who have studied skinks for years, decided to look for clues to the nutrient problem in the structure and chemistry of the yellow-bellied three-toed skink’s uterus.
“Now we can see that the uterus secretes calcium that becomes incorporated into the embryo—it’s basically the early stages of the evolution of a placenta in reptiles,” Stewart explained.
The conclusion they drew from observing both types of skinks and the fossil record showing the frequency with which they occur is that these types of transitions are apparently a lot more common – and a lot more simple – than they had previously thought.