Scientists have never before witnessed the birth of live young and the laying of eggs from the same pregnancy.
This is the first time such an event has been documented in a single litter of vertebrate babies.
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The three-toed skink (Saiphos equalis) is one of only a handful of rare ‘bimodally reproductive’ species, in which some individuals lay eggs and others give birth to live babies. But up until now, no vertebrate has ever been observed to do both in one litter.
“It is a very unusual discovery,” said Dr Camilla Whittington, from the School of Life and Environmental Sciences and Sydney School of Veterinary Science at the University of Sydney.
The three-toed skink is native to the east coast of Australia. In the northern highlands of New South Wales the animals normally give birth to live young, but those living in and around Sydney lay eggs.
“We were studying the genetics of these skinks when we noticed one of the live-bearing females lay three eggs,” Dr Whittington said. “Several weeks later she gave birth to another baby. Seeing that baby was a very exciting moment!”
“The earliest vertebrates were egg-layers, but over thousands of years, developing embryos in some species were held inside the body for longer, until some animals began to give live birth,” Dr Whittington said.
“People mostly think about humans and other mammals giving birth. But there are many species of reptile that give birth, too.”
Dr Whittington said that the unusual observation of both egg laying and live birth in a single litter shows that the three-toed skink is an ideal model for understanding pregnancy.
“Put in the context of evolutionary biology, being able to switch between laying eggs and giving live birth could allow animals to hedge their bets according to environmental conditions,” Dr Whittington said.
“The more we look at these skinks the more we will understand whether our observation is just a bit of an oddity or whether it’s widespread,” she said.