Normally, animals reproduce by either laying eggs or giving birth to live young, but an Australian species of lizard can apparently do both in the same pregnancy. Scientists at the University of Sydney witnessed a three-toed skink lay three eggs before going on to later give birth to a live baby, an event that had never been observed in any vertebrate species before.

If any animal was going to pull off this stunt, it was the three-toed skink. Found along the east coast of Australia, this little lizard belongs to a rare group of species that are "bimodally reproductive." That means that some members of the species lay eggs, while others give birth to live young.

While the species as a whole can "choose" between the two methods, it was generally thought that individual animals were genetically locked into one or the other for life. Populations around Sydney are egg-layers, while those further north have been found to give live birth.

But now it seems that not only can a skink switch between the two, it can use both within the same pregnancy. The team says this marked two world-first observations.

"It is a very unusual discovery," says Camilla Whittington, lead author of the study. "We were studying the genetics of these skinks when we noticed one of the live-bearing females lay three eggs. Several weeks later she gave birth to another baby. Seeing that baby was a very exciting moment!"

To investigate further, the team used a scanning electron microscope to study the egg coverings, and found that they were thinner than those of normal egg-laying skinks. In terms of the structure, they seemed to have a mix of characteristics from both egg-laying and live-birthing skinks.

The team says the discovery makes the three-toed skink a good model for studying the evolution of pregnancy in animals. In evolutionary terms, egg-laying is the older method of reproduction, with many species later developing the ability to carry young to term internally. But as we see around us, laying eggs hasn't gone away either – both methods have their pros and cons.

The three-toed skink appears to be partway through transitioning from egg-laying to live birth, and currently seems to be testing the waters before diving in headfirst. That said, the team isn't yet sure what triggered this particular female to make the switch.

"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," says Whittington.

The research was published in the journal Biology Letters.

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