Lizard produces eggs and live young

Versatility, it is generally acknowledged, is a useful evolutionary trait, but an Australian lizard, the three-toed skink (Saiphos equalis), has been observed taking the idea to extremes.

The species is one of only a handful worldwide known to have a bimodal reproductive strategy. Some lay eggs, while others give birth to live young.

At least one resourceful three-toed skink, however, has been observed doing both – from the same pregnancy.

In a paper soon to be published in the journal Biology Letters, researchers led by Camilla Whittington from the University of Sydney, Australia, detail the case of a lady lizard which, following impregnation, laid three eggs and then, three weeks later, gave birth to a live baby.

It is the only example known of any vertebrate producing both eggs and live young from a single pregnancy.

“We were studying the genetics of these skinks when we noticed one of the live-bearing females lay three eggs,” Whittington says.

“Several weeks later she gave birth to another baby. Seeing that baby was a very exciting moment!”

She notes that her team’s observation marks the skink as one of the “weirdest lizards in the world”, but also a key focus for studying the evolutionary transition from egg-laying to live birth.

Such transitions have arisen independently at least 150 times, she says, but the discovery that S. equalis can do both with equal facility makes it an ideal target for studying the evolution of pregnancy.

“It makes Australia one of the best places in the world to study the evolution of live birth, because we can watch evolution in action,” she explains.

“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.”