Which came first, the lizard or the egg?

Which came first, the lizard or the egg?
Three-toed skink hatching from an egg. This is not an image taken during research for the paper Credit: Nadav Pezaro, Haifa University for the University of Sydney

In a world first, researchers at the University of Sydney have observed a normally live-bearing Australian lizard lay three eggs and then weeks later, give birth to a live baby from the same pregnancy. This is the first time such an event has been documented in a single litter of vertebrate babies.

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 observation will be published in Biology Letters this week, along with advanced microscopy of the egg-coverings.

There are at least 150 evolutionary transitions from egg-laying to live-bearing in vertebrates said Dr. Whittington, who led the study alongside Dr. Melanie Laird, a postdoctoral researcher at the University of Otago, and Emeritus Professor Mike Thompson.

Which came first, the lizard or the egg?
Dr. Camilla Whittington with a Cunningham's skink (a different species from the study). Credit: University of Sydney

"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. 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. "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 said.

Credit: University of Sydney

"Put in the context of evolutionary biology, being able to switch between laying and giving live birth could allow animals to hedge their bets according to ," Dr. Whittington said.

This observation helps make the three-toed skink, which looks like a baby snake with tiny legs, one of the "weirdest lizards in the world", she said.

Further research into this small lizard, which seems to occupy a grey area between and egg-laying, will help determine how and why species make major reproductive leaps.


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Lizards help us find out which came first: the baby or the egg?

More information: Facultative oviparity in a viviparous skink (Saiphos equalis), Biology Letters, royalsocietypublishing.org/doi … .1098/rsbl.2018.0827
Journal information: Biology Letters

Citation: Which came first, the lizard or the egg? (2019, April 2) retrieved 26 June 2019 from https://phys.org/news/2019-04-lizard-egg.html
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Apr 02, 2019
Did the eggs hatch before after or around the same time as the live birth?

Apr 02, 2019
That's a Good question..

Apr 02, 2019
Unquestionably the egg. There were amphibians before lizards.

For that matter, fish lay eggs.

Apr 02, 2019
The simplest philosophical puzzle. If it does not have all the feature of a "fill in the blank" then it is not a "fill in the blank". Evolution will lead to the first "fill in the blank" born from a mother which was not quite "fill in the blank". Live birth vs egg birth does not matter for that philosophical concept. Actually whether the creature is born from live birth or from egg is mostly dependent on whether the creature is warm blooded or cold blooded; cold-blooded creatures lay eggs because they are dependent on their environment for heat, if they were to share that heat with their offspring they could die, and this is the reason these creatures lay eggs so that they can continue to survive and the new creatures being cold blooded are simply dependent on not being eaten and a warm environment. But here is the interesting twist, either dinosaurs were cold blooded and the earth was extremely warm, or all dinosaurs were cold blooded by exceedingly slow (not plausible) or

Apr 02, 2019
Dinosaurs were warm blooded and we are their direct ancestors. The latter is the truth, however where did cold blooded creatures come from; the sea which is where all land creatures came from originally and that is fundamentally the difference between a cold blooded creature and a warm blooded creature; one is designed for the sea, the other for land, and with the land comes additional challenges, organs, complexities, opportunities (flight) and so on and so forth. So you can know that land dinosaurs most were warm blooded at least at the time when the asteroid that wiped them out hit, and then it was colder outside and so the creatures from the sea could not be as large in the same way as you cannot have a jungle in an arid land...

Apr 03, 2019
Liolaemus valdesianus
Zootoca vivipara
Several other lizards are known to switch back and forth between live birth and egg laying.

Apr 04, 2019
"People mostly think about humans and other mammals giving birth. But there are many species of reptile that give birth, too."

And amphibians, and in general the larger clade of stem vertebrates or at least fish - some sharks are especially known for this and of course we have the (male) birth in seahorses.

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