Which reptiles have live birth

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Read more: Lizards help us find out which came first: the baby or the egg? Our research group at the University of Sydney studies the bimodally reproductive three-toed skink, in the hope of understanding how live birth evolved. In northern NSW, the three-toed skink gives birth to live young, but near Sydney, they lay eggs. Even though they reproduce differently, previous research has shown these lizards are a single species. Even the egg-laying members of the species are odd, as the eggs are retained within the mother for a relatively long time.

After being laid, ordinary skink eggs are incubated for at least 35 days before they hatch, but some three-toed skink eggs hatch in as few as five days after being laid. One female even laid eggs and gave birth to a live baby in the same litter. Read more: The first known case of eggs plus live birth from one pregnancy in a tiny lizard. Genes can be expressed switched on to different degrees, and gene expression can stop when not needed.

An egg-laying skink uterus undergoes only a couple of genetic changes between being empty and holding an egg. A live-bearing skink uterus is different. Our research measured changes in gene expression between egg-laying and live-birth in the three-toed skink. We investigated how the expression of all genes in the uterus differed between when the uterus was empty and when it held an egg or embryo. Most reptiles, for instance, deposit their embryos just a third of the way through their development.

In this way, the mother can provide the protective advantages of carrying her young to full term without needing to accommodate a full-size newborn inside her body. Scientists are still learning about the developmental constraints and requirements of these birth strategies. Consider, for instance, the thickness of an eggshell.

In the outside world, though, a thicker shell is helpful to protect against predators. An egg laid too early, then, might be too thin to survive, and one laid too late might be too thick to meet the exponentially growing oxygen demands of the embryo.

In a paper published in Nature in , Organ and his colleagues demonstrated that before a species could evolve live birth, it probably had to evolve the ability to determine the sex of its offspring genetically. The sex of many creatures is circumstantial: Environmental factors, particularly temperature, can determine whether the embryo develops as male or female.

Consider sea turtles. If they laid all their eggs in the water, they would be less likely to get a variety of males and females because the temperature gradient there is much smaller than it is on land. But once a marine species has evolved the ability to determine sex through genes, it no longer needs to venture onto land and can fully adapt to its aquatic life.

The embryo of a three-toed skink just before it is laid in an egg is almost fully formed. Because the commitment to egg laying occurs so late in development, this species has the option for live birth instead.

At the time of that publication, scientists thought that live birth might have evolved among the reptilian ancestors of ichthyosaurs only after they moved from the land to the sea. But the discovery of a million-year-old fossil changed that. That position is telling: Most viviparous marine reptiles are born tail first so that they can continue to draw oxygen from their mother during labor.

The headfirst birth position indicates the ichthyosaur inherited live birth from an even more ancient land ancestor. Whittington and her team study the Australian three-toed skink Saiphos equalis , a lizard with the remarkable distinction of being able to both lay eggs and give birth to live young.

Recently in Molecular Ecology , Whittington and her team describe the differences in gene expression — which genes are switched on or off — between a lizard mother that lays eggs and one that gives birth to live young.

Within a single species, there are thousands of such differences between a female with an egg and one without.

Crucially, the specific genes that get switched on in these cases are very different. But in three-toed skinks, a lot of the genes that switch on when a mother makes an egg also get switched on in mothers with embryos.