Viviparous reptiles that are not snakes are less common; in fact, only a few are known, including two kinds of leg-less reptiles called skinks, the viviparous lizard and night lizards, a group of enigmatic lizards found in the southwestern US and Central America.
Ovoviviparous animals are a sub-type of viviparous animals that also develop their fertilized eggs inside the reproductive tract. The difference is that the young hatch from their eggs inside the mother, then emerge into the world after this internal hatching. In addition to a few reptile species, several species of amphibians and fish, such as the sand shark, are born this way.
Slow worms are leg-less, ovoviviparous reptiles that live in the United Kingdom. Because slow worms develop inside their mother's body, they are shielded from the temperature extremes found in Great Britain. The antenatal anaconda is an ovoviviparous snake that lives in the swamps of northern Argentina. When a female anaconda has young snakes developing inside her, she must maintain the ideal internal temperature. She will typically give birth to anywhere between 15 and 40 young.
Each snake baby is completely independent after birth. Brett Smith is a science journalist based in Buffalo, N. A graduate of the State University of New York - Buffalo, he has more than seven years of experience working in a professional laboratory setting. Previous research has shown that if a Sydney skink was taken north it would still lay eggs, while live bearers transferred south would also continue to reproduce as they previously did.
The University of Sydney study into the three-toed skink will be published in Biology Letters this week. Whittington hopes to map where they lay eggs and where they give birth in further research.
This article is more than 2 years old. Three-toed skink laid eggs followed by a live baby three weeks later from the same pregnancy. The major difference between oviparity and viviparity therefore centers on a strategic evolutionary decision about when the mother should deposit her embryos.
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.
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