National Geographic News
A yellow-bellied three-toed skink.

A yellow-bellied three-toed skink carrying embryos, visible as light orbs inside its body.

Photograph courtesy Rebecca A. Pyles

Brian Handwerk

for National Geographic News

Published September 1, 2010

Evolution has been caught in the act, according to scientists who are decoding how a species of Australian lizard is abandoning egg-laying in favor of live birth.

Along the warm coastal lowlands of New South Wales (map), the yellow-bellied three-toed skink lays eggs to reproduce. But individuals of the same species living in the state's higher, colder mountains are almost all giving birth to live young.

Only two other modern reptiles—another skink species and a European lizard—use both types of reproduction. (Related: "Virgin Birth Expected at Christmas—By Komodo Dragon.")

Evolutionary records shows that nearly a hundred reptile lineages have independently made the transition from egg-laying to live birth in the past, and today about 20 percent of all living snakes and lizards give birth to live young only.

(See "Oldest Live-Birth Fossil Found; Fish Had Umbilical Cord.")

But modern reptiles that have live young provide only a single snapshot on a long evolutionary time line, said study co-author James Stewart, a biologist at East Tennessee State University. The dual behavior of the yellow-bellied three-toed skink therefore offers scientists a rare opportunity.

"By studying differences among populations that are in different stages of this process, you can begin to put together what looks like the transition from one [birth style] to the other."

Eggs-to-Baby Switch Creates Nutrient Problem

One of the mysteries of how reptiles switch from eggs to live babies is how the young get their nourishment before birth.

In mammals a highly specialized placenta connects the fetus to the uterus wall, allowing the baby to take up oxygen and nutrients from the mother's blood and pass back waste. (See related pictures of "extreme" animals in the womb.)

In egg-laying species, the embryo gets nourishment from the yolk, but calcium absorbed from the porous shell is also an important nutrient source.

Some fish and reptiles, meanwhile, use a mix of both birthing styles. The mother forms eggs, but then retains them inside her body until the very last stages of embryonic development. (Related: "Dinosaur Eggs Discovered Inside Mother—A First.")

The shells of these eggs thin dramatically so that the embryos can breathe, until live babies are born covered with only thin membranes—all that remains of the shells.

This adaptation presents a potential nourishment problem: A thinner shell has less calcium, which could cause deficiencies for the young reptiles.

Stewart and colleagues, who have studied skinks for years, decided to look for clues to the nutrient problem in the structure and chemistry of the yellow-bellied three-toed skink's uterus.

"Now we can see that the uterus secretes calcium that becomes incorporated into the embryo—it's basically the early stages of the evolution of a placenta in reptiles," Stewart explained.

Evolutionary Transition Surprisingly Simple

Both birthing styles come with evolutionary tradeoffs: Eggs are more vulnerable to external threats, such as extreme weather and predators, but internal fetuses can be more taxing for the mother.

(Related: "Human Sperm Gene Traced to Dawn of Animal Evolution.")

For the skinks, moms in balmier climates may opt to conserve their own bodies' resources by depositing eggs on the ground for the final week or so of development. Moms in harsh mountain climates, by contrast, might find that it's more efficient to protect their young by keeping them longer inside their bodies.

In general, the results suggest the move from egg-laying to live birth in reptiles is fairly common—at least in historic terms—because it's relatively easy to make the switch, Stewart said.

"We tend to think of this as a very complex transition," he said, "but it's looking like it might be much simpler in some cases than we thought."

The skink-evolution research was published online August 16 by the Journal of Morphology.

12 comments
Steven Noneofyourbuisness
Steven Noneofyourbuisness

It isnt live birth, the baby grows in an egg in its mother the whole time, then when the baby is ready to come out, then it does, it is just a way of protecting its babies.Not live birth.

ভবঘুরে বিদ্রোহী
ভবঘুরে বিদ্রোহী

Come to the point, nearly a hundred reptile lineages have independently made the transition from egg-laying to live birth in the past, and today about 20 percent of all living snakes and lizards give birth to live young only.

It's the fact to discuss, there we found both egg-laying & live birth, a transition period from fish to reptiles & we can demand reptiles to other live-laying animals.


Now we can see that the uterus secretes calcium that becomes incorporated into the embryo—it's basically the early stages of the evolution of a placenta in reptiles,"Stewart explained. 

After getting the evidence all I clearly believe, evolution is exist...

Ronald Jack
Ronald Jack

How can this relate to natural selection

Mike Merritt
Mike Merritt

It's unclear to me if you're saying this is one species that can reproduce both ways. If you move one beach skink to the mountains would it change the way it gives birth? Or are you saying they're basically two distinct species with an indubitable common lineage?

Mike Merritt
Mike Merritt

k, I'm being pedantic, but it seems important.

The following phrase is super confusing to people who don't get evolution: "For the skinks, moms in balmier climates may opt to conserve their own bodies' resources by depositing eggs on the ground for the final week or so of development. Moms in harsh mountain climates, by contrast, might find that it's more efficient to protect their young by keeping them longer inside their bodies." 

Surely you're being elliptical, but even throwing in a phrase like "in a sense" or better "in a metaphorical sense" would help things out here. But better to leave it out entirely. The skinks aren't "opting" for anything. They do what they do, and the consequences are life and death for themselves and their brood. Hence evolution. QED.

Bob Level
Bob Level

"But, but God made everything exactly like it is and nothing ever changes.." Creationists are going to hate this article.

Greg Gee
Greg Gee

"That's not evidence. Show me the evidence."

-Wendy Wright

Vüsalə Yusifova
Vüsalə Yusifova

@Bob Level 

I am a Creationist and Creationists are not against science, but we are against when some scientists make fool of science on behalf of evolution and try to fool people who are unaware (scientists like Charles Dawson - Piltdown Man, or Ernst Haeckel - false embryos and so on.) the history of evoluiton theory is full of fraudulences. Science truly proves Creation.

Dawn Michelle
Dawn Michelle

@Vüsalə Yusifova @Bob Level Wow... so a COUPLE examples of overzealous/fraudulent claims, in the infancy of Evolutionary Science no less, are sufficient for you to simply IGNORE the MOUNTAIN of reputable evidence that has come since? Science DISproves creationism at every turn. Just because you REALLY REALLY REALLY want something to be true does not make it so... and you cannot simply pick and choose what you want to BELIEVE! Science is about fact, not about faith, and frankly whether you believe it or not has NO bearing on the truth.

Ruth Knox-MacBride
Ruth Knox-MacBride

@Dawn Michelle @Vüsalə Yusifova @Bob Level "Mountain of reputable evidence" Dawn Michelle?  The evidence is CIRCULAR REASONING! MICRO evolution does exist, small changes do happen, i.e. wolves and dogs are related, but MAN does NOT come from an ape!  I agree, some people act monkey-brained and that's a fact.  But my God, is NOT a monkey and the Bible says we are made in His image and in His likeness.  Kent Hovind is a GREAT anti-evolutionist scientist and he has done several seminars on the lies of evolution.  Check it out at www.drdino.com

Noah's flood, as referred to in the Bible, caused MUCH of the "evolution evidence" being used to claim evolution as truth.  If the earth is billions of years old, how fast was it spinning back then?  Cause it is losing speed and at the rate it is losing, the earth would have spun too fast for life to exist!  The moon is moving further away from the earth.  If the earth is billions of years old, the moon would have been touching the earth - the tidal waves would have been immense.  The sea is getting saltier and saltier - billions of years ago, it would have been a lake of salt at the rate it is going.  Jesus said:  I am the Way, the Truth and the Life.  If there is no Jesus in your life, there will be no truth in your life even if you REALLY REALLY REALLY want to believe that you can discern truth from lies.  satan is a deceiver and he deceives many.  That's what makes us Christians different:  we walk by faith and not by sight.  And on top of that, God has given us PLENTY of evidence to see Him as Creator.  Any design, demands a Designer.  Creation demands a Creator.  How can NOTHING explode into SOMETHING, hhhmmm???  Something had to exist to cause an explosion.  But nooooo, people prefer cicular reasoning and to deny that earth belongs to God, which means God controls it, NOT MAN!  I'll tell you what the Big Bang was.  I believe that when God opened His mouth and said:  Let there be light, it must have been a really loud noise :)  God SPOKE creaton into existence!  If I'm wrong (and I know I'm not) I have nothing to lose.  But if you're wrong, you have everything to lose.  Eternity is a VERY VERY long time.  May you find Truth.  Shalom.

Ron Coy
Ron Coy

@Ruth Knox-MacBride everything you've listed here as evidence has been debunked. You're wrong. Just... completely wrong.

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