Lizard moving from eggs to live birth

[FaxModem1]

National Geographic

Evolution in Action: Lizard Moving From Eggs to Live Birth
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.

This is interesting. I can imagine just showing this to anyone who has a creationist argument.

[LadyTevar]

This would blow creationists out of the water, all right.

This is fantastic! I wonder if this will continue spreading to more skinks.

[bilateralrope]

This is interesting. I can imagine just showing this to anyone who has a creationist argument.

I'd expect them to scream about microevolution or claim it's a fake.

[Tolya]

No.

"Show me the transitional species!!!" would be the normal way to do it for creationists.

[Alyrium Denryle]

This would blow creationists out of the water, all right.

This is fantastic! I wonder if this will continue spreading to more skinks.

It is actually really common among north american skinks.

[Purple]

@ Tolya

Just for the record, what would a transitional species even be like? I mean we have eggs and we have wild birth. What kind of transitional stage could there be? The best I can come up with is live birth but with every lizard being born with an egg shell cap Calimero style.

[Napoleon the Clown]

I'm guessing that a "transitional species", with regards to eggs vs live birth, would be an animal that does form eggs like most others of its classification but holds the eggs until they hatch before giving birth. There are several species already that do this, Alyrium can likely give quite a few examples without needing to even Google it. I don't remember names off-hand but I think a few snakes do just that.

Something to keep in mind is that every species is one step of many between an established way of doing things and a new way of doing things. We can't really say what new adaptations are currently evolving in a specific species, of course, but there's always something in the pipes, even if it's thousands of years away. Just what constitutes a transitional species will depend on who you ask. I doubt you could get a proper definition out of a creationist.

[Purple]

That is actually way cool.

[madd0ct0r]

I'm guessing that a "transitional species", with regards to eggs vs live birth, would be an animal that does form eggs like most others of its classification but holds the eggs until they hatch before giving birth. There are several species already that do this, Alyrium can likely give quite a few examples without needing to even Google it. I don't remember names off-hand but I think a few snakes do just that.

Something to keep in mind is that every species is one step of many between an established way of doing things and a new way of doing things. We can't really say what new adaptations are currently evolving in a specific species, of course, but there's always something in the pipes, even if it's thousands of years away. Just what constitutes a transitional species will depend on who you ask. I doubt you could get a proper definition out of a creationist.

Sharks are one example that come to mind.

[Ziggy Stardust]

Besides skinks (as Alyrium noted), some species of chameleon are also viviparous (give birth to live young). In addition, a number of snakes are ovoviviparous (hold the eggs inside them until they hatch, then give birth to live young): boas, vipers, garter snakes, slow-worms (actually, I think they are technically a type of lizard, but I don't remember and don't have time to double-check).

Sharks and rays are similar, though they make use of yolk sacs instead of true 'eggs'.

I would bet Alyrium could also dig up some good examples of amphibians.

I am sure that if we look hard enough at invertebrates we could find some good examples, too. I know tsetse flies, as an example, develop the eggs internally and only give birth to a larvae when it is ready to pupate. Essentially, my point is that basically every physically possible reproductive strategy is being tried by some critter out there.

[Alyrium Denryle]

Besides skinks (as Alyrium noted), some species of chameleon are also viviparous (give birth to live young). In addition, a number of snakes are ovoviviparous (hold the eggs inside them until they hatch, then give birth to live young): boas, vipers, garter snakes, slow-worms (actually, I think they are technically a type of lizard, but I don't remember and don't have time to double-check).

They are lizards. And some of those snakes you list are flat out viviparous, with varying degrees of placental function. Garter snakes for example transfer water and gas to their offspring through it, but use yolk for nutrients.

I would bet Alyrium could also dig up some good examples of amphibians.

There is one species of fire salamander, and a few caecilians.

Just for the record, what would a transitional species even be like? I mean we have eggs and we have wild birth. What kind of transitional stage could there be? The best I can come up with is live birth but with every lizard being born with an egg shell cap Calimero style.

Basically, just holding the eggs for a longer period of time. Eventually, the shell is dispensed with, and instead of (in the case of reptiles) attaching to the inner side of an egg (turning reptile eggs will kill them), they attach to the internal wall of the oviduct, over time and selective pressure, they start doing gas, water, and eventually nutrient exchange directly with the mother's blood supply. And Lo. The placenta is born.

[Serafina]

Creationist denial:
"Just because you can show intermediary stages doesn't mean species can change from one state to another, that's never been observed! And if, only in small steps, that doesn't mean you can go all the way!"
/creationist denial

In other words - this will be treated with the same old tactics. Even if you list all the in-between steps, and show each step existing today in related species - they'll still claim that only an uninterrupted real-time observation under laboratory condition of radical changes - not just a advantageous trait or two, but changing between "kinds".

Or, in two words - Moving Goalposts.

[Alyrium Denryle]

Creationist denial:
"Just because you can show intermediary stages doesn't mean species can change from one state to another, that's never been observed! And if, only in small steps, that doesn't mean you can go all the way!"
/creationist denial

In other words - this will be treated with the same old tactics. Even if you list all the in-between steps, and show each step existing today in related species - they'll still claim that only an uninterrupted real-time observation under laboratory condition of radical changes - not just a advantageous trait or two, but changing between "kinds".

Or, in two words - Moving Goalposts.

There is a species of Lacertid living on an island off the IIRC Croatian coastline that was not there thirty years ago. It was introduced, and has become an entirely different species since then, going from a lizard that eats plants to one that eats insects, with all the associated changes in jaw structure, teeth, behavior, and intestinal tract that one would expect from such a shift.

[Eternal_Freedom]

As interesting as that is, I don't think you need to burden creationists with anything as sordid as "truth" or "evidence." THey'll just change the finish line as Serafina said. Hell, some of them probably wouldn't change their tune if Jesus himself appeared and smacked them with a biology textbook.

[Ziggy Stardust]

I believe that would be what is commonly referred to as the Italian wall lizard. This is the original study.

It is an especially great case study because they are only several kilometers removed from another island (off the coast of Croatia, as you said) with similar elevation, temperature,and climate that is also home to a population of the lizard, one that has not become insectivorous. It's not often nature just hands you a control group.

[SMJB]

Hell, some of them probably wouldn't change their tune if Jesus himself appeared and smacked them with a biology textbook.

That mental image made my night.