Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world.

Chris Rehberg / Where Light Meets Dark (http://www.wherelightmeetsdark.com) (Image: Chris Rehberg/Where Light Meets Dark)

(Image credit: Chris Rehberg / Where Light Meets Dark)

Species: Saiphos equalis

Habitat: south-east Australia, caught in the act of evolving

Travel abroad and you are bound to see people doing things a bit differently. If the yellow-bellied three-toed skink were to go wandering, though, it would find members of its own species giving birth in a completely different way.

Skinks from the northern highlands of New South Wales give birth to live young, but those living in the coastal regions near Sydney lay eggs.

The skink is one of only three reptiles in the world that have different reproductive habits in this way. We are seeing them at a halfway stage in their evolution, and that means we can see the evolution of live-bearing in action.

At first glance, the skink looks like a tiny snake, but it has legs – albeit very small ones. It grows to a length of 18 centimetres, if you include the tail, and has a brown back and an orange belly (despite their yellow moniker). It is mostly active at night, and feeds on insects.

Reinventing birth

Although the skinks are almost unique today in being halfway through their evolution towards live birth, in historical terms they are almost ordinary. Live birth has evolved 132 times among backboned animals, and 98 of those occasions were in reptiles.

That suggests that it is rather easy to evolve. After all, to give birth to live young, a female simply has to keep the developing offspring inside herself for longer. But this gives rise to a host of challenges: she must provide it with food so that it can keep growing, protect it from her own immune system, which is liable to attack it, and ultimately eject it in a way that doesn’t kill it.

The skinks have gone a long way down this road. For instance, the density of the blood vessels supplying the uterus increases as the baby grows inside it. The embryo also gets extra food particles, and important ions such as calcium, in the later stages of the pregnancy.

Slinky skink

James Stewart of East Tennessee State University in Johnson City and colleagues have now shown that as the skinks retain their young for longer, they reduce the thickness of the eggshell. This allows the mother to keep the embryo well fed.

However, even the live-bearers have not got rid of the shell entirely. Baby skinks that are born live come out encased in a membrane – all that is left of the eggshell. With a bit of help from their mothers, most of them break out of the membrane within 36 hours.

How did live birth evolve? One group of the egg-laying skinks retain their eggs inside their bodies for longer than the others, and it seems that the live-bearers evolved from these “intermediate” skinks.

Reptiles are more likely to develop live birth if they live in cold climates, where it is a good idea to protect their offspring in their bodies, rather than exposing them to the rigours of the environment too soon. Sarah Smith of Stony Brook University, New York, points out that this explains why it is the skinks who live in the chilly highlands that give birth to live young.

Journal reference: Journal of Morphology, DOI: 10.1002/jmor.10877

Read previous Zoologger columns: Sympathy for the piranha, The world’s most fecund vertebrate, Whale-eater’s helpful sulphur-powered guests, Horror lizard squirts tears of blood, Secret to long life found… in a baby dragon, Eggs with an ‘eat me’ sign, How did the giraffe get its long neck?, The toughest fish on Earth… and in space, Vultures use twigs to gather wool for nests, The biggest living thing with teeth, Globetrotters of the animal kingdom.