Darwin’s Monster: Saiphos equalis, The Australian Three-Toed Skink

Every few years this photo by Rebecca Pyles, published in the National Geographic, goes viral on social media. It seems that most people share it because they’re struck by its tiny little legs. It either looks like a snake evolving new limbs or a lizard en route to leglessness. Now, don’t get me wrong, those limbs are interesting, but more exciting still is what can be seen inside the animal’s belly.

Stated clearly presents: Darwin’s monster, Saiphos equalis, the Australian three-toed skink. Evidence for evolution isn’t just found in textbooks, the fossil record or fancy museums. If you look closely, lessons on evolution can be found through the careful study of any living thing. But there are certain animals alive today, animals with anatomy so bizarre that their lessons on evolution slap us in the face.

These are the creatures that I call “Darwin’s Monsters”. Today’s example slaps us with two completely different evolutionary lessons. One about the origin of snakes and one about the origin of pregnancy. You probably know that tetrapods–four limbed animals with lungs and backbones, humans included–all evolved and split from a single group of lobe-finned fish that started venturing out on land somewhere around 400 million years ago. Our arms and our legs are highly modified fins.

Once this group began diversifying on land, one lineage branched off from lizards and, for some reason, began to lose its legs! It eventually gave rise to the group of animals we now know as snakes. Snakes are technically classified as legless tetrapods, which is sort of funny when you learn what tetrapod means in Greek, but I digress…

Why? Why did snakes lose their legs?

When we study evolution, we usually focus on how new organs and new structures evolve. After all, if nature really is relentlessly “selecting” for individuals with the best possible traits for survival and reproduction, why would animals ever lose entire organs or limbs over evolutionary time? For many years, scientists argued that maybe snakes lost their legs by going through a second, short-lived aquatic stage. Maybe the ancestors of snakes went back to the ocean for a while.

We know that whales and dolphins did this. They still live in the ocean. During their transition back, the hands on their four limbs fused into paddles and their hind legs became dead weight, shrank, and were eventually lost completely.

Most whales still have a pair of small hip bones. They don’t connect to legs anymore, but they still do anchor the reproductive organs, just like they do in other mammals. Today, there are marine snakes that live in the ocean. In the fossil record, we find snakes with shrunken hind limbs that are sometimes found in marine sediments. Both of these facts do lend credibility to the aquatic snake hypothesis; but is this really the best possible explanation? Instead of projecting what we know about whales onto snakes, why not look at modern reptiles currently in the process of losing their limbs today?

Skinks are a diverse group of lizards. Some species have normal sized legs (legs like you’d expect a lizard to have), others are varying degrees of stubby. Some species no longer have any legs at all.

This is not a snake!

So, what selection pressures have been driving these transitions? Well, it’s not life in the water. While some species do swim, fully aquatic skinks are not a thing. Instead, it’s life in the leaf-litter! Nearly all short limbed and no limbed skinks live and hunt either in the leaves or in loose gravels and soils.

It turns out that when you spend all day on your belly chasing bugs between pebbles, through loose dirt, and underneath leaves, shoulders tend to get in the way. Through the study of these and other burrowing reptiles, along with a closer look at fossils and the anatomical details of modern snakes themselves, scientists are now very confident that snakes lost their limbs during an ancient burrowing leaf-litter stage, not a temporary return to the ocean. Scientists don’t have certainty here, but they do have confidence.

Okay, enough about legs. What’s all the fuss about these ghostly shapes inside this animal’s belly? Well, it turns out that this reptile is pregnant. As you probably know, the vast majority of reptiles, birds, amphibians, fish, and even several species of mammal, are egg layers! Therian mammals–the group that includes humans–therians are the oddballs on this planet because instead of eggs, we have pregnancy.

But how? How did live birth evolve from the normal default egg-laying system? So far, the fossil record has not been very much help here. But back in 1925 (so, 100 years ago) a young woman named Claire Weekes was studying reptile reproduction for her PhD. At the time, Claire proposed that egg retention (the holding in of eggs after fertilization) was the most likely transitional form between egg layers and live birthers.

If a mutation caused a mother to retain her eggs after fertilization, and if for some reason natural selection favored that egg retention, retention time can increase through the course of evolution until egg-laying day and egg-hatching day are one and the same! At this point, we would have a simple form of live birth, which could then evolve into all the complex, fancy forms of pregnancy and gestation found in marsupials and placental mammals today.

Okay, that is an interesting idea, but there’s a problem with this model. Egg retention is dangerous! It’s hard to move around if your body is filled with eggs. If an egg dies before birth, it can cause an infection, and holding eggs inside the body until they’re fully mature… Well… that reduces how many clutches you can have each year.

How, then, could egg retention ever be favored by natural selection in place of immediate egg-laying? Claire’s study of reptiles gave her clues about a possible solution. Egg retention is about temperature! In cooler climates, eggs laid directly on the ground can easily freeze or at least drop below ideal growth temperature. Even though reptiles are largely cold blooded, they can keep their internal temperature safe by basking in the sun at day and burrowing to shelter at night.

Claire believed that egg retention could be favored by natural selection if the environment was cold. Sadly, Claire passed away in 1990, 7 years before this report on Saiphos equalis, a paper that confirmed her hypothesis, not just with models or experiments in some laboratory, but with real observations of live animals captured in the wild.

Here and then in later studies, it was reported that in warm coastal populations, the mothers lay relatively normal looking eggs which take up to 15 days to hatch. In colder mountain regions, they hold their eggs in as long as possible, which come out either as clear egg-like sacs and hatch within hours, or open while the eggs are being laid. This is a clunky, primitive, transitional form of live birth!

Here we see normal white eggs from a warm population, compared to the clear egg sac with a fully developed hatchling inside from a mountain population. You can even see his chin and his stubby little arms. Here in one species we see the perfect confirmation of Claire’s evolutionary hypothesis. Not only is the three-toed skink an excellent living example of the transition from egg-laying to live birth, it shows that cold weather really is an environmental pressure that can trigger the evolution of pregnancy. This is the case for reptiles today, and may have been the case for our own early, egg-laying ancestors!

So, to summarize these two lessons: The three-toed skink and its relatives teach us that even though leglessness can evolve in water, it evolves even more readily in leaf-litter! Second, pregnancy and live birth can evolve in response to cold climates.

I am Jon Perry and that is the three-toed skink, one of Darwin’s little monsters, Stated Clearly.