For most of the animal kingdom, babies are born in one of two ways: their parent either lays eggs or gives birth to live offspring.
Recently, a three-toed skink (Saiphos equalis) pulled off an extraordinary feat: It laid three eggs and delivered another baby through live birth in the same pregnancy. That suggests that the lizard species is in a rare transitional form between egg-laying and live-bearing animals, according to a study published in Molecular Ecology last month.
“We affectionately call the three-toed skink ‘the weirdest lizard in the world’ — but it can tell us a lot about the evolution of reproductive strategies,” Camilla Whittington, one of the study’s lead authors and an evolutionary biologist at the University of Sydney, wrote in an email to CNN.
It was the first record of egg-laying and live birth in a single pregnancy
Three-toed skinks, found in Australia, are already a fascinating species for evolutionary scientists, explains Whittington. One reason is that some populations reproduce by laying eggs, while others reproduce through live birth.
The mode skinks use to reproduce generally corresponds with their environment. Skinks around the Sydney area lay eggs, albeit ones with thin shells and embryos that are almost completely developed. In northern parts of Australia, the skinks give birth to live young.
But never before had scientists seen a species lay eggs and experience live birth in a single pregnancy until it was observed in the three-toed skink last year, Whittington wrote. It was the first record of a vertebrate doing so.
Why this could be happening
There are a couple of explanations as to why the skink both laid eggs and had a live baby in the same pregnancy, Whittingon said.
One is that it was a form of “bet-hedging,” meaning that the ability to switch between laying eggs and live birth could provide the lizard an advantage in unpredictable environments.
“For example, if it’s cold or dry, it might be risky to lay eggs in an unprotected nest, and better for the mums to carry the babies until development is complete,” she wrote. “If there are a lot of predators around and pregnant mums find it harder to escape, it might be risky to carry babies to term. Mothers that are able to act flexibly could therefore have an advantage in an unpredictable environment.”
Another explanation is that some feature of the environment could have caused the skink in question to lay part of her clutch abnormally early. Still, Whittington said, two of the skink’s eggs hatched to healthy baby lizards, which means that if this phenomenon happened in the wild, the babies could still be viable.
Scientists don’t yet know what course evolution is taking
The finding could mean that the skink is transitioning to only laying eggs or only experiencing live birth. But scientists say it’s too soon to tell which direction it’s moving in.
In general, animals that give birth to live young have evolved from ancestors that laid eggs and it would be rare for an animal to evolve in the other direction, according to Whittington. And it’s worth pointing out that when this skink does lay eggs, the embyros are almost fully developed and hatch much more quickly than other egg-laying lizards.
Still, it’s “impossible” to determine what course natural selection is taking in this skink species, Whittington said. Their research shows that the uterus of the “transitional” three-toed skinks and live-bearing three-toed skinks function similarly, which could make it possible for the species to reverse from live-bearing to egg-laying, she said.
“At the moment, we can’t rule out the possibility that the transitional animals could be descended from live-bearing ancestors — which is why we are continuing to study these amazing lizards,” she wrote.
“To complicate things, if the environment changes, the direction of selection could change too! In some environments, natural selection might favour laying eggs. In others, it might be more advantageous to give birth to live young.”
And that, Whittington said, is what makes evolution such a fascinating process.