A lizard has pulled off a feat that scientists have never observed before, suggesting that the reptile could be in the middle of an important evolutionary shift.
The three-toed skink (Saiphos equalis), a small lizard native to eastern Australia, managed to deliver members of the same litter both in eggs and through live birth. This means that the species may be an “intermediate form” between egg-laying animals and species that give birth to live offspring, according to a recent study in Molecular Ecology.
While it’s possible that the skink is transitioning between oviparity, the ability to lay eggs, and viviparity, or the ability to give birth, the ultimate evolutionary trajectory of the animal remains unpredictable.
“We don’t know which ‘direction’ evolution is taking,” said corresponding author Camilla Whittington, an evolutionary biologist at the University of Sydney, in an email. “In some environments, egg-laying might be more advantageous than live-bearing. In that situation, we would predict that egg-laying would persist.”
Three-toed skinks are one of a handful of species capable of bimodal reproduction, meaning that some populations are oviparous while others are viviparous. However, it is the only species that has ever been seen employing both modes during the same pregnancy, which demonstrates an unprecedented example of reproductive versatility.
In addition to the Molecular Ecology study, Whittington and her colleagues published a study on Friday in Biology Letters that suggests this skink may employ “facultative oviparity,” meaning that its method of offspring delivery may depend on environmental conditions, or other factors.
For instance, this species was already known to express oviparity and viviparity depending on its location. Three-toed skinks that live in the Sydney metropolitan area lay eggs while their relatives in the highlands of New South Wales, where the climate is cooler and wetter, give birth to live young.
“There’s a prominent evolutionary hypothesis called the ‘cold-climate hypothesis’ that states that live-bearing might be advantageous in cooler climates, and the distribution of these lizards seems to fit that prediction,” Whittington said. “However, there is so much about the biology of these lizards that is still a mystery that we can’t say for sure yet—which is why we are continuing to work on this species.”
The skink’s flexible approach to procreation could shed light on past transitions from oviparity to viviparity, a shift that is estimated to have occurred at least 150 times in vertebrate evolution.
The earliest animals were egg-layers that depended on a larval stage of embryonic development outside the parent’s body. But over hundreds of millions of years of evolution, some animals began to sink energy into in vivo embryonic development, which has its own cost-benefit balance linked to certain conditions.
The three-toed skink may be in the process of undergoing a similar switch from laying eggs to becoming a full-time live-birther. Even when the lizard expresses oviparity, its embryos are relatively well-developed before the eggs are laid, and hatchlings can emerge in mere days.
But it’s also possible that the skink will hold onto its dual reproductive modes “as a bet-hedging strategy in variable environments,” according to the Biology Letters study.
“To complicate the situation, when environments change (as is happening rapidly in the Anthropocene), the adaptive advantages of egg-laying versus live-bearing may also change, which might ‘push’ populations towards egg-laying or live bearing,” Whittington said. “We predict that ‘reversals’ from live-bearing back to egg-laying might even be possible, because of the similarities in gene expression of egg-laying and live-bearing three-toed skinks.”
The new discoveries further cement Australia's reputation as a natural incubator of some of the most bizarre and unique wildlife on the planet. Future research into the lizard’s behavior and abilities could shed light on why some animals remain egg-layers while others made the switch to live young, including humans.