- In a lab at the University of Sydney, a female yellow-bellied three-toad skink first laid eggs, then gave birth to a live baby, all part of the same pregnancy.
- This is the first time biologists have observed both egg-laying and live-bearing in a single litter of a vertebrate animal, researchers say in a new study.
- While the three-toed skink is known to have a dual mode of reproduction — some populations lay eggs, while others give birth to live babies — what mode the skink follows seems to be influenced by genetics and not environmental conditions, previous research has found.
- But the latest study suggests that individuals may be able to “switch” between reproductive modes depending on the situation, researchers say.
The yellow-bellied three-toed skink is not your typical lizard. Vertebrates or animals with backbones usually reproduce in one of two ways: they either lay eggs or they give birth to a live young baby. But the three-toed skink (Saiphos equalis) does both, its mode of reproduction varying with geography. In the warmer, coastal areas around Sydney, populations of the skink are known to lay eggs, while those living in the colder mountains of New South Wales give birth to live young. What mode of reproduction it follows, however, doesn’t seem to be influenced by environmental conditions, previous research has found, because swapping populations doesn’t appear to change how it reproduces, suggesting genetics may be at play.
But a female three-toed skink in a lab at the University of Sydney threw up a surprise. The skink, originally collected from a high-elevation population that’s known to give birth to live young, first laid eggs, then weeks later, birthed a live baby — all part of the same pregnancy. One of the incubated eggs hatched into a healthy baby.
This is the first time biologists have observed both egg-laying and live-bearing in a single litter of a vertebrate animal, researchers report in a new study published in Biology Letters.
“It is a very unusual discovery,” Camilla Whittington, a senior lecturer at the University of Sydney’s School of Life and Environmental Sciences and co-author of the study, said in a statement. “We were studying the genetics of these skinks when we noticed one of the live-bearing females lay three eggs. Several weeks later she gave birth to another baby. Seeing that baby was a very exciting moment!”
What this suggests, the researchers say, is that individuals may be able to “switch” between reproductive modes depending on the situation. But why this happens is unclear.
The researchers speculate that the skinks’ ability to change how they reproduce could be advantageous in “changing or uncertain environments.” In extreme cold or dry conditions, for example, it might make more sense for a skink to carry her offspring to term, the researchers say. At the same time, long pregnancies can take a huge toll on the mother, so laying eggs may be the smarter thing to do in some situations.
Only a handful of backboned species are known to have such dual modes of reproduction. The Bougainville’s skink (Lerista bougainvillii) is one, laying eggs on the Australian mainland but giving birth to live young in Tasmania and the islands of the Bass Strait. Examining such species could reveal more about the evolution of reproduction in vertebrates, the researchers say.
“We suggest that other species in which live birth has evolved from egg-laying relatively recently may also use flexible reproductive tactics,” Whittington and Melanie Laird, a postdoctoral fellow at the University of Otago, New Zealand, wrote in The Conversation. “Further research into this small Australian lizard [three-toed skink], which seems to occupy the grey area between live birth and egg-laying, will help us determine how and why species make major reproductive leaps.”
Laird, M. K., Thompson, M. B. and Whittington, C. M (2019) Facultative oviparity in a viviparous skink (Saiphos equalis). Biology Letters, 15 (4): 20180827. DOI: 10.1098/rsbl.2018.0827