Lizard can switch from female to male before birth, but not vice versa

Some offspring of the spotted snow skink, a lizard found in Tasmania, are born anatomically male but are genetically female

By Christa Lesté-Lasserre

A lizard from Tasmania is the first non-egg-laying animal known to sometimes switch sexes before birth, depending on the surrounding temperatures.

Some egg-laying fish, frogs and reptiles hatch with male sex organs and female chromosomes, or vice versa, when the eggs are exposed to environments that are particularly warm or cool, indicating that their sex changed during incubation.

Now Peta Hill at the University of Tasmania and her colleagues have found the same is true of the spotted snow skink (Carinascincus ocellatus), a lizard found in Tasmania that gives birth to live young.

The researchers trapped 100 newly pregnant female spotted snow skinks at different altitudes in Tasmania and placed them in individual terrariums in their laboratory.

Two groups of 20 skinks had access to a heat lamp over part of the terrarium for either 4 or 12 hours a day, creating temperature zones ranging from 20 to 37°C and falling to around 10°C when the lamp was off. The remaining three groups lived in controlled temperatures of either 33°C, 29.5°C or 26°C for eight hours a day and 10°C for the other 16 hours.

The researchers sexed each newborn skink by examining its sex organs in the first month of life. They also assigned genetic sexes by sequencing DNA taken from the animals’ tails.

All the skinks with female sex organs had XX chromosomes – meaning there were no male-to-female changes. However, 7 per cent of the 423 newborn skinks had male sex organs and XX chromosomes.

These skinks were mostly found in litters in the cooler terrariums and born to females from lower altitudes. Specifically, 20 per cent of the offspring with male sex organs and XX chromosomes were born to females from low altitudes that were in the 26°C terrariums or offered only four hours of heat lamp per day. That was the case for only 2 per cent of the offspring born to females from low altitudes kept in warmer conditions.

The findings are “striking” not only because they are seen in a live-bearing reptile, but also because the phenomenon is so one-sided, says Benjamin Geffroy at the French Institute for the Exploitation of the Sea in Montpellier, who wasn’t involved in the study.

Even so, that might be explained by a generation effect, says Geffroy. Skinks with male sex organs and XX chromosomes – “XX males” – can grow up as functional adult males and might mate with XX females, he says. But because they lack a Y chromosome, all the pair’s offspring would be XX.

“I find it really interesting that we can explain the… bias toward females in warm conditions, if we consider that some XX males reproduce with XX females, leading to more genetic females in the second generation,” he says. A longer study over multiple generations could help support the theory, says Geffroy.

Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2022.0689

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