It only works in one direction — not the other — and it’s not exactly clear why.

In sexually reproducing organisms, the sex of the individual is usually determined by genes — but in some cases, the sex encoded genetically can be overridden by the environment. While the sex chromosomes are fixed at the moment of conception, the enzymes that control the physical differences between male and females can actually change, so you can end up with an animal that is genetically male, but has a fully functional and fertile female body. In reptiles, for instance, temperature is entirely responsible for determining sex. In chickens, temperature has a much less pronounced role, but it can still change the sex of the chick.

This happens most often in reptiles and birds (which can sometimes control the sex of their chicks). But now, for the first time, researchers have noticed this phenomenon in a lizard that doesn’t lay eggs.

Most lizards do lay eggs, but some lizards and snakes (around 20% of all species) are viviparous — they give birth to live young. Skinks are an example of a viviparous lizard.

Skinks are a type of lizard group with over 1,500 species. To learn more about them, researchers captured 100 pregnant female skinks from the mountainous ranges of Tasmania. It was previously shown that skinks live on these mountains at different altitudes, which means that they also live at different temperatures, and researchers wanted to see whether this temperature variation could affect their sex.

The researchers divided the specimens into several groups, keeping them in terrariums at different temperatures.

After the skinks gave birth, the researchers then analyzed the offspring — both anatomically and genetically. All of the skinks with female anatomy had XX chromosomes (so they were also genetically female). But 7% of the male baby skinks also had XX chromosomes — so they were genetically females, but physically males.

The team noted that most swaps occurred in skinks that were exposed to cooler temperatures, and in mothers that were captured at lower altitudes, so would have been most used to warmer temperatures. The researchers, led by Peta Hill from the University of Tasmania, see this as an indication that mothers raised in warmer environments are more likely to have offspring with swapped sex if moved to higher, colder environments.

It’s still not clear exactly why this happens, or why it only appears to work one way, but it shows that this type of swap isn’t only restricted to egg-laying animals. The world of animal gender is probably weirder than you thought.

The study was published in Proceedings of the Royal Society B.