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Lizard gender bends at altitude
A small Tasmanian lizard has startled researchers by revealing that it has different ways of determining the sex of its offspring, depending on the altitude.

At low altitudes, the lizard's gender is determined by temperature, while at high altitudes where the climate is more extreme, it's all down to their genes, according to a study in Nature.

Researchers from Australia, the United Kingdom and the Netherlands studied what happened when the snow skink, Niveoscincus ocellatus, which lives at a range of different altitudes in Tasmania, got pregnant or gave birth under different temperature conditions.

One of the authors, Dr Erik Wapstra from the University of Tasmania says this is the first reptile, and only the second animal species, ever shown to have two types of sex determination in different populations.

The snow skink is a small six centimetre long lizard which lives in coastal and sub-alpine areas beneath rock slabs, eating insects and wild berries. It gives birth to live young rather than laying eggs, producing between one and six babies.

Previous research shows that the sex of baby skinks is determined during the first half of the pregnancy, depending on how long the mother remains in warm sunshine. Warm temperatures generally produce females, while cooler temperatures produce males.
Highs and lows of sex determination

The researchers took heavily pregnant females from both the lowland and highland populations and put them in cages in the lab until they gave birth. The sex ratios of the offspring were recorded.

They also collected females who had just become pregnant - before the sex of the offspring had been locked in - from both populations. Each group was split in two, with one subgroup exposed to 10 hours of sunshine per day, and the other group to only four hours of sunshine per day.

The researchers found that lowland mothers had very different ratios of boys to girls, depending on how much sunshine they had. By contrast, the highland mothers showed no difference in sex ratio of offspring, regardless of how long they spent in the Sun

Based on data collected in the field, the researchers developed a model to predict how these lizards would evolve over thousands of years. The model showed that the divergence of sex determination mechanisms was caused by temperature differences, Wapstra says.

So why develop such a complex system? Wapstra says it allows the skinks to produce more daughters and produce them earlier when conditions are warm.

"If you are born early, you are bigger by the time you reach maturity. This means having better offspring yourself," he says.

But this only occurs in the lowlands, where lizards mature by the age of two or three years.

In the highlands, by contrast, everything is colder and it takes females much longer to mature - about four to five years. Here there's no advantage to being born early - you still end up maturing at the same time, says Wapstra.

So how come lizards don't try to produce big boys?

"Unlike many other animals, such as humans, elephants and chimps, there's no advantage to being a big male because fighting is less important."
Evolution and adaptability

Professor David Lambert from Griffith University in Brisbane has studied evolution in the tuatara, a lizard-like reptile of New Zealand. He says the research illustrates one of the biggest conundrums in biology.

"Sex determination is about as fundamental as you can get," says Lambert. "The classic wisdom would say that when something's really important, it will be conserved evolutionarily. But in fact it isn't the case - sex determination is incredibly variable in nature.

"This example is really interesting because they have both (environment and genes) determining sex in the one species. That's remarkable. It's a classic example of the conundrum."

Reptile expert Professor Rick Shine from the University of Sydney says the study shows that sex determination in reptiles is a lot more complicated, and more responsive to environmental conditions, than is the case in mammals and birds.

"Sex chromosomes are fine if you want to produce equal numbers of sons and daughters, but systems that change offspring sex depending on local weather conditions give you an opportunity to flexibly adapt to new challenges," he says.

"These little Tasmanian lizards show that the evolutionary advantages of switching systems can be so great that we can even see both genetic and environmental sex-determination in different areas within the range of a single species."

Dr Erik Wapstra adds that under a scenario of global climate change, where there would be more warm years than cold years, we may see a growth of more females than males in the population.

"It might not be a bad thing necessarily, because males aren't really that important," says Wapstra.

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