The destiny of many turtles is tightly bound to temperature.

Key points: Some scientists worry rising temperatures will skew the sex ratio in certain turtle populations

Some scientists worry rising temperatures will skew the sex ratio in certain turtle populations Until now it was not thought that such species of turtle could influence their sex at all

Until now it was not thought that such species of turtle could influence their sex at all The discovery may provide some species of turtles a mechanism to help buffer against climate change

Eggs exposed to high temperatures are more likely to be females, and cooler eggs more likely to be males, in some species.

It's a quirk of nature that has some scientists worried that these turtle populations will become skewed towards females as global temperatures rise.

Last year, it was reported that 99 per cent of Australian green turtle hatchlings on beaches in the northern reaches of the Great Barrier Reef were female.

But it appears there is a little wriggle room for at least one species of freshwater turtle.

The embryos of the Chinese pond turtle (Mauremys reevesii) can change the temperature that influences their sexual destiny by moving around the egg, according to a new study published today in the journal Current Biology.

"We have always regarded embryos as passive little organisms that don't have any control over their own environments until after they hatch out of the egg," said study co-author Rick Shine of Macquarie University.

"Our study shows that even a tiny embryo is able to detect the world around it and respond accordingly."

Modelling by the team suggests the ability of an embryo to find the temperature sweet spot could help some species maintain sex ratio balances and help buffer against rising temperatures.

But other scientists say this "fascinating phenomenon" is likely to have a minimal impact in the face of climate change and other threats.

The Chinese pond turtle, also known as Reeves' turtle, lays large eggs in a shallow nest. ( FlickR: Denise Chan )

Temperature, turtles and sex

The sex of a few turtle species, such as Australian freshwater turtles, is determined by X and Y chromosomes just like us.

But the sex of other species of freshwater turtles, all sea turtles, and many snakes, lizards and crocodiles is determined by the environment.

It has long been known that many species of turtle embryos move within the egg during incubation, but the idea that this could actively play a role in determining the sex of the embryo has been hotly contested.

To find out if this phenomenon was possible, the team led by Yin-Zi Ye of the Chinese Academy of Sciences incubated the eggs of the endangered Chinese pond turtle in the lab and in an outdoor pond under normal seasonal conditions.

A temperature of 27.9 degrees C is needed to produce an equal mix of males and females in this species, which is found in Asia.

Embryo of a Chinese pond turtle. ( Supplied: Ye et all/Current Biology )

The researchers detected thermal gradients of up to 4.7C between the two ends of the eggs.

They then injected half the eggs with a substance that blocked the embryo's ability to sense temperature. The embryos within these eggs moved less than eggs that hadn't been manipulated.

When they hatched they skewed towards females under hotter conditions or males under cooler conditions. Meanwhile, an equal number of males and females hatched from the other eggs.

Will it provide a buffer against climate change?

The ability of turtles to use this mechanism will vary from species to species, Professor Shine said.

"It will only work for species that have large eggs in shallow nest in sun-exposed soil, but that's still a lot of species, especially in groups like turtles," Professor Shine said.

"The beauty of embryonic control over incubation temperature is that it allows immediate responses to unpredictable bouts of cool or hot weather."

But, he said, the mechanism would be no match for temperature extremes.

"If conditions get really hot or really cold, then there will be no Goldilocks zone within the egg."

"An embryo may be able to keep a bit hotter or colder than would otherwise be the case, but it won't be able to achieve optimum conditions."

Rory Telemeco of California State University Fresno, who studies how animals adjust to climate change, has previously argued that a vast majority of reptiles would be unable to regulate their temperature in the egg.

An embryo's ability to control its sexual destiny would be limited if the egg was too small, or the embryo too large or young to move around.

"There's this extremely finite set of conditions where it can work," Dr Telemeco said.

"[The researchers] have found a species that meets all those conditions ... which is really impressive and cool, but I don't know if it really changes the idea that the majority of species [of reptiles] aren't going to."

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He said the phenomenon would not work for turtle species that lay large clutches of eggs in deep nests, such as sea turtles.

"As you go deeper in the soil, you stop having any real temperature variation either through the day or with depth that can go across the sides of the egg."

More often, he said, the environment or the mother's behaviour would dictate the sex ratio of the hatchlings.

Factors that can influence nest temperature include local climate and geographic conditions, amount of vegetation and shade, nest structure and laying season.

How big a problem is feminisation?

There is growing evidence feminisation may not be the biggest threat facing turtles and other reptiles, Dr Telemeco said.

"There seems to be a lot of mechanisms in place where historically these organisms have coped with this climate variation and not had big feminisation-induced declines."

Ricky Spencer, a population ecologist at Western Sydney University who studies freshwater turtles, said his unpublished modelling suggests feminisation generally provides more — not less — resilience for turtles to a point.

This is because there will be more females around to breed and they can live for 100-plus years.

"As long as there are some males being produced somewhere within the population they're actually quite resilient."

That's not to say climate change hasn't had a major impact on hatchlings.

All it takes is one sudden spike in temperature to wipe out baby turtles, Dr Telemeco said.

"A lot of nests are failing and nothing about a baby moving in the egg is going to keep them from just experiencing temperatures that cause them to flat-out die."

"The effects of global change on their ability to survive might actually be the bigger challenge for them than becoming feminised," he said.

Dr Spencer said turtles were a barometer of climate change.

Recently there have been reports of turtles nesting earlier.

"By nesting earlier they're nesting in the cooler period," he said.

"So there will be responses to climate change, but whether they can adapt quickly enough that's a different question."

But there are even greater threats, according to Dr Spencer, one of 50 authors on a report about the global conservation status of turtles and tortoises.

Disease, water contamination, pests and human activity are taking their toll on the world's 360 species of turtles and tortoises.

"If you've got a turtle that can produce eggs every year for 100 years it only needs one clutch to really survive ... but if you take out one of those adults that's 100 years or 75 years of reproduction gone."

Professor Shine agreed.