Can they control their gender? Modoki Masuda/ Nature Production/Minden

Turtle embryos may be able to move around within their eggs to seek out hot or cool spots, and doing so might influence their eventual sex.

We know that in some turtles, cooler eggs produce males and warmer ones produce females. That’s a concern, because climate change could result in one-sided sex ratios as temperatures increase. But new work suggests that turtles may have some ability to adapt to combat this.

“Embryos can detect temperature differentials within the egg and move to the optimum position. That behaviour is of course a hallmark of reptiles in their post-hatching life – we’ve simply shown that the ability begins much earlier in life than people expected,” says Rick Shine at Macquarie University in Australia.


He and his colleagues measured the temperature gradients within the eggs of a freshwater turtle, Mauremys reevesii, in natural nests beside an outdoor pond. Previous research suggested that eggs in a natural nest wouldn’t have much of a temperature gradient, but Shine and his colleagues found that the temperature was consistently higher at one end of an egg than at the other, with a maximum difference of 4.7°C.

Read more: Why baby turtles work together to dig themselves out of a nest

“The temperature difference needed to shift from ‘develop as a male’ to ‘develop as a female’ is really tiny – only about one degree Celsius. And there is enough temperature difference between two ends within the same egg for an embryo to change its temperature by that amount,” says Shine.

The team incubated turtle eggs in the laboratory and treated half with capsazepine, a chemical that blocks the embryo’s ability to sense temperature. Then they heated one side of the eggs, creating a gradient. The embryos with blocked temperature sensors stayed in the middle of the egg and produced either all males or females, depending on their incubation temperature. But those with their temperature sensors working as normal moved up to 6 millimetres within the egg, and about half hatched as male and half as female.

This may not be due to any strategic migration within the egg, though. Rapidly growing embryos can turn within an egg, says Gerardo Cordero at the University of Tübingen in Germany.

No muscular capacity

Cordero says that at the point in development when temperature begins to influence sex, the embryos are still in the early stages of developing their limbs. “It’s very clear that embryos just don’t have the muscular capacity to be able to move in the egg,” he says.

Fredric Janzen at Iowa State University is similarly sceptical. He says it is highly unlikely that embryos move in response to temperature in the egg. Not only might the large yolk stand in their way, but the embryos would have to move twice a day to cope with daily temperature fluctuations, he says.

Janzen says field studies have shown that a change in air temperature of just a few degrees above or below average can lead to the young in a turtle nest being all or nearly all one sex. “Clearly, then, the embryos of those species aren’t counteracting even these relatively modest thermal excursions right now,” he says.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2019.06.038