Turtle embryos can influence their own sex

The sex of young turtles can be determined in part by the temperature of the egg they emerge from, new research shows. And it’s not a random process.

Chinese and Australian scientists have found that embryos can move around inside the egg to find a “Goldilocks Zone” – neither too hot nor too cold.

And this, they suggest, may also help turtles offset the effects of climate change.

“We previously demonstrated that reptile embryos could move around within their egg for thermoregulation, so we were curious about whether this could affect their sex determination,” says Wei-Guo Du, from the Chinese Academy of Sciences.

“We wanted to know if and how this behaviour could help buffer the impact of global warming on offspring sex ratios in these species.”

Du and his colleagues incubated turtle eggs under a range of temperatures in the laboratory and outdoor ponds.

They found that a single embryo could experience a temperature gradient of up of 4.7 degrees Celsius within its egg. 

This is significant, he says, because any shift larger than two degrees can massively change the offspring sex ratio of many turtle species.

In half of the eggs, they applied capsazepine, a chemical that blocked temperature sensors, to prevent behavioural thermoregulation. 

After the eggs hatched, they found that the embryos without behavioural thermoregulation had developed as either almost all males or almost all females, depending on the incubation temperatures. 

In contrast, embryos that were able to react to nest temperatures moved around inside their eggs; about half of them developed as males and the other half as females.

Richard Shine, from Australia’s Macquarie University, says finding the “Goldilocks Zone” can shield the turtles against extreme thermal conditions imposed by changing temperatures and produce a relatively balanced sex ratio. 

“This could explain how reptile species with temperature-dependent sex determination have managed to survive previous periods in Earth history when temperatures were far hotter than at present,” he says.

Du notes, however, that this behaviour has limitations. “Embryonic thermoregulation can be limited if the thermal gradient within an egg is too small, or if the embryo is too large to move around or too young to have developed these abilities yet,” he says.

Additionally, he adds, the behaviour cannot buffer the impact of episodes of extremely high temperatures, which are predicted to increase with climate change, Du says.

“The embryo’s control over its own sex may not be enough to protect it from the much more rapid climate change currently being caused by human activities, which is predicted to cause severe female-biased populations,” he says. 

“However, the discovery of this surprising level of control in such a tiny organism suggests that in at least some cases, evolution has conferred an ability to deal with such challenges.”

The findings are published in the journal Current Biology.