A study from Duke University has found that warmer temperatures not only cause more turtle eggs to hatch as females, but these females also have a higher capacity for egg production. The research showed that higher incubation temperatures increase the number of “germ cells” in an embryo, pre-eggs that also play a role in determining 𝑠e𝑥, potentially providing an evolutionary reason for temperature-dependent 𝑠e𝑥 determination and presenting new implications for these species in a changing climate.

A recent discovery sheds light on how and why temperatures determine the 𝑠e𝑥 of turtles

Research from Duke University suggests that not only does a warmer climate increase the likelihood of turtle eggs developing into females, but it also enhances these females’ potential for egg production even before their gender is determined.

This discovery could explain why numerous species besides turtles exhibit temperature-dependent 𝑠e𝑥 determination, a seemingly precarious reproductive strategy that nonetheless endures. Moreover, this revelation could indicate worrying future implications in the context of global warming.

What the researchers found, as published on June 23 in the journal Current Biology, is that the number of “germ cells” — pre-eggs — that an embryo carries is increased by higher incubation temperatures. In fact, they found that those germ cells themselves play a role in the embryo becoming female.

“Sex determination by temperature isn’t just one mechanism,” said senior author Blanche Capel, the James B. Duke Distinguished Professor of Cell Biology in the Duke School of Medicine. “Higher temperatures seem to affect 𝑠e𝑥 determination in incremental ways through multiple cell types in the embryo.”

The more abundant germ cells themselves seem to drive feminization, said Boris Tezak, a postdoctoral researcher in the Capel lab who led this project. “The temperatures that produce females are also the temperatures that increase germ cell number,” he said.

Higher numbers of germ cells are known to control female development in fish as well, Capel said. But to prove the point that more germ cells lead to female turtles, they removed some germ cells from red-eared slider embryos raised at an intermediate temperature that should have yielded 50-50 proportions and saw more males than expected.

Scientists have known about temperature-dependent 𝑠e𝑥 development for decades and have found it in many different parts of the tree of life, apparently because it evolved multiple times in multiple ways.

“It popped up everywhere,” Tezak said. “It seems like a really risky strategy, especially in the context of weather variations and climate change, so why would this system persist?”