Turtle embryos may determine their own sex—by seeking the perfect temperature

For humans, slight variations in temperature don’t mean much. But for some turtles, they mean the difference between whether embryos come out male or female. Now, scientists have evidence that these embryos have some power over their sexual destiny: By moving to slightly warmer or cooler spots inside their eggs, freshwater turtle embryos can help determine their own sex. Not everyone is convinced. But if the new finding holds, this behavior could potentially save some turtle species from extinction by balancing their sex ratios.

A reptile’s sex depends on hormones produced during development. For crocodiles, many fish, some lizards, and most turtles, those hormones in turn depend on external temperatures. Cooler temperatures typically lead to more males, and warmer temperatures generally lead to more females. That means a shift of just 2°C can make all of the offspring one sex. As average global temperatures rise, such a strategy could doom some species, including the already-endangered Chinese pond turtle (Mauremys reevesii). As weather warms, warmer eggs produce more and more females—leaving them fewer males to mate with.

Wondering whether turtle embryos could respond to rapidly changing temperatures, scientists from the Chinese Academy of Sciences in Beijing examined the behavior of the Chinese soft-shelled turtle (Pelodiscus sinensis). They discovered that the embryos could in fact move between cooler and warmer spots inside their paperclip-size eggs, they reported in 2011 in the Proceedings of the National Academy of Sciences .

To find out whether that behavior could change a species’s sex ratio, the same team studied freshwater turtle nests at a commercial turtle farm in Jiaxing, China. After measuring each egg’s temperature gradient, they briefly put the eggs in front of a light to mark the embryos’ positions. The researchers coated half the eggs with a chemical that blocks the embryos’ ability to sense outside temperatures—and would presumably discourage temperature-sensitive embryos from moving.

One week later, the researchers found that a single egg could experience a temperature gradient of up to 4.7°C. The embryos in the coated eggs moved an average of about 2 millimeters—and almost all of them hatched out male. But in the uncoated group of eggs, the embryos had moved, on average, an extra 2 millimeters, and the sex ratio was a near-perfect 50/50, researchers report today in Current Biology . That suggests the mobile embryos were able to find the “Goldilocks zone,” says co-author Richard Shine, a biologist at Macquarie University in Sydney, Australia. That’s a place where the temperature is neither too hot nor too cold (29°C), leading to random selection and a nearly equal sex ratio.

What’s more, when the experiment was repeated in the lab—which varied in temperature but kept constant other environmental factors like humidity and oxygen levels—the results held. “These animals, although tiny little creatures, are able to detect small temperature differences and move to a place that gives them the best survival outcome,” Shine says. “It’s just an extra intriguing ability in a place we would least expect it: a tiny embryo.”

However, this behavior fails to protect the turtles against extreme episodes of climate change, Shine says. “The basic notion is that you can only move in the range that’s available to you within an egg,” he says. If temperatures get too hot, as they are expected to by the end of the century, “then there’s nowhere cool enough [in the egg] to obtain the Goldilocks zone.”

But evolutionary biologist Gerardo Antonio Cordero, a postdoc at the University of Tübingen in Germany, says such conclusions are “bold,” given that the researchers don’t know how long the embryos can move within their eggs. “I don’t want people to get the impression that embryos are moving freely during the [entire] time of development,” says Cordero, whose own 2017 work suggests turtle embryos cannot control their body temperature from inside their eggs. Embryos sit in a yolk sac surrounded by impenetrable membranes, he adds, which makes it difficult to move in such a small space as they grow bigger over time. “The data is very intriguing, and it would be remarkable if that’s the case, but I don’t think it’s so clear cut as the authors make it sound,” he says.

Evolutionary ecologist Fredric Janzen, who has been studying temperature sex-dependent organisms for 35 years, says the findings are “pretty impressive.” But he adds that it’s troubling that the authors imply the embryos have some knowledge about what sex they should become and when to avoid certain temperatures. “We’re talking about embryos at a stage of development where they don’t have fully developed brains,” says Janzen, of Iowa State University in Ames. “How can they coordinate their movements, much less know which movements they ought to be doing?”

He also says turtles have already adapted to climate change by choosing cooler nesting sites or months to lay their eggs, so embryo thermal regulation likely does not—and will not—play a role. “If it’s true, it’s astonishing, I just have no clue how that’s remotely possible.”