How Unborn Turtles Choose To Become Male or Female

While in their eggs, turtle embryos move around to find the perfect spot that's not too hot, not too cold, but just right. And because temperature determines what sex unborn turtles become, the embryos might be able to "choose" their own sex by staying in cooler or warmer areas of the egg, according to a new study.

As reptiles, turtles are ectotherms, or "cold-blooded," and rely on outside sources to maintain their body temperatures. So as juveniles and adults, they have to move between cool and hot spots to stay equilibrated. But what about unborn turtles — do the embryos also seek out thermally optimal environments?

In 2011, researchers found that embryos of the Chinese softshell turtle (Pelodiscus sinensis) appear to move towards sun-warmed areas of their eggs. But it was unclear if the unborn turtles were actively seeking out warm areas to achieve equilibrium, or if their movements were driven by the fluid dynamics of the eggs.

So the scientists decided to revisit the case to find out what was really going on. Wei Guo Du, an ecologist at the Chinese Academy of Sciences in Beijing, and his team began by collecting 125 eggs of the Chinese three-keeled pond turtle (Chinemys reevesii) and dividing them into different groups. They incubated each group differently by warming different portions of the eggs at different temperatures (between 78.8 and 91.4 degrees Fahrenheit, or 26 and 33 degrees Celsius).

LiveScience explains further:

The researchers used bright light to silhouette the developing embryos inside their eggs, measuring where in the egg the turtle was positioned by pinpointing the spot where its neck met its shell. After 10 days of development, half of the eggs were killed with an injection but were left next to the heat sources so the researchers could determine whether any movement observed was active or passive.

The measurements revealed that the live embryos moved toward the heat source when it warmed their eggshells to between 84 and 86 degrees F (29 and 30 degrees C). When the shell was hotter, however, the turtles squirmed away from the heat. Dead embryos didn't move, indicating the embryos were deliberately making these adjustments.

The study suggests that the unborn turtles intentionally moved towards warm areas and away from dangerously hot areas of their eggs, and that their movements were not the result of simple physics, the researchers write in their study, published recently in the journal Biology Letters.

Scientists have long assumed that the nest environment solely controls the temperature of reptile embryos, and thus their sex. But the new work shows that assumption isn't necessarily true and that the developing reptile embryos may exert some control over determining their own sex.

"Our results suggest that animals may actively select their own destiny even at the very early stage of embryos,” Guo Du tells Nature.

Read more over at Nature and LiveScience. Check out the full study in Biology Letters.

Top image via B. Zhao et al., Biology Letters.