For the Chinese three-keeled pond turtle (Chinemys reevesii), self-determination starts early. A study published today in Biology Letters suggests that, as embryos, the reptiles can move around in their eggs to regulate their body temperatures — and, in turn, their own sex.

“Our results suggest that animals may actively select their own destiny even at the very early stage of embryos,” says Wei-Guo Du, an ecologist at the Chinese Academy of Sciences in Beijing and the leader of the team who conducted the study.

The finding counters the long-held assumption that in reptiles the temperature of embryos is determined solely by the environment in which a mother lays her eggs. For C. reevesii, that matters because embryos that develop in cooler conditions are mainly male, whereas those that develop in warmer conditions are mainly female.

Location, location, location

Du and colleagues found that C. reevesii embryos adjust their temperatures by moving to warmer or cooler spots within their eggs, much as turtles move in and out of the sunlight to regulate their body heat once they hatch.

Previous research that examined a different species of turtle also suggested that embryos can move from cooler to warmer areas within their eggs. But Du’s team went a step further, testing whether, like adults, embryos could also move away from dangerously hot areas. The researchers placed 125 eggs in an incubator set at 26 °C and randomly assigned each egg to one of five groups. Those given no additional treatment and those heated to 29 °C on one side, along their full length, stayed near the midpoints of their eggs. Embryos in eggs heated to 29 °C or 30 °C at their pointed ends moved toward those warmer areas. But eggs heated to 33 °C on their pointed ends — exposed to a temperature known to be too toasty for developing turtles — moved away from the hot spot.

To confirm that the tiny turtles were controlling their movements within their eggs, the team incubated 41 C. reevesii eggs so that the surface temperature on one end was kept at 28 °C, the optimal temperature for embryo development. After 10 days, the scientists randomly killed half of the growing turtle embryos; a week later, they measured changes, if any, in the locations of all the embryos, dead or alive. The researchers found that the live embryos moved toward the warm end of the egg, whereas the dead embryos pretty much stayed put — thus implying that the little turtles do actually move themselves within their eggs.

This so-called thermoregulatory behavior within the egg is likely to be widespread in reptiles and perhaps birds, Du says — at least in those with large enough eggs to allow for appreciable variations in temperature. That may come in handy for those species as the climate warms, he adds, because it may help embryos to avoid dangerously hot temperatures.

George Parsons, senior director of the Fishes Department at the John G. Shedd Aquarium in Chicago, also expects that the study’s results would hold among many reptiles, though he’s not optimistic that embryos can save themselves from global warming. “The behavior may buffer climate change effects,” he says, “but only to a certain degree.”

And Parsons would like to see more work on how the new findings relate to sex determination. “Would specific embryos have a predisposition for moving towards or away from heat?”

While praising the study in general, Fredric Janzen, an ecologist at Iowa State University in Ames, questions whether its findings drive the sex ratios of turtles in the wild. Across a variety of turtles that exhibit temperature-dependent sex determination, he notes, the sex ratios of hatchlings are strongly determined by other factors, such as vegetation cover over nests, overall climatic temperatures and yolk steroid hormone levels. “That doesn’t seem to leave much ‘room’ for this embryonic thermoregulation mechanism, if it is general among turtles, to have a substantive effect on offspring sex determination,” Janzen says.

This article is reproduced with permission from the magazine Nature. The article was first published on June 12, 2013.