In some turtle species, sex is determined by temperature within the nest. When it’s higher than 86 degrees Fahrenheit, Chinese three-keeled pond turtle embryos develop as females, Merrit Kennedy notes for NPR. But when it’s lower than 78.8 degrees Fahrenheit, incubating turtles hatch as males.
Thanks to climate change, global temperatures are on the rise, and certain turtle populations are becoming overwhelmingly female. If this trend continues, scientists say, female-dominated turtle colonies could eventually die out, felled by the shortage of males required for reproduction.
Luckily, new research suggests the reptiles have an evolutionary mechanism for preventing such sex imbalances. As scientists from China and Australia report in the journal Current Biology, turtle embryos may be able to influence their sex by moving around inside the egg during incubation.
Per Science magazine’s Katie Camero, the researchers arrived at this conclusion after studying freshwater three-keeled pond turtle nests in Jiaxing, China. The team coated half of the eggs with capsazepine—a chemical that prevents embryos from sensing temperature and, presumably, moving to warmer or colder areas—and left the other half uncoated. Then, the scientists incubated the eggs under a wide range of temperatures. (A single egg could experience a maximum temperature difference of up to 40.46 degrees Fahrenheit.)
Embryos in the first group developed almost exclusively as a single sex, with those in warmer settings hatching as females and those in colder environments hatching as males. Members of the second group, however, were able to find what co-author Richard Shine, a biologist at Australia’s Macquarie University, calls the “Goldilocks Zone,” which is the spot within the egg where the temperature is neither too hot nor too cold, but an ideal 84.2 degrees Fahrenheit. Among these embryos, sex selection was randomized, producing a nearly even split between males and females.
In a press release, Shine says that the Goldilocks Zone may be turtles’ defense against thermal conditions linked with shifting temperatures.
He adds, “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.”
Still, Shine tells Science’s Camero, finding the ideal temperature for random selection won’t always be possible. Embryos can only move within a set area, and if external temperatures reach extreme levels, there will be “nowhere cool enough … to obtain the Goldilocks Zone.”
According to NPR’s Kennedy, Shine and his colleagues emphasize that the embryos probably aren’t making a conscious decision to become male or female. Instead, it’s more likely that moving within the egg is an unconscious evolutionary behavior.
The scientists’ findings have proven controversial. As Gerardo Cordero of Germany’s University of Tübingen explains to New Scientist’s Chelsea Whyte, turtle embryos “just don’t have the muscular capacity to be able to move in the egg” by the point in development at which temperature starts to influence sex.
Cordero, who has previously published research suggesting turtle embryos cannot control body temperature from inside the egg, further tells Camero, “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.”
Speaking with Gizmodo’s Ryan F. Mandelbaum, Jeanine Refsnider of Ohio’s University of Toledo calls the team’s findings convincing but points out that they are only applicable to the Chinese pond turtles studied. More research must be conducted to determine whether the results can be generalized to include a wide range of turtle species.
Regardless of these critiques, Shine concludes to Kennedy, “It does seem as if the embryo has a lot more control over its destiny than we ever expected.”