Sex determination is a choose-your-own-adventure for some species of turtles. Unhatched turtles move around within their eggs, and their movements can determine whether, when they do break free from their shells, it will be as male or female. The fact may improve the prospects of some species in danger of extinction from global warming skewing their sex ratios.
Mammals, with somewhat rare exceptions, determine sex through chromosomes. Although some reptiles do the same, for others sex is based on the temperature at which the incubation takes place, while a few species manage to mix things up in even more complex manners. However, even where sex is based on temperature, it seems there are still some wrinkles we are only just working out.
"We previously demonstrated that reptile embryos could move around within their egg for thermoregulation,” said Professor Wei-Guo Du of the Chinese Academy of Sciences in a statement. “So we were curious about whether this could affect their sex determination."
Du incubated the eggs of Mauremys reevesii, a species of freshwater turtle, in both laboratory and outdoor settings. He found the temperature difference from one end of an egg to the other could be a remarkable 4.7ºC (8.5ºF) at the peak of the day, more than double the width of the band in which the probability of being male or female is approximately even.
While theoretically, this means turtles could influence their sex by moving to the warmer or cooler end of the egg, Du needed to intervene to see if this really happens. In Current Biology he describes applying the chemical capsazepine, which interferes with reptiles’ capacity to sense temperature to half the eggs, reducing movements in response to temperatures.
In a cool month, batches of capsazepine-treated eggs all hatched as males, while in a hot one almost all were female. However, among the non-capsazepine eggs, the ratio was close to even both times. Capsazepine treatment made no significant difference to overall survival, however.
The findings help explain how species that use temperature-dependent sex determination have survived large changes in climate in the past. Turtles are also thought to have shifted between shaded and sunny clutching sites and slowly evolved different temperature ranges in response to changing conditions.
Du calculates this effect means the sexual skew induced by global warming among some species may not be as bad as previously feared. In the light of warnings that just 0.14 percent of loggerhead turtles in the Cape Verde Islands would hatch male by 2100, that’s very good news. However, the paper cautions there are limitations on how effective shell movements can be, depending on the temperature gradient within the shell, and how hot it gets before the embryo can move.