A female painted turtle (Chrysemys picta) is not an over-involved parent. She digs a hole in the dirt, lays a batch of eggs there, and buries them. Then she returns to her freshwater life without giving the nest another thought.
Sometime in the middle of the summer, a significant event happens inside each buried egg: the developing turtle becomes male or female. Its sex hasn’t been determined by its genes like ours is.
Instead, as with many other reptiles, the temperature in the nest tilts the egg toward one sex or the other. Cooler nests produce males and warmer ones make females. If the nest stays within a narrow temperature range, hatchlings of both sexes will crawl out at the end of the season.
But what happens when you randomize nesting sites in an area and compare that with what resident mother turtles are actually choosing to nest?
As it turns out, it’s not so random at all, meaning the mothers are choosing to bias their nests for some kind of advantage depending on the circumstances.
Between the original nest sites and the random ones, there was no difference in the number of eggs that survived all the way through hatching and hibernation. But there was a major difference in sex ratio: while the turtle moms’ nest sites produced roughly equal numbers of boy and girl turtles, the hatchlings from Mitchell’s randomly placed nests were about 80 percent male.
In the long term, turtles that tend to build male-heavy or female-heavy nests will lose out when the population swings in that direction, because young turtles of the opposite sex will then have better mating prospects.