Turtle Moms Choose Their Babies' Genders by Where They Build Their Nests

If turtles had realtors, their motto would also be "Location, location, location!"—but not because they care about a scenic vista. The spot a mother turtle chooses to dig her nest determines whether her young will be males or females. This might even be the most important factor in her decision.

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. The eggs incubate and develop under the soil while the summer wears on. Hatchlings finally chip their way free in the late summer or early fall, but in cooler parts of North America they don't leave the nest right away; they stay hunkered down with their siblings to hibernate until the following spring.

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.

Timothy Mitchell, a Ph.D. student in ecology at Iowa State University, studies a population of painted turtles living in northwestern Illinois. These particular turtles have been under close watch by scientists for more than two decades, but it hasn't become clear whether turtle moms are active in determining their hatchlings' sex—do they choose nest sites that will best balance the sex ratio of their eggs? To find out, Mitchell set up a kind of nest-building competition between himself and the mother reptiles.

Mitchell scoped out 20 nests in his study site, a forested area near the Mississippi River. Right after the mothers left their nests behind, he went in and dug the eggs up. Then he tucked the eggs into artificial, Styrofoam-box nests. Half the eggs from each batch went into a box right next to where their mother had left them, buried at the same depth to create a controlled replica of the original nest. The other half went into a box at a site Mitchell selected at random.

(How do you randomly place a turtle nest? Mitchell used a random number generator to choose a distance away from the original nest, up to 30 meters. Then he flung a pencil in the air and walked in whatever direction in was pointing when it fell. If the resulting location was, say, in the Mississippi, he tried again.)

Just before they were due to hatch, the eggs were dug up and brought to a lab. Mitchell monitored their hatching and then returned the tiny turtles to their artificial nests for hibernation (along with a sprinkling of eggshells, as if the turtles had been there the whole time). He checked on the baby turtles once more in the spring.

Temperature sensors hidden in the nests revealed that sites chosen by turtle moms were a little warmer than Mitchell's randomly selected ones. This meant they were more open to the sun; nests that were shaded by vegetation were cooler.

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.

"This strongly suggests this process of sex ratio selection is influencing where Mom chooses to nest," Mitchell says, "as opposed to selection to have eggs survive."

Wherever she builds her nest within this forest, a painted turtle mother can be assured that her young will survive equally well. But it's in her best interest to keep the sex ratio balanced. 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.

A warming climate is a threat to any species whose sex ratios depend on the temperature. But if female turtles are savvy enough to leave their eggs in exactly the right sex-balancing spot, should we stop worrying about them? "That is still the big question in the field!" Mitchell says. He thinks moms' choice of nest sites will be a crucial part of how this species responds to climate change. But many other factors will matter too, like the fragmentation of the turtles' habitat and how the climate affects their predators. Turtle moms today know how to build a perfect nest for their offspring , but that balance may be as fragile as eggshells.


Image: Timothy Mitchell.

Timothy S. Mitchell, Jessica A. Maciel, & Fredric J. Janzen (2013). Does sex-ratio selection influence nest-site choice in a reptile with temperature-dependent sex determination? Proceedings of the Royal Society B DOI: 10.1098/rspb.2013.2460