Photograph by Joel Sartore
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Male river terrapins use their bold breeding colors to entice females. Once they've mated, females dig nests where eggs incubate at different temperatures—and that can influence hatchlings' sex.

Photograph by Joel Sartore

Is It Hot in Here? Turtles' Sex Can Depend on Temperature

Once river terrapins meet and mate, plenty can still go wrong. If nests full of eggs get too hot or too cold, the sexes may be out of sync.

River terrapins try hard to be fruitful and multiply. Males use their bold breeding colors to entice mates, and their toenails to keep sex partners in their grasp. Females grow larger than males, the better to carry many big eggs. In mating season they couple liberally.

Then females travel long distances, sometimes braving salt water, to sandbanks where they lay and bury several clutches of eggs a year.

Despite such valiant efforts, five of the six species in the terrapin genus Batagur are critically endangered, says Rick Hudson, president of the Turtle Survival Alliance. (Watch video: "Cichlid vs. Terrapin.")

Terrapins lose habitat to sand mining and die as bycatch in fishing nets. Eggs are snatched from nests, to eat or sell; adults are shipped to China, where they're a banquet delicacy.

Even temperature can influence survival: The sex of birds and mammals is determined by chromosomes, but the sex of many turtles is influenced by temperature during incubation. Often hatchlings from cooler settings will be male; from warmer ones, female. (See "Heat Triggers Sex Change in Lizards by 'Turning Off' Key Gene.")

When captive-breeding programs have kept eggs too cool, Hudson says, they've yielded few or no females, which typically lay hundreds of eggs in a lifetime.

Conversely, the advance of climate change could mean warmer incubation locations, a preponderance of females—and a shortage of baby daddies.

The feature Basic Instincts: A genteel disquisition on love and lust in the animal kingdom appears every month in National Geographic magazine.