Every expectant mother gets asked the same question: boy or girl? For mammals like us, it’s an easy call. Two X chromosomes you get pink booties. X and a Y you get blue. But for some reptiles, the answer depends on the weather. Well, on the temperature to be precise. For many lizards and turtles and gators, the sex of the hatchlings depends on the temperature in the nest where the eggs incubate.
 
But why would animals bother with such a seemingly slapdash system for assigning sex? The only logical answer is: there must be some benefit. Each sex must fare better at the temperature that tends to produce that sex. A fine theory, but no one has been able to test it. Until now.
 
In the January 20 online issue of Nature, scientists at Iowa State University describe how they used hormones to produce lizards with the “wrong sex”—animals that came out male even though they were incubated at the female temperature. And vice versa. They found that females born and raised at the female temperature made more babies than females hatched at the wrong, male temperature. Ditto for the males. Call it the Goldilocks principle.  You’re born at a temperature that’s just right—at least if you’re a lizard.

—Karen Hopkin