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This is Scientific American’s 60-Second Science. I’m Karen Hopkin. This will just take a minute.

Every expecting mother gets asked the question: boy or girl? For mammals like us, it’s an easy call. Two X chromosomes¹ you get pink booties. X and a Y you get blue. But for some reptiles², the answer depends on the weather. Well, on the temperature to be precise. For many lizards⁴ 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? And 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 hormones⁵ to produce lizards with the wrong sex:animals that came out male even though they were incubated at the female temperature, and vice⁶ 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 are born at a temperature that’s just right, at least if you are a lizard³.

Thanks for the minute for a Scientific Americans' 60-Second Science. I’m Karen Hopkin.