When it comes to lizard sex, size does matter. But not in the way you might think. Because in the June 4th online issue of the journal Current Biology, scientists say that for at least one type of lizard, larger eggs are more likely to make girls while smaller eggs yield boys.

For many animals, the sex of an individual is dictated solely by its chromosomes. But for small alpine lizards, gender isn’t so cut-and-dried. The scientists had earlier found that nest temperature can influence the sex of lizard hatchlings. Cooler nests turn out more boys—no matter what their chromosomes say. At the same time, the researchers noticed that larger eggs seemed to make more daughters. But they wanted to put their observation to the test. So they took big eggs and removed some yolk—and, voila, they made males. Adding yolk to a little egg was a recipe for a female.

This size manipulation might allow moms to balance their babies’ sexes when a chilly nest would otherwise produce mostly sons. Because when it comes to alpine lizards, sex determination all comes down to a practical yolk.

—Karen Hopkin