First author

In 1977, Eric Charnov and James Bull proposed a now widely accepted evolutionary model to explain why the gender of some species of lizard, turtle, crocodilian and fish is determined not by genetics, but by environmental factors such as temperature. Until now, however, no experimental evidence had confirmed an adaptive benefit — and thus evolutionary significance — of temperature-dependent sex determination (TSD). Daniel Warner, now a postdoc at Iowa State University in Ames, took on a risky PhD project and showed that developing at certain incubation temperatures increases the reproductive success of each sex (see page 566). Warner spoke to Nature about how his gamble paid off.

What hurdles had to be overcome to do this experiment?

For many years it was impossible to properly compare reproductive fitness in TSD species, because there was no way to produce both genders over a range of temperatures. A further complication was that TSD was known only in long-lived species. Recent work demonstrated that chemicals can interfere with hormone synthesis and allow us to override the temperature control of sex determination. And, in 2000, the jacky dragon, an Australian lizard with a 4–5-year lifespan, was discovered to have TSD.

Why take on such a risky project?

Raising 200 jacky dragon hatchlings in semi-natural field conditions over 3 years was a lot of work. I was worried something might go wrong, like feral cats eating my population. But I knew this was my best chance to test Charnov and Bull's hypothesis. My heart was in it, and I really wanted to give it a shot.

Were you surprised by your findings?

After the first major reproductive season, I assigned fathers to offspring using genetic analyses to measure the reproductive success of parents produced at different temperatures. I was shocked by how clearly the pattern reflected the theory's predictions. I was nervous that more data would swamp that pattern, but they actually reinforced it.

How might global warming affect TSD species?

We could see a major shift in sex ratios — which could affect viability, if not cause extinctions, in some populations.

Have Charnov or Bull contacted you?

Yes, they are both aware of this work, and I think they are pleased to see their theory supported. That's really exciting for me. These are the gurus in the field, and I look up to their work.