University of Sydney researchers have finally solved a conundrum scientists have been puzzling over for thirty years: why nest temperature can affect the likelihood of certain reptiles being born male or female.
The simple answer is that in some reptile species, the nest temperatures that produce the fittest and most fertile males are very different to the nest temperatures that produce the fittest and most fertile females, says Professor Rick Shine.
"We've found that some temperatures produce males that are better at growing up, finding a girlfriend and having babies, while other temperatures are better at producing females that will do the same," said Professor Shine.
The research, conducted by Professor Shine and colleagues and published on 20 January 2008 in Nature, provides the first unequivocal demonstration of this evolutionary process.
In mammals and birds sex is determined by genotype at fertilization. Many reptiles, however, hedge their bets, determining the sex of an individual by interaction with the environment, typically temperature. Professor Shine said: "In some reptiles, like turtles and crocodiles, the sex isn't determined until some time after the egg is laid. And in some species, only males or females will be born at certain nest temperatures.

"Thirty years ago two scientists, Ric Charnov and Jim Bull, speculated that this wasn't just a random quirk of selection, but that there was an evolutionary advantage given to males or females incubated at certain temperatures."
Until now it has been difficult to test their speculation with controlled experiments. "The long life spans of turtles, for example, meant we would have needed 100 years to properly test the theory," said Professor Shine.
But the Jacky dragon (Amphibolurus muricatus), a short-lived species of lizard found in Australia, gave Shine and his colleague Dan Warner the chance to design an experiment that could provide an answer. They added hormone treatments to nests at different temperatures to override the natural process of sex determination. They then compared the reptiles born in these nests to those from nests that hadn't been interfered with.
"Both the males and females born in the hormone-affected nests - in other words, at temperatures their sex are not supposed to be born at - were much less fit, and much less likely to have babies, than those born 'naturally'."