Melbourne, January 23: Australian researchers have unearthed proof that an almost three-decade old theory about reptile sex and survival is correct.

Professor Rick Shine of the University of Sydney, and his former student Dr Daniel Warner, now of Iowa State University, have shown that the temperature at which a reptile egg is incubated not only determines sex, but also optimises the number of offspring in future generations.

This is the first time that a team of researchers has made "unequivocal" demonstration that incubation temperatures affect the reproductive success of males and females.

About 30 years ago, US biologists Professor Ric Charnov, now of the University of New Mexico, and Professor Jim Bull of the University of Texas had said that this "environmental sex determination" was not just a quirk of nature.

Indeed, they believed that males or females incubated at certain temperatures had an evolutionary advantage, especially, that they would have an optimised number of offspring.

However, proving their theory had been the "Holy Grail" for evolutionary biologists up to this point.

According to Professor Shine, a major challenge in proving Charnov and Bull’s ideas was to find a species with life span short enough to make it practical to measure the entire number of off-spring it had over its life.

The short-lived Jacky dragon (Amphibolurus muricatus), a common species of lizard found on the east coast of Australia, helped the research team overcome this challenge, he added.

The Jacky dragon produces off-spring within one year of hatching and lives no longer than four years.

Another challenge before the researchers was to artificially produce the "wrong" sex at a given temperature. They hormonally manipulating eggs to produce males and females at temperatures they would not normally be produced at.

Professor Shine has revealed that, though the hormonal manipulation of the eggs had no effect on the health and survival of individual hatchling Jacky dragons, the natural reptiles were better in terms of mating and producing offspring.

The natural males were five to 10 times better in terms of mating and producing offspring, while the natural females produced four to five times more offspring, says Shine.

"Thus reproductive success of each sex was optimised by the incubation temperature that produces that sex in nature as predicted by the (Charnov-Bull) model," ABC Online quoted him as telling Nature magazine.

Bull is said to have written a letter to Professor Shine, congratulating him for this achievement. (ANI)