This dominant 4.5metre male Estuarine Crocodile (Crocodylus porosus) resides a kilometre or so downstream from my own abode on Cooper Creek; a proximity that we never forget!
It has long been known that crocodile gender is determined by temperature. If the temperature of egg incubation is cool, around 30 degrees C, the hatchlings are all female. Warmer temperatures, around 34 degrees C, hatch all males. There is also strong population bias towards females; often as high as 10 to 1.
For about thirty years, this skewed ratio was thought to provide an evolutionary advantage, whereby sex ratio optimises survivorship considerations.
In a recent News in Science article by Dani Cooper, entitled ‘Sex-change lizards settle a hot topic’, Professor Rick Shine of Sydney University and his former student Dr Daniel Warner, now of Iowa State University, report that they have proven this 30-year-old theory.
Studying the relatively short-lived Jacky dragon (Amphibolurus muricatus), which produces off-spring within one year of hatching and lives no longer than four years, the researchers found that hormonal manipulation of gender determination had no effect on the health and survival of the hatchlings, but 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.
It was therefore shown that the incubation temperature that produces that sex in nature optimised reproductive success of each sex.