Sex in dragons: a complicated affair

In most species of animals, an individual’s sex is determined either by their genes (sex chromosomes) or by the conditions they encounter as they are developing. For example, whether a crocodile embryo becomes a male or a female depends on how warm it is within the nest.

But in a large and spectacular lizard from the Australian desert – the Bearded Dragon – things get much more complicated; although sex is usually determined by sex chromosomes,  an unusually warm nest can override that effect. As a result, hot nests produce lizards that are genetically male but develop into females.

New research published today in Proceedings of the Royal Society shows that those sex-reversed 'females' have unusual personalities, as well as unusual genetics.

The University of Sydney’s Hong Li and collaborators, including at the University of Canberra, filmed 'normal' male and female dragons, and their sex-reversed brothers (now sisters) in standard trials to measure boldness, activity level and exploratory behavior. The sex-reversed animals were bolder and more active than 'normal' females and even bolder than most 'normal' males.

So, sex reversal changes a male dragon into a female in terms of reproductive biology – but he/she retains many male-like personality characteristics.

Co-author Professor Rick Shine, from the University of Sydney’s School of Life and Environmental Sciences, remarked that this could make them more successful in the wild - accelerating the rate that a population can shift from one form of sex determination (chromosomal) to a different system (temperature-driven). 

Biologists have generally expected such an evolutionary transition to happen slowly – over thousands of years – but a behavioural difference between genetically-determined and temperature-determined females could give one type of female a major advantage over the other, speeding up the replacement process dramatically.

“One of the most interesting aspects is that, under natural conditions, we can see a process producing individuals with the bodies of females but, at least to some degree, with the brains of males,” Professor Shine said.

“Sex in dragons is clearly a much more complicated matter than we have assumed.

“We don’t know the evolutionary significance of this situation – but field studies on the ecology of these remarkable lizards could answer some very general questions in reproductive biology.”