If you can’t handle the heat… become a female! In the human world, starting out as one sex and becoming another is as yet far from being accepted. In the animal kingdom, however, such a transition is not unusual, and an Australian lizard is the latest to join the ranks of gender bending critters.
The central bearded dragon, with its familiar frilly head and sun-baked colouring, is a popular pet that naturally inhabits the arid areas of Central Australia. But it is likely few people know that their pet lizard might be genotypically male and phenotypically female. In terms of sex determination of the lizard, genotype refers to the sex chromosomes present and phenotype refers to the outward characteristics displayed. In these lizards, as in most other animals, the two normally align. For an individual to display a phenotype different to their genotype is very unusual. Sex determination in lizards is usually genotypically driven alone, or is temperature dependent and is set at fertilisation; the central bearded dragon is displaying a third type of system that is sort of a mixture of the two.
These lizards do have sex chromosomes, labelled Z and W, where: ZZ is associated with maleness, and ZW is associated with femaleness. However, when a ZZ egg is exposed to temperatures of 32°C and above, there is the possibility that the embryo will develop into a female instead of a male, phenotypically. This is thought to be due to a male determining gene on the Z chromosome that can be deactivated by heat.
ZZ female lizards are much more fecund than their ZW counterparts, and have on average around twice as many offspring. This could pose a problem to future lizard populations as ZZ males and ZZ females who mate can only produce ZZ offspring. With rising temperatures, more and more of these ZZ embryos may develop into females instead of males. The outcome could be lizard populations overrun with females and few male bachelors, potentially leading to the extinction of the species.
Central bearded dragons aren’t the only animals to transition between genders; barramundi do this as well, although in a completely different manner. Barramundi are sequential hermaphrodites, meaning that majority of the fish begin life as males and will remain so until they reach sexual maturity. This generally occurs around 3-4 years of age, and then for 1-2 spawning seasons after that they will continue to display the male phenotype. At the age of five, the fish transition from male to female, essentially creating a generation of barramundi cougars who mate with the younger, still-male fish. This system of gender transition is quite common in other species of aquatic fish, especially those found in tropical areas.
We are still coming to accept different ideas of what sex and gender are and how they work. But these two organisms display natural sex determination systems and sex transitioning systems that are incredibly interesting and complex, and perhaps challenge the ideas of what is normal when it comes to sex and gender.