There are two sexes, right? Male and female. And maybe humans conceive of gender as a spectrum, but other animals are just male or female: full stop. Right? Right?!

Wrong. Here there be dragons.

These dragons don't ransack our villages and build mounds of treasure, but they do complicate our binary picture of sex and gender. 

Bearded dragons have an unusual system of sex determination that depends on incubation temperature as well as chromosomes.  In other words, both genetics AND environment determine their sex and a variety of sex-linked traits, such as tail length.

If a genetically "male" egg is incubated in a hot nest, the dragon turns into a sexual female-- a lizard who mates with males and lays eggs! However, these "sex-reversed females" are not typical females; they have many stereotypically "male" traits. They are highly social, and highly fertile, and have long tails. Sex-reversed females may lay eggs, but they don't conform to "female" appearance and behavior.

The careful reader (who also knows a thing or two about lizards!) has probably noticed something strange. First, let's talk lizard chromosomes. As we all know, human males are XY and human females are XX (generally speaking). But lizard males have chromosomes ZZ, while lizard females are ZW (and likewise for many reptiles, including birds). Thus, sex-reversed females who began life as genetic males have male chromosomes ZZ.

A ZZ sex-reversed female mates with a ZZ male and lays eggs... all of which can only possibly have ZZ chromosomes! All of her eggs are genetically male. Therefore, the sex of her babies is entirely determined by temperature.  There is no way sex-reversed females can give birth to genetic females-- only nest temperature can produce females.

What is the moral of this story?

Nature has come up with many ways of determining sex. There are sparrows with four functional sexes, snails who change sex if "that's what their partner is into," alligators and crocodiles whose sex is determined solely by temperature, fish and frogs who change sex at will (have you seen Jurassic Park?), and more. The wonders of nature are myriad.

Overall, on average, most vertebrate species have two consistent sexes with many traits that roughly cluster into two bins. But by no means is a clear binary the rule. The lines blur between sexes, and between sex-associated traits. This is patently, obviously true for bearded dragons... and for humans.

So when you are deciding what you think about sex, and gender, remember Tolkien's words: "it doesn't pay to leave a dragon out of your calculations."