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There are about 30 different species of anemone fish.
We study a species called amphiprion ocellaris,
which really is like the Nemo variety - it looks just like them.
They're very social animals. And it can live for 30 years.
They're genetically monogamous.
So there's usually a reproductive pair living on the anemone.
Females are at the top of hierarchy and are the boss.
They're the larger. They're laying eggs. They're the ones that defend the territory.
Whereas the males are much smaller and, usually, are doing most of the caring of the eggs.
So there are very clear behavioral differences.
They have different roles, in terms of the reproductive functions there.
And part of their natural life history is that they change sex.
So what happens is the female might die, it might get eaten by predator.
In that case, the male will change sex and become female.
And one of the non-reproductive subordinates, the biggest -
they have their own dominance hierarchy among them -
but the biggest of that line will then differentiate to become the male.
So the question we were trying to figure out is,
what changes first?
Does the brain first change from male to female, and then the brain tells the gonads to change?
Or do the gonads change first and then the gonads release steroid hormones, for example,
which then changes the brain?
So the way the experiment worked is that we took two males
and we put them together in an aquarium.
And, when you do that, they'll tend to display,
or sometimes they'll fight a little bit and establish who's boss and who's not.
And the one that was dominant would then slowly start the process of becoming female.
Immediately it will behave differently.
The most obvious behavioral differences is a female is out in the open, defending the nest,
and starts growing significantly -- where it can double or triple its size in several weeks.
So that happens very quickly.
Then the dominant fish will... their brain begins to change;
particularly a part of the brain called the preoptic area,
which is a part of the brain that basically controls your gonads.
What you can see is that in the male brain has a greater number of these very small neurons;
whereas, in the female fish, you see a much greater number of these medium-sized cells.
You can see in this fish that it is transitioning, its brain looks much more female-like, than male-like.
And we see that transition occurs gradually over the course of sex change.
What specifically happens is that there's about a doubling of the number of
neurons that develop in this preoptic area for females.
Females have more neurons in a particular area of the brain,
presumably because they have more sophisticated control of the gonads, in
terms of the development of the eggs, than in the male.
It takes about six months or so before the brain can switch from a male brain to female brain.
They can then cause the gonads to become female. Or they can wait.
And, actually, they can wait for years, we discovered.
So they can persist as female brain and female behavior with male gonads for quite a long time.
There's lots of features that you can identify to say, for sure, that these fish are female, not male.
The female is always the biggest one.
It's always the one that's mainly defending the territory.
It's the one that does the least parental care.
It's very easy to identify. It's very easy to see.
And so, yeah, really, really clear.
It is the female.
The gonads are less important.
It's just the very end piece of being able to lay the eggs or produce the sperm.
It's like that's not the key thing that defines sex here.
It's all in the brain.
It, after all, starred in the brain, right?
Because it's a dominance thing.
So they recognize that they're the dominant one.
And that initiates this feminization of the brain, at a level that is controlling the gonads.
So it's a very basic feminization of the brain; fundamental for the biology these fish.
But that happens first... which was the real surprise.
It's a very clear demonstration that brain sex and gonadal sex can be uncoupled.
And you can be a female and have male gonads essentially.
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