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Behavior is written in the genes.

All of us have innate behaviors;

all animals are born with behaviors.

Behaviors which through evolution
have become more complex.

In many aspects,

behavior in males and females
of the same species is different.

These differences in behavior

are due to differences in the wiring
of the nervous system.

At the same time, these differences
in wiring are established genetically.

Aggression is a behavior
that in general is specific to males.

We see it in species
that are very different,

evolutionarily apart from each other.

The way each one
of these species fights is different

but the mechanisms are similar

and the reasons are almost
always the same.

Males fight for females,
for food, or for territory.

I study aggression in flies.

You are about to see an attack.

One animal is going to stand up

and is going to hurl its body-weight
at his opponent.

The other one, meanwhile,
is going to try to escape.

Why do I study aggression in flies?

Because the behavior,
that I'm interested in understanding,

is the same, and the system
is much simpler.

It's like studying a model.

Sometimes it happens
that the other animal,

instead of escaping,
stays and starts to fight,

and then something
we call "boxing" occurs.

Something crucial about attacking
is that only the males do it.

The one who attacks first
is much more likely to win.

And males only attack other males.

Females never attack.

So, why study behavior in flies?

Because their behaviors
are the same as ours:

Flies sleep, eat, court, fight,
learn, and remember.

The nervous system is much simpler;

and the genes that play a role
in this behavior are very similar.

So, studying aggression in flies
allows us to understand

how this behavior develops
in other species.

A key element when deciding
whether to court or attack

is determining the sex
of the other animal.

All males are programmed to decide
between courtship or aggression,

but the sensory signals they use

to see if what is in front of them
is a male or a female are different;

for example, in our species
visual signals are very important.

This is what a fly has in front of itself.

So, how does it decide?

Evidently, it somehow makes a decision

because what it is going to do
is very different in each case.

When there is a male
entering its territory,

it is going to start attacking

and to chase him everywhere
until the other one decides to escape.

When there is a female,
he extends his wing for her.

As you can see, he literally courts her.

He chases her everywhere,

he sings to her until
he has finally won her over.

So the question is, how does it know?

How does it decide?

What are the key signals that it uses
to decide if it has to court or attack?

If we can identify these signals,
can we change them?

Can we invert this decision?

In insects, like in many species,

chemical signals called pheromones
are very important.

These signals are produced
by one animal and detected by another.

So, then something we decided to do

was to switch the pheromones,
change the scent.

To make it happen,

we manipulated the expression
of one gene, called "transformer".

"Transformer" is turned on in females
and turned off in males.

So, something that we can do
is to mute the "transformer" in females

and force its acquisition in males.

In doing so, we masculinize females
and feminize males.

But not all over, we aren't going
to manipulate the entire animal.

We are going to choose certain parts,

in particular the cells
which produce pheromones.

What we can achieve are females
that have masculine pheromones

or males that have feminine pheromones;

then we are going to see
what is going to happen

when a normal male, unmanipulated,

meets a female
that has masculine pheromones.

This is what happens, it attacks her.

When she enters his territory,
he does something

that he would never do
in front of a normal female,

which is start attacking her.

When we carried out
the reciprocal experiment,

what we found was that males
with feminine pheromones were courted.

What does this mean?

That males are
genetically programmed to court

when they find feminine pheromones

and to attack when they find
masculine pheromones.

But, this behavior,

this innate behavior
that is wired in the brain --

court if there are feminine pheromones,
attack if there are masculine pheromones --

can it be changed?

Could it be that an animal
as simple as a fly

could learn from its experience
and modify a behavior which is innate,

something which is wired
in its nervous system?

To respond to this question,
we did a similar experiment.

Again, we manipulated the "transformer",

but this time, in the females' brains.

These females still have
feminine pheromones

but they act like males.

We muted their "transformer" in the brain
to turn them into aggressive females.

They not only dislike courtship completely
but they also attack.

We then asked ourselves,

what is a male going to do
when it meets a female

that has female pheromones
but is aggressive?

The first thing it's going to do
is court her as much as possible.

It chases and chases her,
extends its wing, sings to her,

but the moment comes
when he decides to change his behavior.

The moment comes
after so much ineffective courting

and in addition to being attacked by her;

he says, "enough is enough"
and ends up attacking her.

This means that even an animal
as simple as a fly

can learn from its experience
and modify its behavior.

And not only that.

When there are fights between two males,
there is always a winner and a loser.

After a male loses many times

what happens is that
he suppresses this behavior.

He stops fighting.

This means that even in an animal
as simple as a fly

not only behavior is in the genes,

but the ability to learn from experience
and modify it is there, too.

Thank you very much.

(Applause)