There's no better poker face than that of the fox squirrel.

These animals just do not produce facial expressions.

For a squirrel, it's all about that tail.

When a predator is around, the squirrel doesn't snarl.

Instead, it whips its tail back and forth to look big and fearsome.

Researchers call this s-shaped movement "flagging."

and it means the squirrel feels really threatened.

At the University of California, Berkeley, animal behaviorist Mikel Maria Delgado wondered what else she could learn from watching squirrels flag their tails.

Could it open a window into squirrels' emotional worlds?

For instance, do squirrels get frustrated, like we do?

So she lured some of the squirrels that live on campus down from the trees.

She taught them how to open a box to find a walnut inside.

Squirrels love walnuts.

That little two-step he's doing... it kind of looks like "Woo hoo!

I'm about to get a nut!"

It's actually squirrel for "Back off."

"This is my nut."

Once the squirrels learned how to open the box, Delgado trained them to expect a walnut each time they looked inside.

And this is key.

Because frustration is usually defined as not getting what you expected.

Then she changed things up.

For some squirrels, she replaced the nut with corn, which squirrels don't like as much.

The squirrels were not amused and the tail flagging began.

Other times, she left the box completely empty: they flagged their tails even more.

Finally, she locked the box.

Flag, flag, and flag.

The squirrels got aggressive, a hallmark of frustration.

The question is, if squirrels do it, and we do it, why do we get frustrated?

Why is it useful, from an evolutionary point of view?

Here's one possible answer: the frustrated squirrels aren't just blowing off steam.

They're gathering up the energy to brute-force a new solution -- kind of like kicking the vending machine when it eats your dollar.

See?

They're trying different ways to open up the box.

Delgado's hypothesis is that frustration might actually be beneficial... that pitching a fit might sometimes be just what squirrels - and people - need to figure things out.