Clip | Fox Tales - Young Foxes Fight for Social Dominance

From the time they are born, young foxes are full of energy. You can often find them wrestling with and biting their siblings. However, this isn’t just innocent play. Pups who are more aggressive edge their littermates out of the best food and sleeping spots, increasing their own chances of survival.

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This might look like cute play, but there''s more going on than meets the eye.

With each bite and grab these pups are battling for dominance.

The victor gains all the rewards.

Like first dibs on food . And the best sleeping spots -- body-building perks that sharpen a pup''s chance of surviving to adulthood.

This battle may finish above ground, but it began for these youngsters, like it does for all red fox pups... weeks earlier underground.

Sandra Alvarez-Betancourt is the first scientist to gain a unique perspective into the underground world of foxes.

She''s spent several years analyzing a thousand hours of fox home movies filmed inside a red fox den in the wild.

Sandra: It's really hard to get to see all these behavior and I was the first one, the lucky one to get to see them and try to look at their behavior and extract as much information as I could from them.

Narrator: Sandra discovered that red fox pups develop their social hierarchy much earlier than anyone suspected.

Sandra: From the third week on, once they have enough strength to walk, actually, they start fighting.

So the first social interactions they show between themselves are aggressive interactions.

Sandra: One of the most surprising things was to see the vixen not interacting with them when they were fighting.

Narrator: These tiny pups'' fight for dominance will establish a pecking order that will last their lifetime.

These are serious battles -- one in five pups... DOES NOT make it of the den alive Sandra: You think oh they wouldn't hurt a fly.

Like, they''re just cute little puppies.

But when you actually see them behaving like this you understand that they are wild animals, and that they have to develop all these social behaviours that are so important for them later in life.

Sandra: Even if she doesn't intervene when they are fighting.

You can see that the mother always cares for them.

She will always make sure that the den is clean.

She sleeps with them to keep them warm and she grooms them.

You also see the tender part, the nice part of them.

It's not just fighting.Yeah, it's a privilege to be able to see these kinds of things.