Women vs. Women

Source

I recently read about an interesting phenomenon.

You walk into a room full of women. You feel eyes watching you, looking you up and down. Even if nothing is said, you can sense that you're being evaluated - your size, your outfit, your hair, etc. On the flip side, you're sitting in a room full of women and another woman walks in. Now you find yourself evaluating her appearance. Why is that? 

The article where I read this suggested that women tend to do this because we feel the need to determine whether the other woman entering the scene is more attractive/put together/beautiful than us. But I want to probe a bit further. Why do we feel the need to do this? It's almost as if we feel threatened by another woman's beautiful appearance.

What's your take? Do you agree?