Not only look at the chimps face... but look at what is going on around... look at the hand gesters too.
This chimp was begging for food.

It takes the entire picture to see what the expression means... what it tells us... how the chimp feels.

This guy looks ANGRY... actually... he was yawning when I took this photo.

Now... these two chimps were very angry with other chimps.

This lil' guy was upset with the other one who had been picking on him.

What was this look all about?

Mwanzii was poking the girls with a palm tree stick.

When you see an expression... you need to look at the entire picture to know what the chimpanzee is feeling. A "grin" is not always an angry expression... it can be a happy feeling too.