A while back I pointed out that a so-called hippo yawn wasn't a yawn - it was a threat display. I said about the photograph in question:
The hippo is showing its fighting tusks, ivory 'teeth' that can tear great chunks out of a rival hippo, or dismember a crocodile . . . or rip you apart if you provoke it. That's what its actions are saying to the photographer. "You worry me. If you come any closer, I'm going to do something about you - and you won't enjoy it!"
There's more at the link.
Courtesy of Daily Timewaster, here's a video of hippos in a zoo being fed watermelons. It offers great shots of their fighting tusks (and everything else in their mouth). The ease with which they crush whole watermelons gives you some idea of the force with which they can bite, too.
Not cuddly critters at all, despite Disney's protestations to the contrary . . .