On average, how wide can hippopotami open their mouths?

Being such large, thick skinned animals they have to have a large gape and long 'tushes' to have any effect on a challenger. With their heads above water, the bulls frequently open their mouths wide as a show of dominance.

Whether intentionally or not this shows off their weaponry. Lions, baboons and certain other mammals do the same thing. The gape is certainly not used in the feeding process as they seldom, if ever, eat vegetation other than short grass and occasionally other plants. They lack cutting incisors and 'crop' the short vegetation with their broad, stiff lips.