Given their diet, why do hippos have to be able to open their mouths so wide, asks Cedric Dallas from Stellenbosch? I imagine that it must have something to do with territorial / mating rituals - but then why females as well ?
Dave Rushworth answers: Your own suggestion concerning territorial behaviour is correct in part. 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.
Being communal animals, the adapted, forward facing 'tushlike' bottom incisors are sometimes used for 'poking' away others in the group to make space. The enlarged canines, which are very sharply honed top on bottom, are solely used as defensive weapons - the only weapons (apart from their bulk) that they have.
To bring these huge 'tushes' to effect they need to open their mouths wide. Although the females, with relatively smaller 'tushes', rarely become involved in territorial disputes they do have to be able to defend themselves (and will do so when necessary) and their young from large crocodiles and herd competition.
Dugongs, which are occasionally seen off our north eastern coast, are distantly related mammals which have much smaller mouths but also graze on aquatic vegetation. All the whales have very large mouths, for varying functions, and there may be some evolutionary association involved.