The Common Hippopotamus (Hippopotamus amphibius) is the third largest land mammal after the African Elephant and the Square-lipped (or White) Rhinoceros. Hippos are semi-aquatic, spending most of the day in water and emerging to graze on grasses at dusk. Territorial bulls control groups of up to 30 females (cows) and their young. Hippos have large canine tusks, and are amongst the most dangerous mammals in the world – they can be aggressive and highly unpredictable. Hippos together in the water in pods are mostly peaceful, unless a foreign male attempts to join the group. The dominant male may warn the intruder with a yawning threat display and bellows, and then might attack. Severe injuries sometimes occur. This small pod of Hippos was in the muddy waters of the Ruighoek Dam in Pilanesberg National Park, South Africa. A young calf was playing with, and probably annoying its mother by repeatedly pressing its open mouth against hers. The dominant bull and another cow was nearby, both partly submerged. The calf persisted, alternating between ‘mouthing’ its mother and resting its chin on her exposed back. Suddenly the other cow nearby charged the pair, and a short but violent fight ensued. The squabble ended quickly, and the mother Hippo retreated with her calf to the dam shallows.