TOM MCHUGH / SCIENCE PHOTO LIBRARY TOM MCHUGH / SCIENCE PHOTO LIBRARY
Hippopotamus yawning. Close-up of the mouth of a yawning hippopotamus (or hippo, Hippopotamus amphibius). This is the hippo's threat display, used for intimidating rivals. Such a yawn displays the hippo's main weapons, its enormous lower canine teeth, which may reach 60 centimetres in length. If the threatened hippo does not back down, a savage fight may result. Hippopotamuses are large water-loving herbivores which live in and around rivers throughout southern and eastern Africa. They spend the majority of the day resting in water, grazing on water plants, but often leave the water at night.
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