Sharon Heald / Hippopotamus (Hippopotamus amphibius) charging, Chobe National Park, Botswana, July.

Enrique Lopez-Tapia / Hippopotamus (Hippopotamus amphibius) charging against boat on Lake Chamo. Nechisar National Park. Ethiopia, November 2014

Ole Jorgen Liodden / Hippopotamus (Hippopotamus amphibius) rearing head out of water, creating spray, Lake Naivasha, Kenya.

Hiroya Minakuchi / Hippopotamus (Hippopotamus amphibius) calves, Okavango Delta, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) bull marking territory by flicking dung, Chobe National Park, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) bull charging in territorial display, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) bull charging in territorial display, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) in territorial display, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius), Khwai River, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) spraying water from nostrils, Khwai River, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) flapping ears, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) juvenile at surface, Khwai River, Botswana