Largest freshwater mammal

The largest freshwater mammal is the common hippopotamus Hippopotamus amphibius, native to rivers, lakes and swamps in much of sub-Saharan Africa, emerging only at night to graze on grass. It weighs up to 3,630 kg (1,300–1,500 kg on average), with adult males continuing to grow in size and weight throughout their lives. Conversely, adult females apparently stop growing once they reach an age of around 25 years old.

Despite looking superficially porcine, as a result of which they were traditionally considered to be most closely related taxonomically to pigs, hippopotamuses have more recently been revealed by genetic comparisons to be most closely allied in fact to cetaceans (whales, dolphins, porpoises), and are believed to have diverged from them around 55 million years ago.

Ironically, despite being semi-aquatic with webbed feet, the hippo is not a particularly good swimmer, and does not float either, so it rarely inhabits deep waters.