If hippopotamuses cannot swim, how did they colonize islands?

1. What they did

Hippopotamuses are widely assumed to have reached islands by swimming, yet several studies suggest they cannot swim at all. The author reviews what is known about how hippopotamuses move in water, and asks how they could then have reached islands such as Cyprus, Crete and Madagascar.

2. What they found

• No published account clearly shows a hippopotamus swimming or floating. The one apparent report, from Central Island in Lake Turkana, is doubtful.
• Hippopotamuses have dense, barrel-shaped, poorly streamlined bodies and dense limb bones packed with cancellous bone, like a diver’s weighted boots. They walk on the bottom of rivers and lakes, and when they lose contact they make short bounding gallops.
• Adults must resurface to breathe every three to six minutes, and juveniles every two to three, so they avoid water too deep to touch bottom.
• They need water daily, their skin cracks in the sun, and they breed slowly (about one calf every two years), so many animals would be needed to settle an island.
• The extinct Hippopotamus antiquus was even larger, with similarly dense limb bones, and was probably also a bottom walker.
• Islands on continental shelves (the British Isles, Sicily, Malta, Zanzibar and Mafia) were joined to the mainland when sea levels fell in the Pleistocene. Cyprus, Crete and Madagascar lie 30 to 430 km from the mainland across deep water.
• Rafting or being swept by currents is unlikely to deliver enough hippopotamuses: even rafting jackrabbits, which breed fast, never colonized California’s Channel Islands.

3. What they concluded

If hippopotamuses cannot swim, they most likely reached Cyprus, Crete and Madagascar by land bridges or very shallow water that let them surface to breathe. Their fossils on islands may therefore point to past land connections not yet detected by geologists, and solving these questions will need biologists, palaeontologists, geologists and biogeographers to work together.