The semiaquatic mammal leverages its own buoyancy and bone density to charge through the water.
The San Diego Zoo’s fact sheet about hippos wasn’t exactly clarifying: “hippo limb muscles are for powerful propulsion through water, but not swimming.”
What is swimming, if not using one’s limb muscles for powerful propulsion through the water? “Well, a boat doesn’t swim,” my colleague Priyanka, pointed out. But hippos don’t have engines, or propellers, or sails. So how, exactly, do they charge through the water so impressively? After contacting (by email) half a dozen zoologists and wildlife parks, I finally had my answer.
“Depending on water level they walk or they swim,” said Dagmar Andres-Bruemmer of the Frankfurt Zoological Society. Except the swimming isn’t really swimming per se, she said. Rather, it’s a kind of gallop."
“For all intents and purposes the hippo does not swim,” said Douglas McCauley, an assistant professor in the department of ecology, evolution, and marine biology at the University of California, Santa Barbara. “It almost always maintains some contact with the bottom and walks or bounces off the bottom using these bottom contact points as a source of propulsion.”
Which means a hippo barreling through the water is often supported on two feet instead of four. It also helps that they can remain submerged for up to 30 minutes. “There are periods in which the hippopotami are ‘in flight’ with no feet in contact with the ground,” the paper says. “In deep water, they locomote by ‘a series of porpoise-like leaps off the bottom’ or in ‘a series of high, prancing steps.’”
Hippos can do all this terrifying prancing because they’ve evolved with just the right combination of buoyancy and bone density to allow it.
“It is quite lovely to see them do this underwater gallop,” McCauley told me. “They remind me of portly astronauts doing a moon walk underwater.”