A Hippo’s Response to an Unknown Caller? A Blast of Poop and a Rowdy Holler

The lumbering animals respond calmly to their grunting and groaning friends, but a stranger’s voice often prompted a loud, filthy territorial response

A hippo wading in water with its head just above the surface and mouth wide open
Hippos may appear inactive, but a recent study shows that they’re listening closely to their surroundings. Paul Souders via Getty Images

Hippopotamuses are best known for their bumbling bodies, massive mouths, and aggressive nature. Fewer are aware that hippos are also chatty species, relying on calls and honks to communicate with others near and far. But until recently, the function of their loud "wheeze honks" has been a mystery. Now, new research published in the journal Current Biology suggests hippos’ distinctive honks allow the animals to tell friend from foe.

“Hippos are quite talkative. They have a repertoire of different calls: wheeze honks, grunts, bellows, squeals,” says study author Nicolas Mathevon, who researches acoustic communication at the University of Saint-Etienne in France, in a press release. “However, the function of these calls has not been studied experimentally. Our study is the first to test experimentally the function of a hippo call.”

To see if and how the animals recognize each other’s voices, the team of researchers worked with wild hippos living in various lakes in Mozambique’s Maputo Special Reserve. Mathevon and his team began by recording calls from seven unique hippo groups. Then, using speakers positioned about 250 feet away from the animals, they played the three types of recordings back to the hippos: one of their own group, one of a neighboring group, and one from a distant group.

Their experiment revealed that hippos reacted definitely to the calls of “friends,” “acquaintances,” and “strangers.” For each recording they heard, the animals responded by some combination of vocalizing, approaching, spraying dung or a combination of everything. The intensity of their reaction was lowest when they heard recordings of individuals from their own pod, and highest when they heard the calls of a stranger, Nicholas Bakalar reports for the New York Times.

“When we played back familiar calls…the reaction was not aggressive. Basically, they just called back,” Mathevon tells Nicola Davis for the Guardian.

The hippos were most likely to spray dung, a territorial marking behavior, when they heard the sound of a hippo that didn't belong to their group. The animals also produced quicker, louder, and more frequent calls when the vocalization came from strangers. 

"In their call, there is information about the identity of the individual—so they have 'voices'—and they are able to recognize each other by their voices," Mathevon says to BBC’s Helen Briggs.

The team noted that hippos’ “wheeze honks” can travel more than half a mile, suggesting the mammals would be familiar with the calls of those living in or near the same lake.

“The most interesting thing to come out of this study is that hippos may have a fine-grained knowledge of the voices of all the individuals around them, and that this knowledge can help them navigate in their social network,” Mathevon says to the Guardian.

Hippopotamuses as classified as vulnerable, and their populations face mounting threats from habitat loss and animal-human conflict. The scientists behind the work are optimistic that their findings could aid hippo conservation efforts, which often include relocating the animals.

"Before relocating a group of hippos to a new location, one precaution might be to broadcast their voices from a loudspeaker to the groups already present so that they become accustomed to them and their aggression gradually decreases," Mathevon says in a press release.