Hippos honk at friends, but tornado poo when strangers are near

Hippopotamuses might look like they’re taking life easy as they cruise sedately in the water, but new research indicates they’re actually listening closely to the grunts and bellows of their neighbours, ready to let loose a territorial poo-tornado on any unwelcome newcomers.

In a study published in Current Biology, researchers have detailed the behavioural responses of hippos to the recorded vocalisations of other hippos. These “wheeze honk” calls carry over long distances, leading researchers to suspect that they play an important role in maintaining social groups.

Working in the Maputo Special Reserve in Mozambique, the team recorded calls from several separate hippo populations, each inhabiting their own lake system. They first played the recordings back to hippos from the same local group to see how they’d react to the calls of their nearest and dearest. They then played calls from neighbouring groups within the same lake, and finally those from unknown hippos in distant lakes.

The hippos were clearly responsive to calls, but responses were very different depending on whether they were hearing hippos that they knew or ones they didn’t.

“We found that the vocalisations of a stranger individual induced a stronger behavioural response than those produced by individuals from either the same or a neighbouring group,” says second author Nicolas Mathevon of the University of Saint-Etienne, France.

After hearing the calls of their closest river-mates, hippos tended to call back and wander towards the call, remaining relaxed.

But upon hearing calls from strangers, hippos became agitated and aggressive, using their tails in whip-like fashion to spray dung in a territorial display.

“In addition to showing that hippos are able to identify conspecifics based on vocal signatures, our study highlights that hippo groups are territorial entities that behave less aggressively toward their neighbours than toward strangers.”

While this is an interesting insight into hippo communication and social dynamics, the researchers say their work could also have important implications for conservation policy.

They explain that hippos are sometimes relocated to boost local population densities in vulnerable areas, but that aggression between existing hippos and newcomers can threaten the success of these efforts.

“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, so their aggression gradually decreases,” Mathevon says. “Reciprocity, in which the animals to be moved become accustomed to the voices of their new neighbours before they arrive, could also be considered.”

The researchers say they will continue to explore the complexities of hippo communication, with future work exploring whether the voices give away other characteristics, such as size, sex or age.