Hippos Recognize the Wheeze-Honk of Strangers and Friends

Hippos recognize each other’s voices and respond less aggressively to animals they know than to total strangers, a new study finds.

The most common hippopotamus call is kind of a wheeze-honk combination. The giant herbivores are typically very chatty and they recognize each other by these noises, which can be heard over long distances.

But they’ll react quite differently when they hear those signature calls from a strange animal, researchers have found.

“Hippos are very talkative. They have a diversified vocal repertoire, with several types of calls. The respective role of these calls are not yet well understood,” corresponding author Nicolas Mathevon of University of Saint-Etienne, France, tells Treehugger.

“Since they form social groups where individuals interact, they need a strong communication system. The acoustic channel certainly plays a big role.”

Mathevon is a bioacoustician, which means he studies how animals communicate through sounds.

“One topic that fascinates me is how sound signals can mediate social relationships. Hippos are fascinating in this respect: they form social groups, with females, males, and young individuals. On the same lake, several groups (or pods) can cohabit,” Mathevon says.

“No one had yet studied the importance of acoustic communication during interactions within and between groups in hippopotamuses. When we decided to study them, a question immediately arose: can they recognize each other by voice?”

Listening to Friends and Strangers

It’s difficult to study hippos because it can be challenging to locate them in the wild, then identify and mark individual animals. So, for their study, researchers worked in the Maputo Special Reserve, a nature reserve in Mozambique that has several lakes where hippos live.

The researchers first recorded calls from each hippo group. Then, they played the recordings for all the hippo groups to see how they’d respond to familiar calls of their own group, neighboring calls of groups from the same lake, and stranger calls from a more distant group.

The animals had different responses to the various calls, answering with calls or by approaching the sounds and/or spraying dung. The responses were different, depending on whether the calls came from hippos they knew or from those they didn’t recognize.

“When we played back calls from unknown individuals, the hippos responded more strongly, i.e. vocalized more, came closer to the loudspeaker (not all individuals, most of the time it was a big one that came), and often exhibited marking behaviour (which in hippos consists of spraying dung all over the place with their short tail),” Mathevon says.

“We really did not know what to expect when we did the first experiments. We were not very surprised since other territorial animals, such as many songbirds, react differently to unfamiliar and familiar vocalizations (e.g. territorial neighbors versus stranger individuals).”

The results were published in the journal Current Biology.

Key for Conservation

Hippos gather in the water in large groups during the day. They look relatively inactive, but Mathevon says that the study results show that they are paying close attention to their surroundings. If they heard a recording from a strange group, they responded immediately.

These findings can be important for research and for conservation, he suggests.

“We think that these findings may inspire conservationists if they need to relocate individuals. It may be possible to get the local hippos used to the voice of the new ones before they arrive (and vice-versa),” Mathevon says.

“Of course, I'm not saying that this measure will be sufficient to suppress all aggression since other sensory signals (chemical, visual) are certainly also involved, but it may help.”