Hippos Use Vocal Recognition to Manage Inter-Group Relationships: Study

A new study shows that the hippopotamus (Hippopotamus amphibius), an iconic African megaherbivore for which little is known about social communication, uses vocal recognition to manage relationships between territorial groups.

“We found that the vocalizations of a stranger individual induced a stronger behavioral response than those produced by individuals from either the same or a neighboring group,” said Dr. Nicolas Mathevon, a researcher with the Equipe de Neuro-Ethologie Sensorielle ENES/CRNL at the University of Saint-Etienne and the Institut Universitaire de France.

“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 neighbors than toward strangers.”

Hippos are tough animals to study because it can be difficult to identify and locate individuals.

Dr. Mathevon and his colleagues managed it by working in the Maputo Special Reserve, Mozambique, an area that includes several lakes inhabited by hippos.

First, they recorded calls representative of each hippo group. They then played the recordings back to all the other hippos to see how they’d react to the calls of their own group (familiar) versus another group from the same lake (neighbor) or a more distant group (stranger).

They found that hippos respond to hearing a played-back call by responding vocally, approaching, and/or spraying dung.

Interestingly, the response varied depending on whether they were hearing hippos that they knew or ones they didn’t.

The overall intensity of the hippos’ response grew when they heard a stranger.

The hippos were also more likely to spray dung, a territorial marking behavior, when they heard the sound of a hippo that didn’t belong to their group.

“When the hippos are in the water, they look pretty inactive,” Dr. Mathevon said.

“But these results show that they really are paying close attention to their surroundings.”

“When they heard the call of another hippo played from the shore, they responded right away.”

“The responses to the sound signals we broadcast were very clear, and we did not expect that.”

The team now plans to learn more about what hippos communicate through their calls.

“We’ll explore how they recognize the sound of other hippos and whether the voices give away other characteristics, such as size, sex, or age,” the authors said.

The study was published in the journal Current Biology.

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Julie Thévenet et al. 2022. Voice-mediated interactions in a megaherbivore. Current Biology 32 (2): PR70-R71; doi: 10.1016/j.cub.2021.12.017