Hippos recognise each other’s voices, study finds

Hippos can recognise each other’s voices and respond less aggressively to the calls of a neighbour compared to those of a stranger, a study has revealed.

The vocal mammal’s “wheeze honk” calls can be heard over long distances and so led researchers to suspect the calls play an important role in the way the animal maintains social groups.

The study, published today in the journal Current Biology, was conducted by a team of researchers in the Maputo Special Reserve in Mozambique. It can be hard to study hippos as they are difficult to identify and to locate individuals, but several lakes in the reserve are inhabited by the animal.

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

“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.”

The team initially recorded calls representative of each hippo group and then played the recordings back to all the other hippos to gauge their reactions to the calls of their own group (familiar) versus another group from the same lake (neighbour) or a more distant group (stranger).

Researchers found that hippos respond to hearing a played-back call by responding vocally, approaching, and/or spraying dung. The response varied depending on whether they were hearing hippos that they knew or ones they didn’t.

The intensity of the hippos’ response grew when they heard a stranger, with an increased likelihood of them spraying dung, a territorial marking behaviour, when they heard the sound of a hippo that didn’t belong to their group.

While in water, hippos can look inactive, but the researchers’ findings show that they are in fact paying close attention to their surroundings. When they heard the call of another hippo playing from the shore, they responded right away.

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

The results will provide the team of experts with useful insights into hippo communication and social groups, as well as the implications of conservation policy. Researchers say that animals often relocate to keep local populations at healthy sizes, but their findings suggest that precautions are needed before making that kind of move.

“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,” Mr Mathevon said.

“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 team of researchers hope to learn more about what hippos communicate through their calls. They’ll also explore how they recognise the sound of other hippos and whether the voices give away other characteristics, such as size, sex, or age.