Edinburgh Zoo announces birth of pygmy hippo calf

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They recognise each other’s voices, allowing them to be selective over how they interact with different hippos. The new study, published in the Current Biology journal, has shown that this ability to recognise voices dictates how the creatures interact with each other. It showed that they are often nice to familiar animals and are more aggressive towards strangers.

Hippopotamuses are vocal animals and have distinctive “wheeze honk” calls, which can ripple over long distances.

Researchers embarked on the study after theorising that their calls may play an important role in maintaining social groups.

When they tested this, they found that the animals are able to recognise each other’s vocal signatures.

Speaking about the findings, Professor Nicolas Mathevon, from the University of Saint-Etienne, France, said: “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.

“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.”

Prof Mathevon is fascinated by bioacoustics, the study of how animals exchange information through sound.

His team of researchers are especially interested in the network of communication used by hippopotamuses where multiple hippos can send and receive information at once, like a form of WhatsApp for animals.

The team travelled to the Maputo Special Reserve in Mozambique – an area that is known to have several hippo-filled lakes – to conduct the study.

They recorded calls made by groups of hippos and played them back to other groups, in order to observe how they would respond.

Prof Mathevon and the other researchers compared the responses between hippos familiar to one another, hippos from neighbouring lakes, and groups that were strangers to one another.

The intensity of the response grew when hippos heard a stranger and the team observed that the animals often sprayed dung – a territorial marking behaviour.

Prof Mathevon explained: “When they heard the call of another hippo played from the shore, they responded right away.

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

He added that while hippos look inactive when they are in the water, they are actually paying close attention to their surroundings.

The findings may influence conservation policy, as it could impact where groups of animals are relocated to protect healthily-sized populations.

Prof Mathevon said: “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.

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

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