Hippos can communicate with one another and tell the difference between the honking sounds of their friends, neighbours and strangers, a study has found.
A team of researchers from the University of Saint-Etienne in France recorded random vocalisations made by groups of hippos at the Maputo Special Reserve in Mozambique.
This area features a host of lakes and different hippo populations, and the researchers played back some of the recordings to the groups using speakers.
They focused on what scientists dubbed “wheeze honks”, a loud sound that travels over vast distances. It has long been assumed to have a social role, but the exact function has been unknown.
In the study, the researchers assumed the “wheeze honk” served as an identity card, telling other nearby individuals exactly who was in the vicinity.
Seven pods, ranging in size from three individuals to 22, were played “wheeze honks” for around half an hour. They heard recordings of noises from a member of their own pod; one from a neighbouring group; and that of a complete stranger.
Researchers set up loudspeakers around 250ft from the water to blast out the hippo grunts.
They then studied the reactions to the sound, and analysed how it differed depending on whether the hippo on the recording was friend or foe.
Hippos responded to the calls in various ways, by shouting back, or marking their territory by spraying their own faeces.
The intensity of their response was greater if they were listening to a stranger’s call, opposed to that of a pod member, the academics discovered.
“We show that the hippopotamus, an iconic African megaherbivore for which little is known about social communication, uses vocal recognition to manage relationships between territorial groups,” the researchers write in the journal Current Biology.
Professor Nicolas Mathevon, co-author of the study, added: “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.
Rare insight into hippo communication
“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.”
Professor Mathevon explained that when hippos are in the water, although they look inactive they are still paying close attention to their surroundings.
The team’s findings not only offer a rare insight into how hippos communicate in social groups, but may also help conservationists protect them.
Hippos, elephants and rhinos, the megaherbivores which roam the African plains, have a crucial role to play in the ecosystems, but it is becoming increasingly difficult for them to survive in the modern world, and their numbers are dwindling.
By understanding their social networks, scientists are hopeful they can better manage populations and relocation events to better protect the species and boost their numbers.
“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,” explained Prof Mathevon.