Hippos Have Something to Say

Hippopotamuses know who their friends are. They can tell the difference between friends and mere acquaintances, and mere acquaintances from total strangers, by the sounds they make.

Nicolas Mathevon, a professor of animal behavior at the University of Saint-Etienne in France, led a team of researchers in the Maputo Special Reserve, Mozambique to listen to hippos. Their study appears in Current Biology:

“Here, we focused on the most common hippo vocalization, the ‘wheeze honk’, a loud call heard over long distances that is assumed to be important for social cohesion and communication between groups, but whose actual function remains unknown.”

The researchers recorded wheeze honks and other vocalizations of seven distinct pods of hippos, then played back the recordings – to the same pod, to neighboring pods, and to distant strangers – while observing and videotaping the responses.

They found hippos could distinguish among friends, neighbors, and strangers by their voices. (It is unclear whether the animals can tell individuals apart.) The big mammals paid close attention and responded immediately to the recordings. They reacted most aggressively when hearing unknown hippos, with louder and more frequent calls, often accompanied by a typical territorial display – defecating while flicking their tails to spread the feces.

That’ll get your attention.

The eavesdropping researchers had to be cautious. Hippos kill 3,000 people a year in Africa. (Compare that to elephants or crocodiles, whose annual kill counts are measured only in the hundreds.) They made their observations from at least 250 feet away using sensitive recording equipment.

The hippo population has been in decline for some time, and understanding how they communicate can be useful for the preservation of the species. The International Union for Conservation of Nature classifies hippos as vulnerable, and the vocalization study could inform conservation policy.

“Before transferring a group of hippos to a new location,” the study concludes, “a potential precaution could be to broadcast their voices from a loudspeaker at a distance from the groups already present so that they get used to them and their level of aggressiveness gradually decreases. Getting the animals to be relocated accustomed to the voices of their new neighbors could also be considered.”

Listen to some hippo sounds here: www.youtube.com/watch?v=-0Z2jcUk380

Photo credit: Nicolas Mathevon