To Hippos, a Wheeze and a Honk Mean More Than Just ‘Hello!’ [Video]

Hippos are among the most hostile creatures in the animal kingdom.

“Apart from mosquitoes, they are the most dangerous animals in Africa, the ones that kill the most people,” said Nicolas Mathevon, professor of animal behavior at the University of Saint-Etienne in France. “People underestimate them. They swim very fast and do not hesitate to attack boats. They are in the water most of the time, but they can get out of the water very quickly. Entering their territory can be quite dangerous.

But you can’t say that hippos don’t give adequate warnings to strangers.

Large mammals make loud noises, and Dr. Mathevon and his colleagues have figured out some of what they mean. Their findings, published Monday in Current Biology, suggest that hippos can tell friends from acquaintances, and acquaintances from strangers, by sound.

Hippos are difficult to study. It is almost impossible to identify and mark individual animals, and difficult to find them even when marked. They spend the night feeding on land, then retreat to the water during the day, gathering in groups with a dominant male guarding a number of females and young animals. Individuals can move from one pod to another – the details of their social organization are poorly understood.

But Dr. Mathevon and his colleagues persisted. They studied animals in the Maputo Special Reserve in Mozambique, where several lakes are inhabited by one or more groups, or groups, of hippos. There, the researchers recorded the cries of the hippos in seven groups.

The researchers made their recordings cautiously, at least 250 feet away, using sophisticated video equipment and a shotgun microphone designed to pick up sounds from a distance. Then they played the recordings back to others while filming their responses.

They discovered that hippos make a lot of noise. Their “hissing horns” can be heard over half a mile away, and their verbal repertoire includes growls, bellows, and squeals. The whistling horn is generally thought to be the way hippos announce their presence, but its social function is unclear.

In 10 separate experiments, scientists played recorded calls to animals using three tests: first playing a hippo’s voice to its own group, then using a call from a neighboring group in the same lake and finally with the sound of a stranger from another. Lake. They made videos of all the answers.

Hippos respond to calls by calling back, approaching the caller, or marking their territory by defecating while wagging their tails to spread the feces. Animals responded in some way to recorded calls from any group, but the intensity of the response was weakest when they heard recordings of individuals in their own group and the strongest. high when they heard the horn of a distant stranger. The reaction to a call from a neighboring group was little different from that of a member of the same group, and only listening to the call of an animal from a group of strangers elicited territorial marking.

In animal behavior studies, it can be difficult to maintain objectivity, Dr. Mathevon said. “There is always a danger in imagining things that don’t exist,” he said. “We must therefore take some precautions”, both in the techniques of data collection and in the methods used to interpret them.

Before each recording, the researchers waited until the animals were calm and quiet. Five different observers assessed the number of hippos and compared their counts. Researchers not involved in the production of the videos scored the animals’ responses on three-point scales using three criteria: approach distance to sound, degree of manure spray marking, and number of vocalizing hippos.

The study may have implications for the preservation of the species. Hippopotamus populations are declining and the International Union for Conservation of Nature classifies them as vulnerable.

“The finding that hippos show stronger behavioral responses to calls from an unfamiliar hippo can be applied to conservation efforts,” said Diana Reiss, an animal behavior researcher and professor of psychology at Hunter College in New York, who n did not participate in the study. “It can be important and beneficial to habituate hippos that may need to be moved to other areas for conservation purposes, to the calls of unfamiliar hippos they will encounter in new locations.”

The study highlights that there is a lot to learn about hippo behavior and group dynamics.

“Hippos have a complicated social system, with various interactions,” Dr Mathevon said. “That usually goes hand in hand with the complexity of the voice system. We show that hippos can distinguish between familiar and unfamiliar voices. But our study is only a first step.