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Hippos are some of the most hostile creatures in the animal kingdom.
“Apart from mosquitoes, the most dangerous animals in Africa are those 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 don’t mind attacking boats. They are mostly in the water, but they can move very quickly out of the water. It can be quite dangerous to enter their territory.”
But you can’t say that hippos don’t give enough warning to strangers.
Large mammals make loud noises, and Dr. Mathevon and his colleagues have deciphered some of what they mean. Results, Published Monday in Current BiologyThey suggest that hippos can distinguish their friends from acquaintances and strangers they know based on their voices.
It is difficult to study hippos. It is nearly impossible to identify and mark individual animals, and it is difficult to find them even when marked. They spend the night feeding on land and then retreat to the water during the day, congregating in groups with a dominant male guarding a number of females and young animals. Individuals can move from one compartment to another – the details of their social organization are poorly understood.
But Dr. Mathevon and his colleagues insisted. They studied the animals in the Maputo Special Reserve in Mozambique, where one or more groups of hippopotamuses, or pods, lived in several lakes. There, the researchers recorded the hippos’ calls in seven pods.
Using advanced video equipment and a shotgun microphone designed to pick up distant sounds, the researchers carefully recorded from at least 250 feet away. Then they played the recordings to the others while they videotaped their responses.
They found that hippos make a lot of noise. Their “snarls” can be heard half a mile away, and their verbal repertoire includes grunts, bellows, and squeaks. The growling horn is generally considered to be hippos’ way of announcing their presence, but its social function is unclear.
In 10 separate experiments, the scientists played recorded calls to animals using three tests: first by playing a hippo’s voice to their own group, second using a call from a neighboring group in the same lake, and finally by another stranger’s voice. goal. They videotaped 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 their droppings. The animals responded somewhat to recorded calls from any group, but the intensity of the response was lowest when individuals heard their recordings from their own pod and highest when they heard a distant stranger’s horn. The response to a call from a neighboring group was slightly different from someone from the same group, and only hearing an animal’s call from a group of strangers provoked territorial marking.
Dr. Mathevon said it can be difficult to maintain objectivity in animal behavior studies. “Imagining things that don’t exist is always a danger,” he said. “Therefore you must take some precautions” both in the techniques of data collection and the methods used to interpret it.
Before each recording, the researchers expected the animals to be calm and quiet. Five different observers evaluated the numbers of hippos and compared their numbers. The researchers, who were not involved in the making of the videos, scored the animals’ responses on three-point scales using three criteria: distance of approaching the sound, degree of marking by spraying manure, and number of vocalizing hippos.
The work could have implications for species conservation. Hippo populations are declining, and the International Union for Conservation of Nature classifies them as vulnerable.
“The finding that hippos show stronger behavioral responses to an unknown hippo call can be applied to conservation efforts,” said Diana Reiss, an animal behavior researcher and professor of psychology at Hunter College in New York, who was not involved in the study. “It can be important and beneficial to familiarize hippos that may need to be relocated to other areas for protection to the calls of alien hippos they will encounter in new places.”
The study highlights that there is much to learn about hippo behavior and group dynamics.
Dr. “Hippos have a complex social system with a variety of interactions,” Mathevon said. “This often goes with the complexity of the sound system. We show that hippos can distinguish between familiar and unfamiliar sounds. But our work is only the first step.”
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