The sophisticated human language is something special. However, not as special as was thought for a long time. Many animals also communicate via sounds and share the most amazing things with each other, as bioacousticians are gradually finding out. Dolphins, for example, recognize each other by individual names, elephants use infrasound to tell each other it’s time to leave over long distances, and baboons have a variety of friendly grunts, meaning “I won’t hurt you,” “I want to cuddle you,” or “I want to touch your child”.
For most animal species, however, we are still far from understanding the meaning of their vocalizations. Hippos, for example, are known to make sounds all the time, both above and below water. But what the animals communicate with it has so far been a mystery to humans. The biologist Julie Thévenet from the French University of Saint-Etienne has now investigated what a vocalization that is particularly common among hippos is all about: the “wheeze honk”, in German something like “Keuch-Huper”, which can be heard for miles.
In the animal world there are dear and evil enemies
For their study, which is currently in the specialist magazine Current Biology was published, Thévenet and her colleagues first recorded wheeze-honk calls from hippos from various lakes Maputo Special Reserve on, a nature reserve in Mozambique. The scientists then selected seven groups of hippos, to which they played the calls of different species. They observed how the animals reacted. They treated five groups in succession with the wheeze honk of an unknown species from a distant lake, the wheeze honk of a “neighbor” from a nearby lake, and the call of a hippopotamus from the same lake. Two groups only got the call of the distant stranger and that of the group member.
“The reactions of the animals were clear,” says Thévenet’s colleague Nicolas Mathevon. “We didn’t expect that.” The hippos reacted to the calls of unfamiliar conspecifics from a distant lake in a clearly aggressive manner: they roared back and began spraying droppings to mark their territory. These reactions were much more moderate to the sounds of animals from a neighboring lake, and when the call was made by a group member, the hippos often answered, but usually did not begin to mark their territory.
The scientists conclude from this result that hippos can recognize each other on the wheeze honk. It’s something like a “vocal signature,” Mathevon suspects. The study also shows that hippos that live together in a lake see themselves as a group with a common territory. That wasn’t entirely clear until now.
Like the hippopotamuses in the current study, many “territorial” species that claim and defend a particular territory respond more aggressively to an unknown conspecific than to a known animal from an adjacent territory, the researchers write in Current Biology. Behavioral scientists speak of the “Dear Enemy Effect”.
Hippos can talk above and below water at the same time
The wheeze honk studied in the current study is just one call out of a vast repertoire of many different sounds that hippos use to communicate. The animals can growl, grunt, snort and even scream. Sometimes they make crying noises, but usually only briefly. And when they interact with other hippos, clicking sounds are often heard.
Apart from the unusually large variety, the “language” of the hippos has another special feature: it works both on land and under water. Some of the lute is only used underwater, others are intended exclusively for communication on land. And about a third is “amphibious,” meaning the sounds can propagate in water and air at the same time. For this form of communication, the animals half-submerge so that their eyes and nostrils are above the surface of the water, and their mouths and throats are below.
Although human language is much more differentiated than the hippo’s vocalizations, humans would fail at the task of speaking above and below water at the same time. Among other things, because sound waves break at the boundary between water and air. The supposedly unique human ability to learn and understand foreign languages, on the other hand, also exists in the animal kingdom. The mourning drongo, an African passerine bird, can imitate the alarm calls of about 50 different animal species. When he calls like a meerkat warning his fellow snakes, or like an excited crested lapwing, the alarmed animals will flee, dropping their food. Then the language-gifted bird eats it.