Researchers from the AGH University of Science and Technology in Kraków used special microphones to record sounds made by hippos. According to Marcin Zastawnik, acoustic researcher from the AGH University of Science and Technology, sounds were recorded both in the water and in the air. "Hippos communicate at low frequencies, often in infrasound" - says the scientist and explains that people are incapable of hearing these sounds.

Rafał Okoński from the Warsaw Zoo says that some of the sounds made by hippos can be heard. He describes them as "sounds similar to a croak interrupted by hiccups, and this is the range that we hear normally every day and can register". But perhaps some of the hippopotamus communication escapes our attention. Okoński explains that hippopotamuses have a special anatomical structure - they can receive the vibrations of the water environment, and thus the sounds, through their... jaws.

According to Zastawnik, sometimes a hippopotamus stretches, and then the fish - which surround the animal under water and clean its skin - flee from the animal. And this may suggest that the animal makes sounds that we do not hear.

The acoustics expert adds that during the recording in the zoo, it will probably not be possible to determine whether hippopotamuses actually use infrasound in communication. Sound recordings will be analysed later - after the sound is processed. The researcher explains that in order to hear infrasound, the speed of the recording should be increased. Then the human ear can begin to register previously inaudible sounds.

"In the future, we plan to perform similar measurements also for giraffes" - he says and explains that these animals also communicate using very low, specific sounds.

Accodring to Rafał Okoński, "hippos are currently found only in Africa. There are two species - common hippopotamus and pygmy hippopotamus". Two common hippopotamuses live in the Warsaw zoo - a male and a female.

PAP - Science in Poland, Ludwika Tomala

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