Credit: PAP/ Adam Warżawa 09.06.2009
Underwater, no one can hear you scream. But the sounds grey seals make can be heard. Dr. Łukasz Nowak at the University of Twente in the Netherlands has now carried out research into what acoustic phenomena allow seals to make sounds both above and under the water.
He said: “The seal's body works like a loudspeaker. Grey seals are a bit like old married couples. They very rarely talk to each other. Specifically, when the mating season comes. In the case of seals, this period is at the beginning of the year, right after young seals are born and the females are ready to get pregnant again. Then the communication between males, females and young is very intensive.”
In his research, the scientist focused not on what these sounds mean, but how they are made. He made all his recordings available in open data files.
The way seals make sounds is interesting for acousticians because these animals live both in water and on land. And unlike humans, they can not only make noises that travel through the air, but also sounds that can be heard underwater, despite the extremely large differences between these environments. Another interesting thing is that grey seals are comparable in size to humans, and the frequency of their sounds is well perceived by the human ear.
Dr. Nowak studied the sounds made by grey seals in the seal sanctuary at the University of Gdańsk Marine Station in Hel. He distinguished three groups of sounds differing in acoustics and presented hypotheses on how these sounds could be generated.
He said: “Seals had to adapt to acoustic communication, to communicate both above and under the water.
“We humans usually vibrate the column of air we exhale from our lungs when we want to say something. We use the nasopharynx as a filter that we can tune. Our vocal systems are designed to emit sound mainly through the mouth, where the air is released. The vibrations of the chest, for example, are irrelevant in the emission of sound.”
In the case of water, this method of creating sounds will not be effective because the sound does not generally pass into the water from the air. Structural sounds generated in vibrating solids (e.g. tapping a door with a hand) are transferred better in water than aerodynamic sounds caused by vibrations of the air (such as the human voice).
Therefore, in order to transmit sound signals to each other under water, seals must change the air vibrations into the vibrations of their body. Tissues have mechanical properties quite similar to those of water. And from them, vibrations - and thus sounds - are transferred to the water quite well.
Sometimes some of the seals' sounds are accompanied by bubbles (which means that the sound is produced when exhaling). And some are not. Based on the structure of the latter sounds, the researcher guesses that the animals must pump the air one way and the other. The resulting sound causes the seal's body to vibrate, and the body transmits these vibrations to the water.
It is different, however, when the seal is on the surface. Then, a large part of the sound is emitted through the nostrils.
Dr. Now said: “Grey seals live between water and land. They communicate in the range of acoustic frequencies that we can hear with the naked ear. The properties of their vocal systems - unlike, for example, dolphins, which use ultrasound - are similar to human ones.”
His research appeared in the journal Bioacustics.
In the database provided by the scientist, you can watch videos with recordings of seal conversations, as well as listen to audio recordings of both underwater sounds and sounds made on the surface.
PAP - Science in Poland, Ludwika Tomala
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