Unknown component discovered in whale song

In addition to the sound pressure, humpback whales also perceive the vibration of the water particles

Humpback whales are known for their widely resounding songs. Now biologists have discovered a previously unknown component of this communication. © NOAA
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More than just sound: In the songs of the humpback whales, there is a previously hidden component. Marine biologists have discovered that marine mammals not only send out sound waves but also create vibrations in the water particles. These particle movements can still be measured hundreds of meters away - and could therefore represent a hitherto overlooked channel of walk communication.

Whales and other marine mammals communicate with complex sounds - and even learn new "dialects". Humpback whales, in particular, are known for their widely audible chants. The long tones and stanzas can be heard over several kilometers, because the pressure waves of the sound are effectively transmitted by the water.

Whales are eavesdropping on Hawaii

But as it turns out, the whale song has an additional, previously unknown component. Aran Mooney of the Woods Hole Oceanographic Institution and his colleagues discovered this by accident when they wanted to overhear humpback whales off the coast of the Hawaiian island of Maui. They had not only hydrophones in the water, but also sensitive accelerometers. These register, if and how quickly the water particles in their environment change their movement.

Mooney illustrates the difference between sound waves and particle motion using the example of one standing next to a car standing, loud music sounds. "What we hear are the pressure waves of sound, " explains the researcher. "What makes our seat vibrate is the movement of the particles." The music not only changes the air pressure, it also causes the air particles to get carried away and swing for a short time.

Surprisingly far-reaching

According to common wisdom, sound pressure from whale singing also produces particle movements of this kind, but these spread out under water for a maximum of a few meters - so it was thought. But as the researchers pushed past some submerged humpback whales, they registered something surprising: ad

"We did not expect to measure particle motion farther than a few meters from the whale, " Mooney says. "But as we pushed farther and farther away, the particle movement registered by the device was loud and clear." Even at 200 meters away, the song of the humpback whale produced distinctly measurable tempo changes in the water particles.

Humpback whale off the coast of Maui, where marine biologists listened to these whales. T. Aran Mooney, Woods Hole Oceanographic Institution / NMFS

Additional communication channel

According to the researchers, this suggests that the whale song is even more comprehensive and complex than previously thought. "It's a whole other aspect of sound that we never thought the whales could use, " says Mooney. "Our results demonstrate that humpback whale particle motion is a significant audible signal for nearby swimming animals."

The subtle vibration of the water could help whales find their singing companions better. Because the movement of the water particles makes it even better than the sound pressure to determine the exact distance and position of the source, as the marine biologists explain: "With sufficient amplitude of the signal, the whales could localize their species and their movements consequences."

Perception of the skull

The whales would not be the only animals that perceive and use this "side effect" of sound: hippos are already known to register such water vibrations over their bodies they h This will make you underwater, even if your ears are just above water.

The researchers suspect that the humpback whales can perceive the movements of the water molecules, especially at low frequencies, directly via their skull bones. "Earlier studies have suggested that guiding sound and vibration over the bone helps them to listen, " says Mooney.

Man-made noise maybe twice as damaging

But the discovery of this hitherto unknown component of whale song also arouses concern. Because the noise that we humans make by marine engines, sonar or seismic exploration, not only produces sound waves. The machines that are used for drilling or other underwater work also cause vibrations and thus low-frequency particle movements.

"Most man-made underwater noise is low-frequency and it's doubling every decade, " explains Mooney. If the whale movement of the particles is indeed another important channel of their communication, the anthropogenic vibrations could disturb them far more than previously thought. "That could be a big problem for the whales, " says the researcher. (Royal Society Biology Letters, 2016; doi: 10.1098 / rsbl.2016.0381)

(Woods Hole Oceanographic Institution, 03.11.2016 - NPO)