Whimsical: whale shined with rocket scanner

Whole-body computed tomography reveals something new about the minke whale's behavior

The hearing of minke whales is particularly sensitive in different frequency ranges. © danielbenhain / iStock
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Glass whale: Researchers have examined the entire interior of a minke whale with an industrial computer tomograph. With the detailed, anatomical data they could simulate a hearing test. Accordingly, the hearing of minke whales is polarized to their predator number one and can also perceive directions. With their results, which have now been presented at a conference, the scientists also want to better understand the impact of noise on marine mammals.

Whales are known for their large repertoire of complex sounds: with their long groans and high-pitched squeals, they can communicate underwater for several kilometers. The reason: Water transmits the sound waves much better than air. Especially humpback whales and bowhead whales are true vocal artists. However, scientists have been puzzling for a long time about whether and how whales can perceive the direction of sounds.

In the rocket scanner

To find out more about whale hearing, Ted Cranford of San Diego State University and his colleagues have been examining the inside of a minke whale. The nearly 3.50-meter-long whale had already stranded in 2012 on the coast of Maryland in the northeastern United States and euthanized on the recommendation of a veterinarian. Subsequently, the whale was transported away and frozen as a whole.

Representation of minke whale within the industrial CT scanner. © Ted Cranford, San Diego State University

"We asked to borrow the whale and scan it in an industrial computer tomograph designed for solid fuel rocket engines, " says Cranford. The researchers had to wait for this opportunity for a full two years. The scans of the whale's interior then served as input for a simulation that replaced a hearing test.

"Our techniques make it possible to simulate the biomechanical processes of sound perception and to assess the curvature of the Wales on the basis of the anatomical details, " explains Cranford. "Scanning the entire minke whale allows us to predict how well the whale can hear over a range of frequencies." Display

Minke whales listen to their predators particularly well

Previous research had pointed out that minke whales communicate primarily with sounds in the range between 50 and 300 hertz and with short click sounds between 1, 000 and 2, 000 hertz. Minke whales are likely to be particularly sensitive in these areas to hear their conspecifics. "It was therefore very surprising that our model predicted a sensitivity of between 10 and 40 hertz, " says Cranford.

But why do the sea gulls listen so well in this area? The researchers suspect that the minke whales are particularly sensitive to these frequencies in order to better hear their most important predator the killer whale. In the wake curve of fin whales, however, the researchers did not find this sensitivity to high-frequency tones. The fin whale is about twice as large as the minke whale and is usually not on the menu of the orca.

In addition, the simulations suggest that minke whales hear sounds best when they emerge right in front of them. The whales may therefore perceive the direction of sounds and thus locate their source - for example, other whales or oncoming ships.

Man is too loud

Many whales communicate with each other with low-frequency tones. However, they are not the only giants in the ocean that emit such sounds: ship propellers and other human activities also produce noise in the same frequency range. Researchers fear that this noise pollution has a detrimental effect on marine organisms and the ecosystem.

"According to some estimates, man-made noise in the oceans has doubled every decade for the last 50 years, " says Cranford. "It is therefore crucial that we understand how marine vertebrates receive and process low-frequency sounds." Only then can one evaluate the impact of the noise and how effective the countermeasures are are, so the scientist. (Experimental Biology, 2018)

(Experimental Biology 2018, 24.04.2018 - YBR)