Humpback whale offspring "whispers"

Young animals use very soft sounds to avoid robbers

A humpback whale female with her cub in the Exmouth Gulf in Western Australia. © Fredrik Christiansen
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Whispers instead of singing: Humpback whales are known for their widely resounding songs. But when the female whales have young, they are very quiet: mother and child communicate then only "whispering", sometimes even only by touch, as underwater shots reveal for the first time. The reason for the "whispering": louder calls could attract orcas and other predators - and for those the walkalb would be a welcome feed.

Humpback whales are long-distance hikers: even if the whalers have offspring, they migrate thousands of miles from their winter quarters in the tropics to the summer quarters in the polar regions. For the whale calves, this means: After their birth in tropical sea bays, they must grow as fast as possible and get fit for the long journey.

Mother-child couples overheard

But how the whale calves manage to do this, how much and when they are nursed by their mothers and how the two communicate with each other has been largely unknown. "We know next to nothing about the early stages of life of the wild whales, " says Simone Videsen of the Aarhus University. "But just this time is crucial for the survival of the whale calves on their long migration to the summer feeding grounds."

To find out more, Videsen and her colleagues have now temporarily equipped eight newly born humpback calves with adhesive sensors that record their movements and vocalizations. For the first time, they were able to overhear the communication of wild humpback whale mothers and their young.

Whispers instead of shouts

It turned out: Walmütter and their boys communicate with each other in a special way. When both dive together, they do not make loud calls, as happens more often among adult humpback whales. Instead, the wallowing whale "whispers" with a weak grunt and squealing just so loud that the mother hears it and both do not lose themselves in the water. display

That's how the quiet squeal of the Walkalb sounds:

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"The calf volume was more than 40 decibels lower than that of the humpback whales singing in the same area, " Videsen and her colleagues report. "At the same time, the calf lutes were also 20 to 70 decibels quieter than the social calls of the adult humpback whales." The whisper of the whale calves is therefore masked after a few dozen meters from the ambient noise.

The soft whispering is mainly used to protect the Walkalb from predatory Orcas. Fredrik Christiansen

Protection from listening orcas

And the recordings revealed something else: To show the mother that it wants to sleep, the Walkalb does not use any acoustic communication at all. Instead, it slides under them and signals by touching that now a plentiful flow of milk quite sch nw re: "We heard a lot of scraping sounds, like two balloons, which are rubbed together "Videsen reports. "That was probably the calf that nudged his mother when it wanted to suck."

The reason for this almost "stealthy" communication: "Acoustic signals can be overheard by robots or adult humpback whales and inadvertently reveal the location, " say videsen and their colleagues. Especially orcas often hunt humpback whales and are therefore a great danger to the young. Although the humpback whales are not a direct threat to the whale hunters, they are a source of annoyance to mother and calf.

Threatened by the sea noise

However, the very quiet communication between Walm ttern and their carvers also means that they can be particularly sensitive to ambient noise. The knowledge is therefore important for the protection of these whales. "Because mother and child communicate in a whisper, for example, ship noise could easily overbear these quiet calls, " says Videsen.

In the worst case this would mean that the whale calf loses the connection to his mother during diving. If the humpback whales adapt to it, and become louder themselves, orcas might be able to track it down. "In both cases, an increased noise would have negative consequences for the chance of survival of the walkers, " the researchers said. (Functional Ecology, 2017; doi: 10.1111 / 1365-2435.12871)

(British Ecological Society (BES), 26.04.2017 - NPO)