Whales have been fine-tuning their sense of hearing for millions of years.
Very different and much superior to that of humans, it is their main instrument for feeding in the aquatic environment, so unfriendly to sight and smell, and for detecting threats in time.
But for just 70 years, the seas have been filled with a new noise for them, sonar.
Now a study shows that they respond to its waves in the same way that they do when hearing their great predator, the killer whales: by stopping eating.
For a decade, a group of researchers has followed specimens of four species of whales in the North Atlantic Ocean and the South Arctic, studying their behavioral changes.
43 of them had been placed with locators capable of recording sound, acceleration or pressure.
With all this they wanted to know, in addition to where they were, what they heard, when they submerged, to what depth or for how long.
Aboard a scientific ship, they followed them for a dozen campaigns, exposing them to recordings of killer whales and noting when they received sonar emissions from ships they passed.
The results of their work, recently published by the scientific journal
PNAS
, show that the whales stop diving in search of food when they hear the killer whales.
On average across all specimens and species, the reduction in immersion time was 81.7% compared to normal sound conditions.
Analyzing the times when they also recorded sonar pulses from a nearby ship, they found that the drop was almost identical, 79.5%.
“Sonar is a loud sound that represents an unknown threat, to which animals respond based on their adaptive tendency to avoid predators”
Patrick Miller, cetologist at the University of Saint Andrews (UK)
The marine biologist at the University of Saint Andrews (United Kingdom) and lead author of the study Patrick Miller highlights this coincidence: “We were surprised that the whales in our study responded to sonar as strongly as to the sounds of their predators, the killer whales” .
Do they confuse both acoustic signals?
"No, we don't think the whales think the sonar is an orca, because they have very good hearing and the sounds are very different," Miller replies.
This researcher, dedicated to studying the acoustics and behavior of cetaceans, thinks rather that "sonar is a loud sound that represents an unknown threat, to which animals respond based on their adaptive tendency to avoid predators" .
A proof in line with what Miller says is that the degree of response of the four species of whales, although different, remained similar to the two stimuli, killer whales or sonar.
Thus, the sperm whales reduced the time of their dives by half when it came to sound waves of human origin and by 48% when sensing the killer whales.
Humpback whales reduced their activity by up to 80% (a little more when perceiving sonar) while pilot whales limited their search for food to the same level for both animal and human sound sources, around 70%. .
Finally, the northern beaked whales completely stopped submerging while hearing both the animal and artificial sounds.
These differences in the response threshold seem to have an adaptive origin and, again, would have killer whales as an agent or selective pressure.
For sperm whales, thanks to their large size, and for pilot whales, which defend themselves in groups like dolphins, killer whales are a relatively minor threat.
However, for humpbacks, especially when they have young, they are a real danger.
The worst is the boreal beaked whales.
Neither the smaller size nor the lack of gregariousness protects them.
That is why they blend in with the soundscape and try to go unnoticed.
The way to do it is to stop using their echolocation system: with it they detect schools of fish, but they also enter the orca's
radar
, so they go into silent mode and let themselves be carried away by the waves.
The authors of the study believe that their results could serve to anticipate the impact of sonar on the attitude of other species of whales.
They have investigated four that are very different from each other.
Three are from the group of odontocetes (toothed whales), which use echolocation.
The other is a baleen baleen whale that does not have this acoustic system.
The four use different frequencies of the sound spectrum and there are two large ones, one intermediate and one relatively small, such as pilot whales.
Some are solitary and others are in groups.
“Generations will pass until they learn and adapt in the best way to respond to anthropogenic noise and each generation of whales lives long”
Patrick Miller, University of Saint Andrews and co-author of the study
It remains to be seen when the whales will learn that sonar is not dangerous, at least not like orcas.
“It will take generations for them to learn and adapt in the best way to respond to anthropogenic noise, and each generation of whales lives long,” Miller laments.
Work has focused on a region relatively free of orcas and ships until recently.
But climate change is favoring the expansion of both ever further north.
It would be necessary to know what is happening in other latitudes and with other species.
Nor should we forget, as the authors of the study point out, sonar poses other threats, such as its relationship with strandings or physiological deterioration derived from the stress caused by encounters with ships or so much human noise in the seas.
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