Noise from ships changes the way that whales and dolphins communicate with each other, according to two studies published this week.
Marine biologists know that an increasingly cacophonous ocean, filled with the din from shipping, seismic surveys and sonar, is undoubtedly impacting the animals that live there. Loud noises could be damaging the hearing of marine mammals that are dependent on sound as a way of making sense of their three-dimensional world, and sonar blasts have been linked to the mass stranding of whales around the world.
Now, two independent teams of scientists have shown that whales and dolphins alter the sounds that they make in response to the whirr of ships.
University of Maryland marine biologist Helen Bailey and her team recorded the whistles of bottlenose dolphins (Tursiops truncatus) off the coast of the U.S. state of Maryland and published their work Oct. 24 in the journalBiology Letters. The researchers analyzed the characteristics of those calls and took note when they changed. They found that when there was more ambient sound, mostly from passing boats and ships, the dolphins switched to higher frequencies and streamlined their whistles.
“It’s kind of like trying to answer a question in a noisy bar and after repeated attempts to be heard, you just give the shortest answer possible,” Bailey said in a statement. “Dolphins simplified their calls to counter the masking effects of vessel noise.”
A second team of scientists, led by biologist Koki Tsujii of the Ogasawara Whale Watching Association and Hokkaido University in Japan, looked at the changes in the songs of whales — humpbacks (Megaptera novaeangliae), in this case. Publishing their results in the journal PLOS ONE, also on Oct. 24, they recorded the changes in the songs sung by male humpbacks as a ship carrying passengers and cargo motored by.
The team’s recordings of 26 singing whales revealed that when the ship passed and the singing whales were less than about 1,200 meters (3,940 feet) from the ship, they tended to cut their songs short. What’s more, they often wouldn’t start up again for half an hour. They also weren’t as likely to hang around and sing at distances of less than 500 meters (1,640 feet) from the ship’s path.
The researchers note that, because only the males sing in an effort to attract females, they can’t be certain of how the noise impacts young and female humpbacks. But it’s clear that the sounds are changing behaviors, the authors said.
“Humpback whales seemed to stop singing temporarily rather than modifying sound characteristics of their song under the noise, generated by a passenger-cargo liner,” the authors of the PLOS ONE paper said in a statement. “Ceasing vocalization and moving away could be cost-effective adaptations to the fast-moving noise source.”
The team suggests that more research would help determine if these sounds increased the stress levels of the affected humpbacks.
The authors of the dolphin study raised similar concerns.
“These whistles are really important,” Bailey said in the statement. “Nobody wants to live in a noisy neighborhood. If you have these chronic noise levels, what does this mean to the population?”
She and her colleagues suggest that, in the presence of these levels of ambient sound, dolphins might have trouble getting their full message across to other members of their species.
“[The] noise-induced simplification of dolphin whistles may reduce the information content in these acoustic signals and decrease effective communication, parent-offspring proximity or group cohesion,” the authors write.
Banner image a sounding humpback whale by John C. Cannon/Mongabay.
Fouda, L., Wingfield, J. E., Fandel, A. D., Garrod, A., Hodge, K. B., Rice, A. N., & Bailey, H. (2018). Dolphins simplify their vocal calls in response to increased ambient noise. Biology Letters, 14(10).
Tsujii, K., Akamatsu, T., Okamoto, R., Mori, K., Mitani, Y., & Umeda, N. (2018). Change in singing behavior of humpback whales caused by shipping noise. PLOS ONE, 13(10), e0204112.
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