When we’re talking with friends and a truck rumbles by or someone cranks up the radio, we talk louder. Now scientists have found that North Atlantic right whales do the same thing in their increasingly noisy underwater world.
Marine biologist Susan Parks found that right whales call louder when the level of background noise goes up.
“It’s not a very surprising result, in a way,” said Parks’ colleague Mark Johnson, an acoustic engineer at Woods Hole Oceanographic Institution (WHOI). “What’s surprising is that one can actually measure it.”
Parks drew on whale sound data that she, Johnson and biologists Peter Tyack of WHOI and Douglas Nowacek of Duke University compiled between 2000 and 2005. Parks earned her Ph.D. in 2003 from the MIT/WHOI Joint Program and is now on the faculty at Pennsylvania State University. The team's findings were published July 7, 2010, in the online edition of Biology Letters.
The scientists had attached temporary recording devices called “D-tags” to the backs of right whales. The tags record the whales’ calls and the background noise around them, as well as the animals’ depth, orientation, and direction of travel.
“We had been interested in looking at how right whales responded to the sounds of ship noise,” said Johnson. “What she did was go back and look at that data and see if there was other information in there that might have a conservation value.”
Such “meta-studies” are especially valuable when working with an endangered animal such as right whales, he said, because they allow researchers to mine new information from years of old data without tagging the whales again.
Parks analyzed the right whales’ “upcalls,” which were first described in the 1970s by her postdoctoral mentor, Chris Clark of Cornell University. Whales use upcalls to let other whales know they are around. Upcalls are easy to recognize: They start at a low pitch and then swoop upward.
But even with distinctive calls and a big stockpile of data, it wasn’t easy coming up with enough recorded calls to do the analysis. “Right whales aren’t very communicative,” said Parks. They don’t call often, and the researchers wanted to get multiple calls from individual whales in different levels of background noise. In the end, they assessed up to 18 calls from each of 14 whales.
Every one of the 14 whales got louder as ambient noise level went up. Their age and gender didn’t matter. They all maintained a level of about 10 to 12 decibels above the background noise. One whale called at 117 dB when the background noise level was 105 dB, 132 dB when the background noise rose to 120 dB, and 138 dB when background rose to 126 dB. Two whales that were recorded only in high-noise surroundings were practically shouting during every call, reaching nearly 150 dB. (Parks cautioned that decibel levels underwater cannot be directly compared to decibel levels in air. So while 150 dB in the ocean is loud for a whale, it is not comparable to 150 dB in air—a noise level that most humans would find very painful.)
Parks also found that whales recorded in the 2000s started their upcalls at a frequency between 100 and 200 Hertz (roughly an octave below middle C). By contrast, in recordings made in the 1950s by WHOI whale song pioneers Bill Watkins and Bill Schevill, right whale upcalls started about an octave lower, at 50 to 100 Hertz. Parks speculated that over time, whales may be able to change their call frequency to make themselves heard above rising background noise. Still, a noisier environment could affect the ability of the endangered whales, whose population is fewer than 400, to find mates and reproduce.
“To the extent they can, animals will try to keep up with [changes in] their environment,” Johnson said. “The difficulty occurs when they are challenged by both a reduced population, as is the case with the right whales here, and sudden changes in their environment, or difficult-to-combat changes—so, for example, shipping noise that is so loud or so frequent that they are unable to fully compensate for it, so their communication range reduces.”
Parks said having to shout to be heard could affect the whales in other ways as well. Calling louder takes more energy. It might also distort the meaning of the call, what it sounds like to other whales. “We don’t know what a whisper or a shout sounds like from a whale,” Parks said. “We can tell for humans, but not for whales.”
The sparse record of calls also might be significant. Parks said she wonders whether the low call rate of right whales is a species characteristic or if it is related to their increasingly noisy environment. Does excess noise discourage them from calling? “We don’t know if they just stop at some point,” she said. “Like being in a loud room, you talk louder to be heard—but if it’s noisy enough, you just stop trying.”
—Cherie Winner, with additional reporting by Kate Madin
This work was supported by the National Oceanic and Atmospheric Administration, the Office of Naval Research, and the National Oceanographic Partnership Program.