When you dive into the frigid waters of the Pacific Ocean off the coast of southern California, the first thing you notice is the silence. Other than the bitter cold. Your body begins to adapt to the chilly water as blades of slimy kelp brush across your ankles. You spit out the bit of brackish saltwater that inevitably seeps into your mouth. Then you quickly dunk your head into the sea so that you might wet your hair and wipe it away from your eyes. It's in that moment - when you're entirely submerged under the rolling waves - that you notice the silence. You can almost hear the oscillating thuds of the waves breaking against the sand. As your heart beats faster to push warm blood into your arms and legs, perhaps you might even be able to hear your own heartbeat. Even against the auditory backdrop of the pounding of the waves and your heart, you can't help but perceive the quiet. If only it were so for the blue whales that call this corner of the ocean home, at least for part of the year.
Each summer, groups of endangered blue whales (Balaenoptera musculus) pass along the coast of Southern California between San Diego and Los Angeles. It isn't a secret that the ocean is a noisy place if you're a whale. In addition to the natural soundscape of the ocean, whales can hear sounds that have human origins, like sonar, passing ships, or underwater explosions. Considerable scientific attention has been paid to the effects of high-intensity anthropogenic noise on the communication abilities of whales and other marine mammals. After all, these animals communicate over vast distances by producing clicks, whistles, and songs. Previous findings have confirmed that the presence of ships interrupts blue whale songs. And some whales have been observed increasing the amplitude of their foraging calls in noisy environments, in an effort to aid others in distinguishing their communication from the undersea cacophony. Imagine having to pick out the sounds of only the cellos from amid an entire orchestra.
What these studies have in common is they all examined the impact of human noises that happen to fall into the same auditory frequencies as blue whale calls. In a new paper published today in the journal PLoS ONE, researchers from the Scripps Institute of Oceanography at the University of California, San Diego, describe the effects of mid-frequency sounds, outside of the natural range of the calls and songs of what David Attenborough called the "largest animal that exists or has ever existed."
Both male and female blue whales that pass through California waters each summer produce characteristic low-frequency sounds known as "D-calls," which have frequencies less than 100Hz. Scientists believe that these calls are used to alert others to the presence and location of a food source. Over the course of two summers, the Scripps scientists observed the response of blue whales to mid-frequency sounds created by sonar, which occur in the 1000Hz to 8000Hz range. Would the presence of these sounds alter or disrupt the communication behavior of blue whales, even though they are well outside the range of sounds produced by the whales themselves?
In all, there were 4643 hours worth of data to analyze. D-calls were heard 48% of the time, while sonar was picked up 9% of the time. The researchers also collected data on naturally-occurring noise to use as a comparison point, which included the audible effects of wind, rain, earthquakes, as well as the communication of other animals, such as dolphins.
The researchers discovered that hearing sonar reduced the whales' likelihood of producing D-calls by half. In other words, the whales were twice as likely to produce their low-frequency D-calls at baseline than when they could hear the mid-frequency sonar.
While the human-produced noises that were investigated in this study did not fall into the frequency range of baleen whale calls (the group of whales that includes blue whales), their clear response to mid-frequency sonar suggests that they can hear sounds in frequency ranges outside of the ones that they use to communicate. The researchers hypothesize that one reason this might be the case is that having an extended hearing range may allow blue whales to hear, and thus avoid, their predators. Killer whales, whose vocalizations do overlap with mid-frequency sonar, frequently prey on blue whales.
We become frustrated when poor cell service results in dropped calls on our mobile phones. But we can just press a button to redial a phone number and finish placing our orders for pizza delivery. Imagine if a noisy environment not only resulted in less efficiency in finding dinner, but also in the increased likelihood of becoming someone else's dinner! This is the reality in which blue whales live, thanks to human intrusion into the marine soundscape. We think of the ocean as a calm, quiet, peaceful place. For some species, it can be quite the opposite.
Melcon ML, Cummins AJ, Kerosky SM, Roche LK, Wiggins SM, et al. (2012) Blue Whales Respond to Anthropogenic Noise. PLoS ONE 7(2): e32681. doi:10.1371/journal.pone.0032681
More on animal communication in noisy environments:
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Image by Ed Yong, used with permission.