A hearing test for wild belugas
With the loss of sea ice, the Arctic Ocean is becoming an increasingly industrialized — and noisy — space. For beluga whales (Delphinapterus leucas), which rely on sounds to find food and navigate dark waters, more background noise could limit their range of hearing.
Before they can determine these impacts, scientists needed to know more about what belugas can hear. Previous studies had small sample sizes and none of them examined belugas in the wild. So, a team led by Aran Mooney from Woods Hole Oceanographic Institution gave hearing tests to seven healthy, wild belugas in Bristol Bay, Alaska.
The whales were captured and released during a separate health assessment project over the course of two weeks during the summer of 2012. Scientists and locals used small boats to spot and guide belugas into shallow water, where they placed a belly band stretcher with flipper holes underneath the whales. Then the researchers used suction cups to attach a small speaker to the lower jaw — a region of acoustic sensitivity — and placed electrode sensors just behind the blowhole and on their backs.
A hearing test for whales is a lot like a hearing test for humans: play a sound, then measure the response. The team tested beluga hearing by playing 20-millisecond tone bursts with a 30 millisecond break in between, and then they measured the whale’s neurophysiological responses. They found that all the belugas can hear pretty well. The final tally: They all heard sounds from the 45 to 80 kHz range up to 128 kHz, with surprisingly little variation in their hearing ability and sensitivity. In addition to audiograms, researchers also obtained baseline health information with ultrasounds and saliva, mucous, and blubber samples.
Bristol Bay belugas are one of six subpopulations in U.S. waters, and they live in a relatively quiet, acoustically pristine environment. By contrast, belugas living in Cook Inlet near Anchorage contend with a lot of military and commercial activity, and those animals are declining at a rate of about two percent per year. Now that researchers know how to measure what wild belugas can hear, they can test Cook Inlet belugas to see if noise is a major stressor. In turn, that information could help researchers design noise-limiting measures to protect belugas back in Bristol Bay, where increased shipping and mining activity is expected to raise noise levels. — Janet Fang | 22 May 2014
Source: Castellote, M. et al. Baseline hearing abilities and variability in wild beluga whales (Delphinapterus leucas), Journal of Experimental Biology (2014). doi:10.1242/jeb.093252
Images: J. Helgason via shutterstock.com (top) & Aran Mooney, Woods Hole Oceanographic Institution (middle) & Alaska Department of Fish and Game (bottom)