Bobbing around in the northern Pacific Ocean, spotted seals are most at home in the Arctic water, only pulling themselves onto the sea ice to rest, nurse their young and moult. But their pristine environment is coming under increasing threat from human activity. ‘The ocean is becoming louder and louder’, says Jillian Sills from the University of California at Santa Cruz (UCSC), USA, who explains that these animals probably depend a great deal on their hearing as they spend much of their time hunting in the dark northern water where vision is often of limited use. ‘We are interested in understanding how Arctic seals perceive the surrounding environment’, says Sills, who explains that some hearing data exist for seals from temperate locations. As little was known about the hearing abilities of seals from further north, she and her colleagues Brandon Southall and Colleen Reichmuth were keen to understand how noise might affect the hearing and communication of northern populations. Fortunately, two orphaned spotted seal pups – Amak and Tunu – had recently been rescued by the Alaska SeaLife Center before being rehomed at the UCSC Long Marine Laboratory, so the team embarked on a comprehensive study of the youngsters' hearing in a bid to discover how vulnerable they might be to noise pollution (p. 726).
Sills recalls that the playful pups were eager to learn and picked up new tasks quickly. Having trained the animals to touch a nearby plate with their noses in return for tasty fish rewards when they heard a hearing test tone, the team then tested the animals' hearing sensitivity. ‘[Harbour] Seals have quite different hearing capabilities in air and under water’, explains Sills, so the team tested Amak and Tunu's underwater hearing from 0.1 to 72.4 kHz and were impressed to see that their hearing spanned a remarkable seven octaves. And when they retested the animals' hearing in the air, they were surprised to find that the spotted seals' performance was much better than they had anticipated. ‘The conventional view was that seals had good underwater hearing and poor in-air hearing abilities, because they had to sacrifice the latter during their transition to semi-aquatic living’, explains Sills. However, Amak and Tunu were able to respond to airborne sounds over a span of four octaves. ‘Our work indicates that spotted seals hear nearly as well as hearing specialists in both environments’ says Sills, adding that the youngsters' hearing thresholds in air were the lowest ever measured for a marine mammal.
But how good was the seals' hearing in noisier environments? Measuring the quietest level at which the seals could reliably detect single tones against background noise, the team discovered that the youngsters' hearing was equally good in air and water. However, when the team recorded the seals' reaction times to sounds in peaceful and noisy situations, they found that sounds have to be much louder (large amplitude) against background noise in both air and water before the animals perceive them as having the same loudness as sounds played against silence.
Having discovered that spotted seals' hearing is remarkably sensitive in both air and water, Sills is keen to find out how seismic guns – used in prospecting for oil – might affect the animals' delicate hearing. She adds that it is essential that we understand how noise affects the hearing of Arctic residents so that regulators can predict its impact to ensure that the spotted seals' hearing comes to no harm.