Some animal ears hear only loud sounds; others are extremely sensitive. Most louder man-made ocean noise, including tanker engines, low frequency sonar, and ATOC, are near or below 100 Hz. So one of the most important hearing question is: How well doanimals hear low-pitch sounds?
Burnyce is one of the only elephant seals in the world who wears headphones. Marine biologist Ron Schusterman and his research group at Long Marine Lab at the University of California, Santa Cruz, strap headphones to Burnyce's head to help them study elephant seal hearing. Schusterman and his research group are testing her ability to hear different sound volumes and pitches. Whenever the blindfolded elephant seal hears a sound she nudges a rubber-ended lever with her nose, and if she's correct, Schusterman rewards her with a fish. And recently they completed the first ever study of elephant seal, sea lion, and harbor seal hearing at frequencies below 100 Hz (about equal to the lowest note on a cello).
They found that elephant seals hear better than other seals at these low frequencies. This could be due to the larger size of the elephant seal's bulla - the inner ear chamber. A large bulla, like a big stereo speaker, can resonate better at lower pitches.
Unfortunately, scientists know far less about the hearing ability of other species. A few labs have tested the hearing of bottlenosed dolphins and harbor porpoises. Most of them can hear lower frequency sounds, but their hearing is far worse in that range. With larger whale species, logistical problems prevent scientists from learning more about their hearing ability. It's fairly difficult to bring a full grown whale into a tank and reward him with 100 pounds of krill every time he presses a lever.