Australian sea lions and long-nosed fur seals hear best near 3.2 kHz in first in-air tests

A paddle press is a behavioural answer. Each animal was trained to push the paddle only when it heard a tone, and keepers rewarded correct responses with fish ^[4]. Dr. Benjamin Pitcher, a behavioural biologist at Taronga Conservation Society Australia ^[11], said: "The seals and sea lions were given the choice to participate voluntarily in the hearing tests using positive reinforcement, allowing us to measure what they could actually perceive while keeping the animals engaged in the process" ^[8].

The sample is small. Four animals took part, two of each species, all at Taronga Zoo Sydney ^[13]. Lead researcher Christine Erbe described the Australian sea lion as endangered, with "an estimated population of only about 12,000 animals" ^[7]. For that species, the hearing curve now on record comes from two individuals. Two captive animals cannot show how much hearing varies across a wild population, or whether these two hear better or worse than most.

The headline figure is a peak. Both species were most sensitive near 3.2 kHz ^[1], the frequency at which the quietest tone still drew a paddle press. The published summary gives that peak but does not report threshold levels in decibels or the full span of frequencies tested. An assessor needs those numbers to set a vessel's or a construction site's noise against the curve.

Both audiograms were measured in air ^[3]. For the fur seals, the team compared the hearing curve with noise from boats, ships, aircraft, trains and construction, and found that many of those sounds would be audible to animals hauled out on land ^[12]. The summary describes that comparison for the fur seals only.

Erbe, of Curtin's Center for Marine Science and Technology ^[11], presented the result as a precondition. "Before we can understand whether a noise is affecting an animal, we need to know whether it can hear that noise in the first place," she said ^[5]. "These hearing tests give us that baseline and allow us to start asking how noises from ships, boats, aircraft and coastal construction might affect seals and sea lions" ^[6].

I think her framing is the right way to read these papers. They tell a regulator which sounds these animals can detect in air, and Pitcher said the findings "could help inform environmental impact assessments and noise management as shipping, coastal development and offshore industries expand" ^[9]. Hearing a sound and being harmed by it are separate measurements. Pitcher said the second is still to come: "We now know what these animals can hear in air. The next step is to understand how noise affects them underwater, and when it begins to interfere with their behavior or hearing" ^[10].