Study of Yangtze Finless Porpoise Reveals Not All Cetaceans Hear Alike

A new study from Woods Hole Oceanographic Institution (WHOI) and marine biologists in China highlights the differences in dolphins' ability to hear across species, reshaping the previous "one-size-fits-all" approach taken on the hearing ability of the cetaceans.

In the heavily trafficked, murky waters near China's Three Gorges Dam, the Yangtze finless porpoise faces an aural assault beneath the water, with the noise of shipping, deep-water dredging and underwater construction creating a cacophony beneath the waves.

Aran Mooney, a biologist at WHOI and a lead author of the study published online this week in the Journal of Experimental Biology, said his team wanted to gain a better understanding of how the Yangtze finless porpoise -- a highly endangered species --  is affected by all the noise.

Only about 1,000 Yangtze finless porpoises (Neophocaena phocaenoides) are left in the wild, the researchers say, but what we know about how the mammals hear is based off of more commonly studied cetacean species such as the bottlenose dolphin.

"This can be a problem when natural resource managers and regulators base aquatic noise pollution policy decisions on data from a limited number of 'representative species' when there are over 70 species of toothed whales or odontocetes that live in a variety of aquatic habitats," the researchers said in a statement.

Mooney and his colleagues' research demonstrates that the size and shape of toothed whales' heads across species can result in significant differences in how the creatures receive sound and how sensitive they are to a range of frequencies.

"We've learned that there's more variation than we've taken into account on how different species hear," Mooney said.

Toothed whales, such as the Yangtze finless porpoise, do not have external ears. The porpoises hear when sound reverberates though their head, throat, jaw and acoustic fat present in the mandible.

The research team conducted hearing tests on several Yangtze finless porpoises that had been taken from the river and relocated to a research facility at the Institute of Hydrobiology of the Chinese Academy of Sciences in Wuhan, China.

By using acoustic sensitivity examinations and CT scans, the researchers were able to gain a better understanding of how the finless porpoises process sound. They found a marked difference in how the finless porpoise hears compared to the common bottlenose dolphin.

"The exam results showed that the finless porpoises are sensitive to sound nearly equally around their heads while bottlenose dolphins and beluga whales exhibit a substantial 30-40 decibel difference in sound sensitivity from their jaw to other parts of their head," the researchers said.

The Yangtze finless porpoise hears omni-directionally, which the researchers say can lead to trouble in discerning signals amid the constant clutter of noises underwater.

"In a noisy environment, they'd have a hard time hearing their prey or their friend. It makes it more difficult for them to conduct basic biological activities such as foraging, communicating, and navigating in the river," Mooney said.

The find suggests there may be further auditory variations among cetacean species. Mooney said that wildlife management strategies should take these species-specific variations into consideration.