Critically Endangered Yangtze Finless Porpoise Faces Threats from River Noise

First Posted: Oct 21, 2013 11:07 AM EDT

There may be another threat to the Yangtze finless porpoise, one of the most endangered species in the world. It turns out that noise from shipping traffic could be impacting this animal's ability to hunt for and find prey in the wild. This, in turn, could greatly affect species populations.

Currently, there are only an estimated 1,000 Yangtze finless porpoises alive today. In fact, the finless porpoise shares the same habitat as the Baiji river dolphin, seemingly the first toothed whale that has become extinct by humans. This makes it especially important to understand exactly what risks this creatures may face.

Like all toothed whales, the Yangtze finless porpoise does not have external ears. Instead, it hears when sound reverberates through its head, throat, jaw and acoustic fat within the mandible. In order to better understand the process of hearing and how excess sound might affect this species, the researchers conducted hearing examinations on two Yangtze finless porpoises. Originally from the wild, these finless porpoises have since been brought to reside at the Institute of Hydrobiology in Wuhan.

"Porpoises, like babies, can't tell us if they hear in their left or right ear, so we measure their hearing physiologically from the surface of the skin," said Aran Mooney, one of the researchers, in a news release.

Scientists transmitted broadband clicks and low, mid, and high frequency tones within a normal threshold through silicon suction cup sensors. These sensors were placed on nine parts of the animal's head and body. The researchers then non-invasively recorded the porpoises' neuron responses.

So what did they find? It turns out that porpoises are sensitive to sound nearly equally around their heads. That's a sharp contrast to bottlenose dolphins and beluga whales, which exhibit a substantial 30 to 40 decibel difference in sound sensitivity from their jaw to other parts of their head.

So what does this mean exactly? Because of the Yangtze's finless porpoise's morphology, it's very likely that the animal hears omni-directionally. This means that it may have difficulty discerning signals among a clutter of constant noise. This, in turn, could make it difficult for the species to navigate and hunt.

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

The findings reveal a little bit more about the risks that the finless porpoise faces. Moving forward, the research could help inform conservation decisions and might allow scientists to better preserve this species into the future.

The findings are published in the Journal of Experimental Biology.

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