The Yangtze River, the longest river in Asia, was once home to two different species of dolphin: the Yangtze finless porpoise and the Baiji dolphin. While the latter was declared extinct in 2006, the former may be heading in the same direction and their ears might be to blame.

There are about 1,000 Yangtze finless porpoises, which inhabit the high traffic waters near the Three Gorges Dam in China. Known for its mischievous smile, the finless porpoises had their hearing examined by scientists to see how it affected their ability to navigate, communicate and hunt for food.

“We want to understand how they may be impacted by noise,” said Aran Mooney, a biologist at the Woods Hole Oceanographic Institution (WHOI) and a lead author on the study published in the Journal of Experimental Biology, said in a statement.

While previous studies have focused on bottlenose dolphins and their ability to communicate with sound, the latest findings were one of the first to show how the size and shape of the finless porpoise’s head is connected to how they hear noises. The results can influence the way regulators make aquatic noise pollution policy decisions in the future, the scientists said.

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

Scientists conducted hearing exams on two Yangtze finless porpoises, similar to ones given to infants. Researchers used silicon cup sensors attached to nine parts of the porpoise’s head and body to emit broadband clicks in low, mid and high frequencies to see how they reacted. Like other toothed whales, the finless porpoises do not have external ears. They hear sound by reverberations felts through their heads, throats, jaws and acoustic fat in their mandibles.

Results from the hearing exams showed the finless porpoises were sensitive to sound around their heads. “Porpoises, like babies, can’t tell us if they can hear in their left or right ear, so we measure their hearing physiologically from the surface of the skin,” Mooney said.

CT scans on the two porpoises revealed that the animals’ acoustic fat pads -- an area of the dolphins’ body that gives them the ability to hear and process echolocation -- are thicker and more disc-shaped compared to those found in other toothed whales. The scans show the porpoises hear omni-directionally, making it hard for them to distinguish sounds among a bunch of different sounds.

“Now that we have some hearing data, we are working on modeling how the conformation of these pads and their dimensions and shapes relate to the frequencies and sensitivities,” Darlene Ketten, a biologist and director of the Computerized Scanning and Imaging (CSI) facility at WHOI, said.

According to the International Union for Conservation of Nature and Natural Resources (IUCN), Yangtze finless porpoises are considered critically endangered. Major threats to their livelihood include habitat loss, boat traffic and pollution. At times, the animals get caught in boat propellers and fishing gear frequently used in the Yangtze River. Boat noise also affects their ability to hear and forage for food.

“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.

Further studies on other toothed whales, such as the Risso’s dolphin, harbor porpoise, and white-sided dolphin, and continued research on the Yangtze finless porpoises are needed to completely understand the auditory differences among the different marine animals, he added.