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Scientists have found new evidence that might help explain why dolphins beach themselves.
In a study published this week in the journal PLoS One, researchers found severe to profound hearing loss in 57 percent of bottlenose dolphins and 36 percent of rough-toothed dolphins that were tested after they stranded.
Researchers did not find hearing loss in other stranded dolphins and whales, including two sets of Risso's dolphins that beached themselves at Marco Island in 2005 and on Bonita Beach in 2007.
Dolphins use echolocation, a sort of sonar-like process, to navigate and to find food, so scientists have long guessed that hearing loss might turn dolphins off course.
The study, led by researchers at Mote Marine Laboratory in Sarasota and the University of South Florida in Tampa, is the first strong evidence of the link, USF biological oceanographer David Mann said.
"It was a big surprise to see that many,:" said Mann, lead author of the study.
By comparison, Mote's longest-running bottlenose dolphin study in the world since 1970 has found hearing problems to be generally absent in the Sarasota Bay population, the study said.
Scientists test dolphin hearing by putting sensors on the dolphins' heads to measure brain activity in response to sounds. Human infants' hearing is tested the same way.
A team of 16 scientists in the United States and the Caribbean measured hearing in 36 dolphins and toothed whales that were at rehabilitation centers or aquariums after beaching themselves from 2004 through 2009.
Their aim was not to try to explain the strandings but merely to gather data about what frequencies marine mammals can hear, Mann said. Once they started testing, though, they noticed the unusually high percentages of stranded dolphins with hearing loss.
The study included seven bottlenose dolphins, including one found tangled in fishing gear, and 14 rough-toothed dolphins. Hearing loss was also found in the one short-finned pilot whale examined.
Hearing loss was not found in any of seven Risso's dolphins tests nor in two pygmy killer whales, a spinner dolphin, an Atlantic spotted dolphin and a beaked whale.
Five Risso's dolphins beached themselves off Marco Island in July 2005. Only two, nicknamed Bonnie and Clyde, survived to be tested for hearing loss.
Bonnie later died in rehab, but Clyde was released back into the wild and tracked with a satellite tag to waters off Maryland.
In the Bonita Beach strandings in 2007, two pairs of mother and calf Rissos dolphins beached themselves.
One pair did not survive, but the other pair was released back into the wild, the first time a mother and dependent calf were successfully released back into the wild in the Southeast region.
Causes of hearing loss in marine mammals include noise from shipping, underwater explosions or seismic testing, birth defects, age and antibiotic drug treatment.
Some emerging research shows that exposure to the chemical PCB could hinder hearing development in dolphins like studies have shown in rats.
Hearing loss effects more than just dolphins' ability to find their way around, the study suggests.
Two young rough-toothed dolphins, thought to have had hearing defects since birth, were found with sponges in their stomachs, an indication they might have been unable to hunt for food like fish or squid. Mann said the study shows the importance of testing rescued dolphins and whales for hearing loss before making the decision to release them back to the wild.
Full Article: http://www.audiologytalk.com/news/news_show.php?id=78
