Dolphins and whales are stranded on beaches around the world on a daily basis. When USF researchers decided to find out why, their studies pointed to hearing loss as a contributing factor.
During the course of their study, which was conducted from 2004 to 2009, a team of 16 scientists from the USF College of Marine Science and various marine science institutes in the U.S. and Caribbean examined 35 dolphins from seven different species and one beaked whale that were found stranded or severely entangled in fishing gear.
They found that approximately 57 percent of the bottlenose dolphins and 36 percent of the rough-toothed dolphins had significant hearing deficits.
“Hearing is very important for their livelihood, for echolocation,” said Jessica Powell, one of the study’s researchers who works for the National Oceanic and Atmospheric Administration (NOAA). “It’s part of how they speak.”
Echolocation allows dolphins and whales to determine distance and direction by bouncing echoes off of nearby objects, she said. If they suffer from hearing loss, their directional skills would be severely challenged and could land them on beaches.
Hearing tests were conducted on the animals through the use of Auditory Evoked Potential (AEP) measurements. Danielle Greenhow, a postdoctoral marine science student who worked on the study, said the same process is used to test hearing in human infants by transferring noises to the inner ear through an apparatus in the jaw.
Electrodes were suction-cupped to the surface of the animals and measured their brain activity, as well as “background noise,” which include heartbeats and breathing.
“By playing a series of frequencies from the jaw bone and measuring the output of the brain, we can deduce what is the lowest level they can hear each frequency,” Greenhow said.
The frequencies played ranged from 10 to 100 kilohertz. The intensity was measured in decibels and was abnormally high so that the dolphins that exhibited hearing loss would notice the tones.
Five factors contribute to hearing loss in mammals: intense chronic noise like boat engines, transient intense noises like explosions, age-related hearing loss, congenital hearing impairment and ototoxic drug treatment. Greenhow said humans could cause some of these factors.
“It’s not only important to monitor the causes of these strandings,” she said, “but also to take a step back and find out if there are possible implications for anthropogenic (human produced)causes.”
During the study, Greenhow said she was able to contribute to a scientific first – conducting the tests on a short-finned pilot whale in a country off of the Venezuelan coast where a mass-stranding event occurred.
“It was the first pilot whale (tested), using auditory evoked potentials, anywhere,” she said. “They’re large animals. We were just lucky that it turned out to be a juvenile, so we were able to use our methods to test it. A larger animal is going to have a thicker skull, a larger head. It’s harder to get at a clear brain wave pattern.”
The studies findings were published in a Nov. 2 report that was mainly authored by David Mann, a biological oceanographer in the College of Marine Science.
Powell assisted in writing the report, and said that its purpose is to encourage all scientists, researchers and humanitarians to test the hearing of whales and dolphins before and after strandings.
“I think it’s important to understand what (hearing capabilities) these animals are coming ashore with. It’s also one of the first papers out there to document hearing loss in stranded animals,” she said. “We just wanted to publish a paper that kind of gave everybody a heads up. You can never know what caused a stranding; it could be a number of things working together. But hearing tests are something that need to be done on a regular basis during stranding events.”
Not all of the animals studied exhibited hearing loss. No impairments were detected in seven Risso dolphins, two pygmy killer whales, an Atlantic spotted dolphin, a spinner dolphin and a beaked whale. Nonetheless, Greenhow said hearing tests should be conducted on all rescued animals before being released back into the wild.
The team is currently trying to create a simpler, smaller and faster AEP system that could become a regular part of a veterinary examination.
“Having animals tested before they go to rehabilitation can help rehabilitators make a decision on whether it will be successful,” she said. “If an animal has really poor hearing, a doctor can probably determine at that point whether that animal can be released and still survive in the wild.”