Few events in the marine world are as upsetting to the public and as mystifying to experts as dolphin and whale strandings, but a new study by a team of scientists led by the University of South Florida and Mote Marine Laboratory has discovered that hearing loss may play a role in some of the animals' distress.
In a study published in the new edition of the journal PLoS One, the researchers found severe to profound hearing loss in 57 percent of the bottlenose dolphins and 36 percent of the rough-toothed dolphins studied after the animals stranded.
Given that these species rely on echolocation for orientation and feeding, the researchers believe that hearing loss could play a significant role in some strandings, said Dr. David Mann, a USF biological oceanographer and the paper's lead author. The finding might also provide good cause for veterinary experts, scientists and resource managers to rethink the rehabilitation and release of dolphins given the role hearing loss might play in their trauma.
During the study, a team of 16 scientists from marine science colleges and institutes across the United States and the Caribbean examined 36 dolphins and toothed whales in Florida and at a number of aquariums and rehabilitation centers. The animals had been found stranded or entangled in fishing gear between 2004 to 2009, and ranged in age from calves to adults, including one dolphin believed to have reached a ripe old age, given his lack of teeth.
The scientists found strong trends among the bottlenose and rough-toothed dolphins suffering from hearing loss, and the only short-finned pilot whale examined also had profound hearing loss. Interestingly, no hearing impairments were detected in any of the seven Risso's dolphins from three different stranding events, or from two pygmy killer whales, one Atlantic spotted dolphin, one spinner dolphin, or a juvenile Gervais' beaked whale that were also part of the study group.
The researchers tested the animals' hearing using "auditory evoked potentials." So-called AEPs are commonly used to measure hearing in human infants. Sensors are placed on an animal's head to measure brain activity in response to a sound.
The method was used to measure hearing in the stranded cetaceans and compare that to measurements obtained during health assessments of the free-ranging dolphins living in Sarasota Bay, which are the subjects of the Sarasota Dolphin Research Program. The Program, a collaboration between Mote and the Chicago Zoological Society, began in 1970 and today is the longest-running study of a dolphin population anywhere in the world.
More than half of the dolphins tested had been brought to Mote's dolphin hospital for treatment. Dr. Randall Wells, director of the Sarasota Dolphin Research Program , said hearing loss has long been discussed as a potential reason for dolphin strandings.
"Cetacean hearing problems were hypothesized to be a cause of strandings even before the 1970s, when Mote became one of the first organizations studying why marine mammals strand, through necropsies and treatment of live-stranded cetaceans," Wells said. "Finding that many stranded cetaceans do indeed have hearing problems is an important advancement in our understanding of these phenomena."
The finding also has implications for how stranded dolphins and whales should be treated, Mann said. "If you have a dolphin that comes in with high-frequency hearing loss, there may not be much point trying to release it," Mann said. "All stranded rehab animals should have their hearing tested â€" because rehabilitation and release is time and money intensive, and everyone wants to do what is in the best interest of the animal."
As a result of the study, the researchers urge marine mammal veterinarians to conduct hearing measurements both before and after drug treatment. Rescued dolphins are sometimes treated with a class of antibiotics that could damage crucial hair cells in the ear that allow the animals to hear. Mann said dolphins released back into the wild with damaged hearing may be at risk.
Mann said while hearing may be a factor for some stranded animals, other dolphins and whales might still have a multitude of health issues. That's why it's important that hearing tests be incorporated into the animals' care and rehabilitation.
There are five main contributing factors to hearing loss in marine mammals:
*Intense chronic noise, from things such as shipping *Transient intense noise, such as explosions and underwater seismic testing *Age-related hearing loss *Congenital hearing impairment *Antibiotic drug treatment
The group also pointed to evolving studies by USF researcher Eric Montie showing that exposure to the chemical PCB in marine mammals may play a role in hearing development in dolphins as it has been shown to in rats.
The researchers did not know the noise exposure history in the dolphins found to have hearing loss, although they suspect that two rough-toothed dolphins that stranded when they were young might have had hearing defects from birth.