AEI lay summary of this recently published scientific paper (download paper):
Mann D, Hill-Cook M, Manire C, Greenhow D, Montie E, et al. (2010) Hearing Loss in Stranded Odontocete Dolphins and Whales. PLoS ONE 5(11): e13824. doi:10.1371/journal.pone.0013824
This paper generated a wave of press coverage upon release, most of it suggesting that the research found that stranded dolphins are predominantly deaf, and often focusing on the idea that exposure to shipping and other ocean noise is the likely culprit. I must admit that when I first saw the headlines, I too thought this might be the smoking gun implicating chronic ocean noise in population-scale impacts, but as usual, the popular press had vastly oversimplified and distorted the actual findings.
Baby dolphin found stranded in Uruguay, November 2010
Deafness found in a less than a quarter of stranded animals
While, as noted in some press coverage, 1200 0r more dolphins strand each year in the US, this study looked at just 35 stranded cetaceans of 8 species that stranded between 2004 and 2009. The big headlines focused on the fact that 57% of bottlenose dolphins had profound hearing loss. This represents 4 of 7 individuals, who were deaf or close to deaf; one was clearly very aged (it had no teeth left), so the authors suggest this individual was clearly experiencing age-related hearing loss. One other species showed hearing loss in a significant proportion of individuals studied: 5 of 14 Rough-toothed dolphins had profound hearing loss (36%); two of these are considered likely to have been born with hearing loss (see below).
By contrast, none of the 7 Risso’s dolphins studied had hearing loss, and only 1 of the other 6 individuals of other species were afflicted with hearing troubles. When we also consider the individual noted above whose causal factors may be less mysterious, the study as a whole seems to imply that deafness from unknown (or possibly noise-related) sources affects less than a quarter of the animals studied. Still, a quarter is a large proportion, far more than studies of wild populations suggest have hearing troubles; so the question of why these animals are deaf is surely an important one.
Newly independent sub-adults are the most common deaf stranders: born deaf or severely hearing-impaired?
Perhaps the most striking trend noted in the results is that 5 of 12 sub-adult individuals were deaf or nearly deaf, while only 5 of 18 adults were hard of hearing. The authors note that it is likely that at least two of the 5 sub-adults had congenital hearing loss (i.e., impaired hearing from birth); about 2 of 1000 humans are born this way. They go on to remind us that mother dolphins can support young for years via lactation, and “it is possible that a dependent calf with congenital hearing loss could survive with the help of its mother for a long period. Once it is weaned, however, it would not be able to forage successfully on its own.”
Noise: possible factor for some, unlikely for others
The researchers mention chronic noise as a possible source, but only briefly, noting that some of the deaf dolphins lived in areas where they may have been exposed to ship and boat noise for much of their lives, but that other deaf individuals were from locations where they were “unlikely” to have experienced elevated chronic noise levels.
PCBs and hearing loss
The final causal factor, which the authors seem to give more attention to than noise, is chemical exposure. “Cetaceans, particularly odontocetes, have been shown to accumulate very high levels of polychlorinated biphenyls (PCBs), which can be maternally transferred to offspring. Evidence from studies with rats suggests that these chemicals may affect how hearing develops. In fact, it has been shown that developmental exposure of rates to PCBs results in severe hearing loss.” These relationships have not yet been studied in cetaceans. And, there is no indication that these animals were tested for PCB levels.
Noise cannot, of course, be discounted as a factor in hearing loss among cetaceans. While it appears to take relatively high noise exposure to cause hearing damage, repeated or chronic exposure to close-passing motorboats or large ships could accumulate over time to reduce hearing sensitivity. It is altogether possible that some of the individuals studied here had experienced intense noise exposures, we cannot know that with any assurance. The patterns found in this study, however, seem to point to birth defects and chemically-mediated disruption of the hearing system as important factors as we consider possible causes of cetacean hearing impairment.