Whales and other cetaceans live in an almost exclusively acoustic world. Dependent on their hearing to hunt, communicate with other members of their species and interact with their environment, whales rely heavily on their auditory sense to survive. Yet we know little about these animals' acoustic processing, much less how our activities impact on them. So when two pygmy killer whales were rescued by the Mote Marine Laboratory Dolphin and Whale Hospital after stranding on a Florida beach in June 2008, Eric Montie from the University of South Carolina Beaufort, USA, and David Mann from the University of South Florida, USA, grasped the opportunity to find out more about the hearing of these elusive creatures (p. 945).
Montie recalls, ‘Charlie Manire called David and said, “We've got these two stranded pygmy killer whales, are you interested in doing some hearing tests on these animals?”; they were trying to get an idea of whether or not there were any hearing deficits because you don't want to release a cetacean into the wild that has severe hearing loss or is deaf.’ Given this rare chance, Montie and Mann tested the pygmy killer whales' hearing while Manire cared for them. The team measured the animals' brain electrical activity as they played a series of 14 ms beeps through the whales' lower left jaw, starting at the lowest pitch of 5 kHz up to the highest pitch of 120 kHz, at various sound pressure levels (volumes) to test the whales' hearing sensitivity. Then they measured the brain's response at several locations on the surface of the whales' heads as an auditory signal travelled through – and was processed by – different brain regions.
Although both of the animals were most sensitive to frequencies ranging from 20 to 60 kHz and could hear frequencies as high as 120 kHz, their high-frequency hearing was not as good as that of other toothed whales.
Next, the team CT-scanned the head of one of the pygmy killer whales to find out more about the animal's hearing. Reconstructing a 3D image of the whale's brain and auditory system, the team could see that structures in the pygmy killer whales' auditory system were similar to those of other toothed whales. The jawbones were hollow and packed with fat that pressed against the tympanoperiotic complex to transmit sound to the middle and inner ear for processing.
With a clear picture of the whale's hearing system and acoustic electrical activity patterns recorded from the brain at several sites across the animal's head, Montie and Mann realised that they could begin to identify which regions of the brain were involved in processing information about the whale's complex acoustic environment. Analysing the strength of the brain stem's response over the surface the whale's head, Montie and Mann conclude that the auditory nerve and two brain regions – the inferior colliculus and medial geniculate body – generate some of the electrical signals and process the complex sounds that steer whales through life.
Having tested the hearing of these two pygmy killer whales, Montie is keen to investigate the effects of man-made pollution on dolphin and whale hearing. ‘I have a training in marine toxicology so I'm very interested in whether or not the pollutants that marine animals accumulate throughout their lifetime, and then transfer to their young through milk, affect how they hear,’ says Montie. He explains that some man-made pollutants can affect the thyroid system, which could affect hearing development in young cetaceans, with potentially catastrophic effects for future generations.
- © 2011.