Abstract

With the first dolphins fishermen collected for Marineland of Florida in the late 1930's, A.F. McBride documented the interesting and essential capability that we know as echolocation. Over almost 50 years since that time, trained cetaceans working in cleverly designed experiments have allowed us to elucidate many features of this remarkable proprioceptive sense. We now know that echolocation was crucial to the evolutionary success of dolphins as ocean predators.

Since the classical study of C. S. Johnson at Point Mugu, California, in the mid-1960's, we have understood that the essential element in this dolphin "ESP" is a keen sense of hearing. Delphinoid cetaceans, specifically white whales and bottlenose dolphins, have a broad band hearing range from under 100 Hz to around 150 kHz, roughly 8 times the span of human hearing. At their most sensitive frequencies, these animals have hearing thresholds near 10-14/W/m2 compared to about 10-12/W/m2 for humans at the most sensitive frequencies.

Without having brought these animals into our facilities for close observation and study, humankind would still be largely ignorant of the echolocation capability and keen sense of hearing that delphinoids and other cetaceans possess.

Empowered with knowledge about this keen hearing, many environmentalists are justifiably concerned about the increase of human-made noise in the sea that might adversely impact these sea mammals. There is a great deal we must learn about how human-made sound in the sea might impact marine mammals. Evidence suggests that what we know about noise effects on human and land mammal hearing may not be applicable to understanding noise effects on sea mammals.

We have prepared a 13 -minute video document of one experiment aimed at beginning the process of building a solid scientific structure on which to base decisions concerning human-made sound in the sea and marine mammals.

In examining the potential impact of human-made sound on sea mammals, it was considered that whale hearing sensitivity might diminish with increasing ambient pressure. To test the effect of depth, two white whales made 885 dives to a platform at 5, 100, 200 or 300 m in the Pacific Ocean. Each stationing on the platform up to 12 minutes at a time, whales whistled when they head a 500 ms tone from a hydrophone. With increasing depth, air density increase in the middle ear, sinuses, and nasal cavity changed each whale's whistle response, but did not attenuate hearing as it does in the aerial ear (of humans and other land mammals tested in pressure chambers) due to middle ear impedance changes. The findings support theories that sound is conducted through whale head tissues to the ear without the usual ear drum/ossicular chain amplification of the aerial middle ear. These first ever hearing tests in the open ocean demonstrate that whales hear as well at depth as near the surface; therefore, zones of influence for human made sounds are just as great throughout the depths to which whales dive, or at least to 300 m.

Acknowledgements

This work was supported by Office of Naval Research N0001496WK30349. I thank our DEEPBEAR team of Donald Carder, Rob Smith, Tricia Kamolnick, Mark Todd, Mark Beeler, Monica Chaplain, Scott Shaffer, and the NRaD training staff and veterinarians for supporting this project.