On a personal level, this paper is related to Ken Norris' thinking about echolocation in marine mammals. He, as well as many others, initially believed that pinnipeds probably used echolocation. He changed his mind in September of 1966 after he heard a presentation by R. J. S. at the Animal Sonar Meetings in Frascati, Italy, about the unlikelihood that California sea lions use echolocation as compared to relying on other sensory modalities, particularly vision, to navigate and forage. As was characteristic of Ken, he saw the humor in all of this and proceeded to draw a fine cartoon (which he showed to R. J. S. later in private). It depicted R. J. S. as Don Quixote and Tom Poulter as himself, fleeing the point of a lance aimed at his rear.
DARK ADAPTATION AND VISUAL SENSITIVITY IN SHALLOW AND DEEP-DIVING PINNIPEDS1
Version of Record online: 26 AUG 2006
Marine Mammal Science
Volume 15, Issue 4, pages 1303–1313, October 1999
How to Cite
Levenson, D. H. and Schusterman, R. J. (1999), DARK ADAPTATION AND VISUAL SENSITIVITY IN SHALLOW AND DEEP-DIVING PINNIPEDS. Marine Mammal Science, 15: 1303–1313. doi: 10.1111/j.1748-7692.1999.tb00892.x
- Issue online: 26 AUG 2006
- Version of Record online: 26 AUG 2006
- Received: 5 January 1998. Accepted: 5 May 1999
- visual sensitivity;
- dark adaptation;
- elephant seal;
- harbor seal;
- California sea lion
Pinnipeds forage almost exclusively underwater. Consequently, observing them is difficult and relatively little is known of how they use their senses to locate prey, avoid predators, and navigate while diving. Vision has been presumed to be of primary importance, although previous measurements of visual functioning in pinnipeds have been restricted to just a few shallow-diving species. As diving pinnipeds experience rapid changes in light levels during descent/ascent and low light levels at depth, it has not been clear whether they possess visual capabilities adequate for use while diving, particularly in the case of deep-diving species. To examine this issue, behavioral psychophysics have been used to assess and compare the dark adaptation rates and relative light sensitivities of a deep-diving pinniped (northern elephant seal, Mirounga angustirostris), two shallow-diving species (California sea lion, Zalophus californianus, and harbor seal, Phoca vitulina), and a human subject.
In comparison to the human subject, both the California sea lion and the harbor seal dark-adapted relatively quickly and were more light sensitive. These findings suggest that both of these species are well suited for vision in the moderately dim shallow-water environments in which they dive to forage. In contrast, the elephant seal reached complete dark adaptation in less than half the time taken by the other pinnipeds, and it was significantly more light sensitive. Unlike the shallower-diving species, the visual abilities of the elephant seal are commensurate with the extreme conditions experienced while deep diving. Thus, we conclude that elephant seals are sufficiently adapted to rely on vision underwater, even while diving to depths in excess of 1000 meters where bioluminescence may be the sole source of ambient light.