Sharks: Navigation Depends on Noses?

What makes sharks swim straight? Researchers say they may be following their noses. For instance, sharks with a blocked sense of smell wandered the ocean in a recent study, whereas those with clear olfactory passages proceeded fairly directly home.

The scientists hypothesize that the sharks in the study, leopard sharks, are guided by the scent of chemical gradients picked up from the "coastal productivity" that occurs alongside shore, which is different from the smell of the open ocean.

Researchers from the University of California San Diego and elsewhere published this and other findings recently in the open-access journal PLOS One.

In the study, 25 leopard sharks were captured near the shore. Around half had their noses blocked with harmless cotton balls or petroleum jelly, as Discovery reported. The sharks were all moved 9 km offshore, let go, and tracked using acoustical methods for about four hours each.

In the resulting scenario, the sharks without impairment of their sense of smell were 62.6 percent nearer to shore at the end of the four hours, having followed fairly unwavering paths. Those that were, well, all stopped-up with cotton or petroleum jelly were only 37.2% nearer to shore than their starting point in the open ocean, and they had followed wandering paths, according to a release.

Andrew Nosal, a post-doctoral researcher at the Scripps Institution of Oceanography and the Birch Aquarium in La Jolla, Calif., said in the article: "Although chemical cues apparently guide sharks through the ocean, other sensory cues likely also play a role. Future work must determine which environmental cues are most important for navigation and how they are detected and integrated."

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