Marine scientists now believe that sharks – just like migrating birds, wildebeest and salmon – are using their noses to find their way home after long trips away.
It has long been known that sharks have a keen sense of smell to detect prey, but two new experiments are the first to prove that sharks also smell tiny amounts of chemicals in the water as a homing device.
In a paper published today in PLOS One, Andrew Nosal, a post-doctoral researcher at the Scripps Institution of Oceanography and the Birch Aquarium in La Jolla, Calif., found that the leopard sharks who were taken several miles off the San Diego coast were twice as likely to find their way home if they were able to smell.
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Those with their noses blocked – by harmless cotton balls and petroleum jelly – sort of wondered around, according to Nosal.
"What was amazing is that sharks that could smell find found their way back to shore no problem," Nosal said.
The experiment is fascinating in itself, using 15 leopard sharks that could smell, and 11 with blocked nostrils.
"We kidnapped them from their home, put them in a small holding tank, on the way out in the boat," he said. "We wanted to make sure the sharks couldn't retrace their steps. We covered the tank with opaque tarp so couldn't see sun, we aereated from a scuba tank, so there were no chemical cues. And we hung a strong magnet so no chance they could use geomagnetic cues. Yet upon release they could come straight back."
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Every summer, thousands of leopard sharks congregate in the shallow waters off La Jolla Shores Beach, a popular place for children and families and who can watch the sharks with a mask and fins.
These pregnant female sharks have found a sweet spot of sorts where currents arising from an underwater canyon cancel each other out, giving the sharks a less turbulent, quiet, and slightly warmer spot to incubate their babies.
Nosal has been studying the leopard shark for several years. He says that coastal sharks that migrate, such as the leopard sharks, have larger olfactory bulbs than those who do not. The olfactory bulb is a sense organ in the brain that processes chemical signals through the nose.
"Sharks with coastal migration those tend to have larger olfactory bulbs than expected for their size," Nosal said. "All these things pointed to smell as a way to navigate and it hadn't been demonstrated until now."
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So what are they smelling? Nosal said they could be following gradients of plankton or amino acids that make up prey in the water. The sharks' home area likely smells different than the open ocean several miles out to sea.
We are hypothesizing that these sharks are following chemical gradients," Nosal said. "These gradients are correlated with coastal productivity, we have upwellings that fuel a lot of growth and that sustains a lot of organisms. There could be chemicals associated with increased productivity."
Researchers in Florida's Tampa Bay found similar results in tracking young blacktip sharks.
Jayne M. Gardiner, assistant biology professor at the New College of Florida, published a paper in 2015 in Integrative and Comparative Biology that found sharks with blocked noses also had trouble finding their way back to their home territory.
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Gardiner said that more evidence is coming in that smell is just as important as other environmental clues to help sharks migrate from places where they feed to places where they raise theier young.
"We have bits and pieces from various animals, but we don't have the whole story yet," Gardiner said. "Most of us are in agreement is that it's a fairly similar story across different animals."