科学美国人60秒:Sharks Head Straight Home by Smell

Some sea creatures can find their way through thousands of miles of seemingly featureless oceans. Even more impressive is the route that they take.  

“Well, we’ve known for a long time that sharks are capable of long-distance migrations, and in some cases those migrations occur along very straight paths.” Scripps Institution of Oceanography biologist Andy Nosal. “And this has always begged the question: how exactly do they know where they’re going?...So, there have been a lot of hypotheses floated over the last several decades, including the idea that these sharks are using, for example, geomagnetic cues, chemical cues and others. But none of these have really been systematically tested in the field.”

Nosal and his team suspected that the navigational secret of some sharks might be their sense of smell. They use their keen noses to find food, of course. And other fish, like salmon, are known to use olfaction to navigate.

To see if his hunch was right, Nosal scooped up some adult female leopard sharks in their preferred environment, waist-deep water off the San Diego coast. He attached a small radio transmitter behind their dorsal fins. And he blocked the sense of smell in half of the sharks by shoving cotton balls soaked with petroleum jelly in their nostrils.

Nosal and his crew dropped the sharks about six miles away at a spot in the open ocean. The researchers then tracked the sharks as they tried to swim back home. Four hours later, the sharks that could smell were two-thirds of the way back home—and had swum in very straight paths. But the ones with stuffed noses took erratic routes and only made it about half as far.

Nosal thinks they all eventually got back home. “And we know that because even though their movements, even though they didn’t get as close to shore and even though their paths were more wind-y, their movements were still biased towards shore. On average they still finished closer to shore than when they started.” And those jelly-soaked cotton balls eventually disintegrate, returning the sharks to normal. The experiment is the first to show that sharks can indeed sniff their way back home. [Andrew P. Nosal et al, Olfaction Contributes to Pelagic Navigation in a Coastal Shark, in PLoS ONE]

But it also shows that they must have some kind of back-up system that lets them that lets them navigate non-nasally. For Nosal, the next questions are: What’s the back-up system? And if sharks navigate by smell, what exactly is it they’re smelling that leads them—by the nose.

—Jason Goldman