Leopard sharks find their way with their noses, say scientists

New research suggests that the olfactory sense plays a key role in leopard shark navigation.

|
Kyle McBurnie
The olfactory sense may play a key role in leopard shark navigation, according to a new study by researchers from Scripps Oceanography and the Birch Aquarium.

Forget Google Maps – these sharks get directions from their noses.

A recent study led by Andrew Nosal, a postdoctoral researcher at Scripps Institution of Oceanography and the Birch Aquarium, found that scent could be an important navigational tool for leopard sharks.

In a controlled experiment, sharks with unimpaired olfaction were able to swim back to their coastal habitats more efficiently than their scent-compromised counterparts. Dr. Nosal and colleagues published their findings Wednesday in the journal PLOS ONE.

Leopard sharks favor coastal waters and enclosed bays, but are capable of swimming to distant ocean sites with remarkable efficiency, often following nearly straight routes. But until now, it was unclear what force was guiding them.

“We were not necessarily surprised to find that this remarkable navigation ability is mediated by smell, at least in part,” Nosal says. “Sharks were already known for their keen sense of smell, and we knew other animals use smell for navigation, such as salmon, birds, and insects. It had simply not been experimentally demonstrated for sharks in the open ocean until now.”

To test this hypothesis, researchers captured 25 leopard sharks near the shoreline and released them more than 5 miles offshore. Roughly half the sharks had their sense of smell temporarily impaired (by stuffing petroleum-jelly-soaked cotton wool into the shark's nostrils). Nosal and colleagues then tracked each shark’s return journey using acoustic technology.

On average, control sharks ended up 62.6 percent closer to shore after four hours. By comparison, impaired sharks only made it 37.2 percent closer to shore. The unimpaired sharks also took significantly more direct routes than the impaired sharks.

“We were amazed by the sharks’ ability to navigate back to shore – along nearly straight paths, no less – after being taken from their home, just outside the surf, and released in unfamiliar and hostile territory in the middle of the open ocean. Even sharks that happened to swim away from shore initially made corrective U-turns within 30 minutes of release.”

But while it’s clear that scent is an important navigating tool for these sharks, researchers are still tracking down the specific mechanisms at work.

“We did not determine what exactly the sharks were smelling, but they were likely able to detect and respond to various chemical gradients that run perpendicular to shore,” Nosal says. “Along the coast, productivity is high – meaning lots of life – due to upwelling, which is the process that brings cold, nutrient-rich water to the surface and fuels plankton growth.”

“This is why seawater off California is often a greenish color close to shore, but becomes increasingly clear as you move offshore,” he adds. “Thus, we suspect the sharks are smelling something associated with productivity. For example, they may be detecting dissolved amino acids, the building blocks of proteins. The concentration of dissolved amino acids would increase with proximity to shore due to higher productivity there.”

Other fish have the ability to detect productivity – salmon, for example, can home in on dissolved amino acids in river water – so it’s possible that sharks could do the same. Leopard sharks might also be following a compound called dimethyl sulfide, which is also associated with high productivity.

“Planktivorous sharks, including basking and whale sharks, are believed to cue into dimethyl sulfide gradients to locate plankton blooms on which to feed,” Nosal says. “There is no reason to think sharks could not use dimethyl sulfide gradients to navigate more generally.”

Many shark behaviors, like feeding, use multiple senses in tandem. So while smell is certainly important to shark navigation, it is most likely not the only sense at work.

“Even the movements of sharks with their sense of smell blocked were biased toward shore, suggesting other senses also play a role,” Nosal explains. “These could include hearing low-frequency sounds emanating from crashing waves or detecting geomagnetic fields.”

But further research will be necessary to determine how these senses are integrated hierarchically, Nosal says.

“Which senses are most important?” Nosal wonders. “Which senses are backups to other senses? This will also require more fieldwork and controlled laboratory studies.”

You've read  of  free articles. Subscribe to continue.
Real news can be honest, hopeful, credible, constructive.
What is the Monitor difference? Tackling the tough headlines – with humanity. Listening to sources – with respect. Seeing the story that others are missing by reporting what so often gets overlooked: the values that connect us. That’s Monitor reporting – news that changes how you see the world.

Dear Reader,

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for $15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to CSMonitor.com.

QR Code to Leopard sharks find their way with their noses, say scientists
Read this article in
https://www.csmonitor.com/Science/2016/0107/Leopard-sharks-find-their-way-with-their-noses-say-scientists
QR Code to Subscription page
Start your subscription today
https://www.csmonitor.com/subscribe