Leopard sharks find their way with their noses, say scientists
Forget Google Maps 鈥� these sharks get directions from their noses.
A recent study led by Andrew Nosal, a聽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 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.
鈥淲e were not necessarily surprised to find that this remarkable navigation ability is mediated by smell, at least in part,鈥� Nosal says. 鈥淪harks 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.鈥�
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).聽Nosal and colleagues then tracked each shark鈥檚 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.
鈥淲e were amazed by the sharks鈥� ability to navigate back to shore 鈥� along nearly straight paths, no less 鈥� after being taken from their home, just outside the surf, and released in unfamiliar and hostile聽territory聽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.鈥�
But while it鈥檚 clear that scent is an important navigating tool for these sharks, researchers are still tracking down the specific mechanisms at work.
鈥淲e 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,鈥� Nosal says.聽鈥淎long the coast, productivity is high 鈥� meaning lots of life 鈥� due to upwelling, which is the process that brings cold, nutrient-rich water to the surface and fuels plankton growth.鈥�
鈥淭his is why seawater off California is often a greenish color close to shore, but becomes increasingly clear as you move offshore,鈥� he adds.聽鈥淭hus, we suspect the sharks are smelling something associated with productivity. For example, they may be detecting dissolved amino acids, the building blocks of proteins.聽The concentration of dissolved amino acids would increase with proximity to shore due to higher productivity there.鈥�
Other fish have the ability to detect productivity 鈥� salmon, for example, can home in on dissolved amino acids in river water 鈥� so it鈥檚 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.
鈥淧lanktivorous sharks, including basking and whale sharks, are believed to cue into dimethyl sulfide gradients to locate plankton blooms on which to feed,鈥� Nosal says. 鈥淭here is no reason to think sharks could not use dimethyl sulfide gradients to navigate more generally.鈥�
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.
鈥淓ven the movements of sharks with their sense of smell blocked were biased toward shore, suggesting other senses also play a role,鈥� Nosal explains.聽鈥淭hese could include hearing low-frequency sounds emanating from crashing waves or detecting geomagnetic fields.鈥�
But further research will be necessary to determine how these senses are integrated hierarchically, Nosal says.
鈥淲hich senses are most important?鈥� Nosal wonders.聽鈥淲hich senses are backups to other senses? This will also require more fieldwork and controlled laboratory studies.鈥�
