Animals do the most amazing things. Read about them in this series by Janaki Lenin.
Marine animals’ ability to navigate across open seas has long been a mystery. Scientists now have an inkling of how sharks find their way. They follow their noses.
Underwater navigation is complicated because the terrain has no landmarks or paths. Everything looks the same. In the shallows and just below the surface, the sun may indicate where east lies, but it provides no more clues to underwater travellers.
How do white sharks find their way between California and Hawaii, a distance of more than 3,600 km? What map do they follow during their voyage? After all, they could easily go off course and arrive in Polynesia, 4,000 km to the south.
In 2007, Jayne Gardiner and Jelle Atema from Boston University, Massachusetts, suggested odour is the sharks’ primary sense, but that they may also rely on vision, water currents, electroreception and touch. Since sharks can’t tell in which direction an odour is coming from, they depend on the lateral line, according to the researchers.
The lateral line is a series of electrosensory organs located under the skin on the snout and along either side of the shark’s body. So crucial is this sense that sharks were unable to smell their prey when scientists neutralised their lateral lines.
“Odour itself is not directional,” Atema told The Wire. “Only odour gradients are directional. But it is well-known that extracting a reliable odour concentration gradient is very difficult, particularly over large distances.”
It’s one thing for sharks to use their proverbial sense of smell to track down delectable morsels. How would they smell a coast thousands of miles away?
The olfactory bulb, a part of the brain that decodes scents, in some species of sharks doesn’t scale in proportion to body size. Animals that travel long distances like tiger sharks and white sharks have outsized olfactory bulbs than the more sedentary reef sharks.
Based on this observation, neuroscientist Lucia Jacobs speculated in 2012 about the primary function of olfaction. “Instead of the main olfactory system simply telling the brain what something is, I proposed it is telling the brain both ‘what’ and ‘where,’ which is a much more powerful message,” Jacobs told The Wire. “When you have a map, you not only can store information on it but you can also extrapolate across space and time. So an odour plume, whether in air or water, can act as a sensory highway, which an animal can use to calibrate its navigation cues from other senses, such as vision.”
“In water, however, vision is only short-range. So for long-distance navigation, I proposed that olfaction could be a very important method for navigation.”
Andrew Nosal from the University of California, San Diego, and his colleagues conducted an experiment to test this hypothesis. They chose the leopard shark – a species with large olfactory bulbs – which congregates off the Pacific coast of North America.
At an average length of about a metre and half, leopard sharks are no danger to humans. They swim along the bottom of muddy or sandy flats, kelp beds, rocky reefs and the open coast. Although they stick close to shore, they also make occasional forays into the open sea.
Nosal and his team caught 11 female leopard sharks along the coast. To make sure the animals didn’t pick up any geomagnetic cues, the researchers hung a strong neodymium ring magnet 5 cm above the bottom of the holding tank. Ostensibly, the swinging and spinning magnet garbles the sharks’ magnetic reception. An opaque tarpaulin covered the holding tank so the fish couldn’t see where they were going. If the water in the holding tank was aerated by changing it, the sharks could pick up chemical cues from it. So the research team used compressed air. Finally, to disorient the animals, the team spun the boat in figures-of-eight before heading 11 km northwest to the release site.
At the release site, they plugged the noses of these sharks with petroleum jelly-soaked cotton wool balls. Sharks don’t breathe through their noses so the cotton wool doesn’t suffocate them. It merely prevents them from smelling and disintegrates over 24 hours. The researchers tagged the sharks with acoustic transmitters that were timed to pop off from the fish and float in four hours.
When Nosal and team released the animals, they expected the leopard sharks to make a beeline for the nearest shore, a distance of 9 km. The open sea is hostile zone for these creatures of the coast. It’s not a place to find clams, crabs and fish, and there are numerous predators abroad.
Unable to smell, these sharks found their way to shore only 37.2% of the time. They swam slowly and in convoluted paths, seemingly unable to find their way.
Earlier this year, Jayne Gardiner and her colleagues published the results of a similar experiment on young blacktip sharks in the Gulf of Mexico. The youngsters who couldn’t smell had a hard time returning to shore.
Nosal’s team caught another 15 sharks and put them through the same treatment: the magnet in the tarp-covered holding tanks, figure of eight spins, the works – except for one thing. They didn’t plug the sharks’ noses. The behaviour of these sharks would offer useful comparison to the disoriented ones with stuffed noses. The researchers called these sham-controls.
As Nosal expected, the tracks of these sham-control sharks ended 62.6% closer to shore in four hours.
What did the sharks smell that guided them to the shore?
The researchers don’t know for sure. Sharks perceive amino acids in faint traces. Nosal and his colleagues suggest amino acids released by plankton along the coast may act like beacons, guiding shore-bound leopard sharks. Salmon are thought to smell these same organic compounds to find their way to the streams where they were spawned.
Another fragrant candidate is dimethyl sulphide, a compound produced by phytoplankton and other marine algae. Seabirds, marine mammals and plankton-eating sharks use it to find their way.
River mouths, rocky shoreline, reefs, and kelp forests release different combinations of compounds. Every location along the coast may have its own signature smell.
How do sharks navigate by recalling the smell of their destination?
“When we released the sharks from shore, two things could have happened,” Nosal told The Wire. “One way olfaction could play a role is that the sharks simply detected a low concentration of some odourant. That indicated to the sharks that they were far offshore. Then perhaps they used other senses to find their way back to shore. In this case, olfaction is only important when the shark is first released.
The other possibility is that after they were released, they started swimming. If they were swimming in the direction of shore, the odourant concentration would get higher, and the sharks would know they were swimming in the right direction and continue that track. If instead the sharks detected the odourant concentration was decreasing as they swam, they would know that was the wrong direction and turn in a different direction and swim for a while to see if the concentration started to increase, i.e., the smell gets stronger.
“We actually observed this in several of the sham-control sharks. When they happened to be released in an offshore direction, they swam in that direction, but then within 30 minutes made a sharp U-turn and started to swim toward shore,” explained Nosal.
Besides odours, noise could also aid navigation. The low-frequency pounding of surf could indicate the shore. At the experiment location in La Jolla, surf noise is approximately 50 to 300 Hz, well within the hearing range of sharks. The experiment’s subjects swam near the bottom where sound travels slowest and farthest.
One of the cool findings of the experiment was a few of the sharks with unplugged noses “swam very constant depths toward shore, typically about 20-30 meters below the surface, even though bottom depths were 500 meters.” Nosal continues. “However, once the sharks passed over the continental shelf they made an abrupt and apparently deliberate dive down to the bottom as if they knew a bottom was there. Surely they could not have seen the bottom from 50 metres above. I wonder if perhaps they were using acoustic cues. I imagine the acoustics are fundamentally different over the shallow continental shelf than over the deep ocean and perhaps the sharks could hear that difference.”
The case for olfactory navigation
In 2014, Atema and his colleagues from Florida, U.S., suggested that when specific senses are disabled, sharks successfully identify, track, and capture prey by switching to alternate sensory systems.
Indeed, Nosal speculates the sharks with stuffed noses eventually found their way to shore using other cues. Although they couldn’t smell the chemical cues, perhaps they could taste them. However, Atema thinks taste is highly unlikely. Sharks could use the sun to provide directional information. But some leopard sharks appeared lost on sunny days. While other sharks navigated successfully on overcast or foggy days.
The sense of smell in animal navigation hasn’t been researched adequately. Homing pigeons were studied for many decades. Recent research shows the birds smell their way more than sense the earth’s magnetic fields.
Nosal and his team weren’t done. They picked up another 10 sharks and dropped them 19 km away, farther from the shore than the others, without messing with their senses. Since these sharks had numerous cues, the researchers expected them to find their way without a problem. But only six reached the shore in four hours. And surprisingly, four appeared lost and moved in tortuous paths as if they had stuffed noses. The researchers wonder if only experienced sharks are able to navigate accurately.
If 40% of normal sharks that had all senses about them were lost, is the case for olfactory navigation solid?
“There will always be outliers in data sets, which is why we average the results from individuals within groups and use statistics to compare them,” says Nosal. “We attribute [the lost normal sharks] to the farther location, where chemical cues may have been patchier such that only certain sharks and certain days were successful. So, the conservative conclusion that olfaction contributes to navigation is really based only on the sham-control and [deprived of smell] sharks released from the closer location. We cannot provide any definitive mechanism, but only hypothesise a few possibilities like cross-shore chemical gradients associated with coastal productivity [of the organic compounds].”
Navigating 19 km ought to be no big deal for a species that crosses the San Pedro Channel, separating mainland California and Santa Catalina Island, which is is 32 km wide at its narrowest and approximately 800 metres deep.
As the researchers say, they haven’t gotten to the bottom of shark navigation. This is the first study to use wild sharks to demonstrate if the sense of smell plays a role in open-sea navigation.
Search for an exact mechanism
“The novelty is that this study was done in the field with animals that can be tracked,” says Atema, who wasn’t involved in Nosal’s experiment. “Based on our own larval reef fish and shark work, I (and others) find it hard to believe that there can be a chemical gradient useful for navigation over 9 km with a predominantly longshore current. Currently, there is no evidence for an odour plume in that area of the ocean. However, that does not mean there is no useful chemical information. It means we now need to find evidence for or against such a spatial gradient. And that is exciting. I can think of a number of possibilities.”
Nosal says he plans to determine the exact mechanism for navigation in future, including what other senses are involved and how they work in tandem.
Does the lateral line that is so vital for sharks to find their prey play no role at all in navigation?
“The lateral line plays a role in sensing currents and integrating current direction with odourants,” says Nosal. “So, at very small scales, sharks could perhaps detect a scented water plume, like a small current. The mechanism we have proposed is that the sharks may be following larger scale chemical gradients, which would not require the lateral line.”
Lucia Jacobs, who hypothesised about the role of smell in navigation, found this study interesting. “I have suggested that how an animal navigates will depend on the spatial scale of the stimuli,” says Jacobs. “Rapidly swimming or flying animals should in fact rely more heavily on olfactory navigation because it may take long distances to map an odour plume accurately. So this could be a very interesting behaviour to study in sharks.
“Finally, as others have said, sharks are fascinating because despite the length of time they’ve been around, no other group has managed to displace them. They’re so good at what they do. Perhaps a big part of this is their olfactory navigation system.”
The study was published on January 6, 2016, in the journal PLOS ONE.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.