Updated: Dec 13, 2021
Many species of sharks are capable of incredible, long-distance migrations; traversing borders of multiple countries, crossing ocean basins and nearly circumnavigating the globe. Many are even capable of returning to the exact same location, with perfect accuracy, year after year (this phenomenon is known as "site fidelity" or "philopatry"). It has long baffled scientists how sharks know where they are going during these migrations and how they find their way... Some have suggested they navigate using the sun or the stars, or using underwater bathymetry in the oceans depths... But some recent work has given us a little more insight into how sharks are capable of such amazing feats... It seems they use the Earth's magnetic field to navigate their way around the globe!
Our planet Earth is made up of semi-solid rock, which is constantly shifting and flowing. The core, made up primarily of the metals iron and nickel, is under extreme pressure and incredibly high temperatures. The heat which naturally escapes from this core causes "convection currents" (flow of a liquid as a result of forces), which generate electrical currents. It is these currents which cause the planet as a whole to have a magnetic field called the "geomagnetic field". It is this geomagnetism what gives us our magnetic north that a compass points towards.
There is some argument about whether human beings can detect this magnetic field somehow, but one thing is for certain; many different species of animals, like pigeons, sharks and even bacteria, can sense geomagnetism...
Sharks have an absolutely remarkable sensory system! Including all five senses that human beings enjoy, they have two additional senses. They are also able to detect water displacement and changes in pressure through their "lateral line", and are also able to sense electromagnetic fields through their "ampullae of Lorenzini". These jelly-filled pores, present in high concentrations around the snout and face, are capable of detecting minute electrical and magnetic fields within the water, and transmitting this signal directly to the brain. This allows them to sense the muscle contractions and even the heart beat of their prey, which is especially helpful if said prey is very good at hiding in coral or sand. It also allows them to detect geomagnetism (Keller et al, 2021).
It has long been speculated that this electromagnetic sense might be responsible for how sharks navigate over long distances. So, in order to test this hypothesis, a group of research scientists performed "magnetic displacement" experiments on bonnethead sharks (Sphyrna tiburo), which they had caught in the Gulf of Mexico and transported back to their lab. They placed the sharks in a tank which was situated between electromagnets (known as "Merritt coils") that could generate an electromagnetic field. They started by simulating the same electromagnetic conditions experienced at the location where they had collected the sharks, and then changed the field intensity so that it mirrored conditions 600 km south and then 600 km north of the capture site. These areas had a relatively weaker and stronger magnetic field intensity respectively. They then set up a camera and watched how the sharks reacted to the different magnetic fields (Keller et al, 2021).
They found that when the sharks were exposed to the control magnetic field, which mirrored the foraging grounds where they were caught, they swam around the tank in random directions. If you take a look at the diagram below - where each circle represents the direction that the sharks chose to swim (called the "inclination angle") and each dot represents a different individual shark - you can see that under the control conditions [middle] and the stronger northern magnetic field [top], the spots are scattered. Under these conditions each shark tried to swim a different way. However, when the sharks were exposed to the weaker southern electric field [bottom], we see all the spots concentrated in one corner, meaning that they all tried to swim in a northwards direction (Keller et al, 2021).
These findings suggest that, when they were exposed to the weaker magnetic field intensity, which they would experience when they were further south from their foraging grounds, the sharks used the geomagnetism to turn themselves around and head northwards. So even when they were displaced, the bonnetheads were using geomagnetic fields to orient themselves, in order to try to find their way home (Keller et al, 2021).
This work shows us that these little sharks possess what is known as an "innate magnetic map". This means that bonnetheads are born with the capability to detect magnetic fields and and they just innately know how to respond to these cues for navigation! So cool! How do they know!? (Keller et al, 2021).
"Bonnethead sharks derive spatial information from geomagnetic cues."
- Keller et al, 2021
Whatsmore, it also shows us that these sharks are capable of learning! The researchers theorised that the reason the bonnetheads swam in random directions when they were exposed to the stronger, northern magnetic field, was because they learn how to navigate using geomagnetism based on their own experiences. In their natural habitat, these sharks would figure out they needed to head back north if they wandered out of their foraging grounds and would learn to identify the associated changes in geomagnetic intensity. However, the stronger geomagnetism associated with the more northerly direction would be unfamiliar to them because they have never swum up onto the land and into the middle of the continental United States! Under these unfamiliar conditions, they did not know how to orient themselves towards home (Keller et al, 2021).
We are only just beginning to understand how sharks use their electrosense for navigation. I suspect there will be many other fascinating findings, in many other species, where we learn that sharks are able to use this super-power to perform many other amazing feats. There are always more mysteries to solve...