Sharks use earth's magnetic field as GPS
Birds, sea turtles, lobsters, newts and even bacteria are known to be able to sense the earth's magnetic field and navigate using the ability. While some animals use "magnetoreception" to derive only directional "compass" information and orient themselves north or south, others are known to also derive positional "map" information to know where in the migratory route they are situated. In essence, the ability works as a sort of GPS, thanks to the fact that the magnetic field of the earth varies predictably across the surface of the globe (Lohmann et al., 2007).
"The vector of the geomagnetic field provides animals with directional information, while intensity and/or inclination provide them with positional information."
Behavioural experiments done by Bryan Keller et al. has now found evidence for magnetoreception in the bonnethead shark. The ability was previously known to exist in the yellow stingray, a species closely related to, and belonging to the same subclass as sharks (Elasmobranchii). In the study report, the researchers indicate two features of sharks that made them prime candidates for the experiment. Firstly, tracking hammerhead shark movements in the ocean has revealed that they swim in the same direction for long distances and that the trajectories they follow are associated with geomagnetic features (magnetic maxima and minima). Secondly, bonnethead sharks show site fidelity - individuals return to their favourite site in estuaries or bays after migration (this is apt, as we shall see, for the experimental design used in the study).
For the experiment, the researchers brought 20 juvenile bonnethead sharks to the lab (captured from the coast of Florida), where they were allowed to freely swim in a tank that was surrounded by Merritt coils - an apparatus that simulates a customizable uniform magnetic field in the tank. The orientation of the sharks was recorded while being subjected to geomagnetic fields from:
1. the site of capture,
2. 600 kms south of the site, and
3. 600 kms north of the site
The researchers found that orientation of the sharks was random when the magnetic field resembled that of the site of capture, but was significantly northward when the magnetic field was south of the capture site; indicating that sharks can sense geomagnetic fields and tend to move toward their favourite site.
The orientation was once again random when the sharks were subjected to a northern geomagnetic field. The researchers speculate that this could be because the orienting behaviour is partly learned and because they have no experience being this far north of their capture site due to geographic restrictions.
Sharks in the Gulf of Mexico could learn that fields weaker than those at the capture site indicate more southward locations but would never experience stronger fields than the capture site and thus may not know how to respond to such conditions.
Read more about the study here.