Lots of animals use the earth’s magnetic field to help them find their way around. They range from newts, lobsters and ants to birds, fish and marine turtles. So it looks as if magnetic navigation has a very ancient evolutionary lineage.
But we still don’t know for sure how it works.
As I explain in ‘Incredible Journeys’, there are two main theories - which are not mutually exclusive.
The first is fairly simple. The idea is that animals make use of particles of magnetic minerals (typically magnetite) inside their bodies. As the animal moves through the earth’s magnetic field these particles are subjected to minute forces that twist or pull on them.
If the animal has sensory nerves hooked up to the particles, it may be able to detect these forces and infer something useful about the character of the surrounding field. Some scientists think that a mechanism like this could even enable an animal to work out roughly where it is, though that is quite controversial.
A very different theory has been attracting a lot of attention recently (partly perhaps just because it’s so difficult to pin down). The idea here is that a light-dependent subatomic process may be involved.
Certain molecules (cryptochromes) react to the impact of a photon of light by briefly generating a so-called ‘radical pair’ of electrons. In theory, the behaviour of such a ‘radical pair’ could be affected by the orientation of the surrounding magnetic field. If these extremely subtle changes could somehow be picked up by the animal’s nervous system, they might provide the basis of a biological compass.
It’s even been suggested that migrating birds (which have cryptochrome molecules in their eyes) might ‘see’ the surrounding magnetic field superimposed on their visual field - a bit like a pilot’s head-up display. Wow! Well, at the moment this remains only a promising theory.
But there is another intriguing possibility that I allude to briefly in my book. It hasn’t received much attention until recently but it’s just received a bit of a boost. This involves the principle of electromagnetic induction.
If an electrical conductor is moved through a magnetic field a current is induced within it (this is how dynamos work). Oddly enough, it looks as if the fluid-filled semi-circular canals of the inner ear may operate in a similar way.. A new piece of research from David Keays’ lab in Vienna suggests that electric currents induced in the semi-circular canals of the homing pigeon could be the basis of the magnetic compass which the birds use.
It’s early days, but if this turns out to be right, it will be a very significant breakthrough.
Watch this space…