by Andre on 2 March 2005
For the last couple of days I’ve had electricity and magnetism on the brain so I wanted to write something about E+M in biology, but not just about membrane potentials or inter-molecular interactions. The first thing that came to mind, after electric eels, was magnetic navigation in birds.
I had heard that birds (and apparently newts) can sense Earth’s magnetic field and use it to navigate, but I had no idea if this was or reality. It didn’t take long though to find this article published last year in Science by Cochran et al. (subscription required) where the authors describe their experiments on Catharus thrushes (nocturnal migratory birds). Experimental birds were “treated” with an eastward pointing magnetic field during twilight and then released. The treated birds flew west instead of north. So birds do migrate based on magnetic cues.*
But how do the birds sense a magnetic field? My first guess was that their tongues are actually bar magnets, but no one had ever noticed before. Being scientifically inclined, I probed cyberspace to see if anyone had a picture of a thrush’s tongue, but this was a mistake since thrush is also a white mouth fungus. People who have done some serious thinking about this problem naturally had some better ideas.
The first is not sooo different from mine. Researchers posit the existence of small particles of magnetite which would experience a detectable torque from the Earth’s magnetic field. A study last year in Nature, Mora et al. (subscription required) suggested that this is the mechanism responsible for orientation in homing pigeons. By selectively cutting nerves in the beak the authors also ruled out that the magnetoreception is directly linked with the birds’ visual system.
While this may be true for homing pigeons, an interesting piece of evidence that leads to the other major model for magnetorecption is that in many birds the ability is linked to the visual system. Before outlining their model, Ritz et al.** list a number of studies that found that some species of migratory birds lose magnetosensitivity while they are exposed to particular colours of light. The candidate model outlined in Ritz et al.’s paper relies on the fact that certain radical-pair reactions have magnetically dependent lifetimes that could be detected by the bird. They speculate that this could be perceived by the bird through a direction dependent modulation of the eye’s sensitivity.
So it seems that the jury is still out on the precise mechanism responsible for magnetosensitivity, but there’s no doubt that some birds use it as a navigational tool. It’s probably mistaken to look for the mechanism of magnetosensitivity anyway since different species seem to have evolved different detection systems, but this is common in biology. In fact, this contingent aspect of biology is the source of its richness and one of the reasons I find it so interesting. I look forward to seeing where this research leads.
*The authors went further, concluding that the birds use the setting sun to calibrate their magnetic compass. This explains, for example, why birds don’t confuse their direction when crossing the magnetic equator. It also should sooth people’s fear (at least in the case of thrushes) that a reverse of Earth’s magnetic field will wreak havoc on migrating birds.
**I liked this paper because it carefully addressed the problem at several relevant scales. It includes quantum mechanical calculations of the radical pair lifetime, describes a physiological model that would allow the effect to be perceived, and finally suggests a candidate molecule for the magnetoreceptor. The paper flows well, in my opinion, because of this well thought-out progression. It doesn’t feel like a least publishable unit.