How do birds sense the Earth’s magnetic field?
If there is a physical property in the world that provides useful information, chances are that at least some animals will have evolved a sensory system to exploit it. The Earth’s magnetic field is no exception: it provides useful and reliable information to navigate the globe. There has been extensive research into whether and how animals may use the magnetic field for navigation, and although most agree that it is being used, the nature of how this information is processed can still be a matter of heated debate.
Thorup and Holland in a recent commentary on animal navigation in Journal of Experimental Biology state that:
[…] there are more reviews published on the subject than there are experiments providing evidence for the hypothesis at present
One more study by Zapka and collaborators has now been added to the mix, published in Nature last week. Here, they looked at the neural basis of the magnetic-based orienting behaviour in migratory European robins.
For birds, there are two theories of how magnetic information may be detected (magnetoreception), and data in favour and against each are not scant.
One theory proposes that there are specialized structures in the upper bill of the bird that can detect magnetic information, which is then carried to the brain through a branch of the trigeminal nerve. The other theory says that magnetic information is sensed through specialized molecules in the back of the eye, and that the information is processed by the visual system.
Several lines of evidence support the first theory. First, you can find magnetite in the upper bill of pigeons, in structures that appear to be contacted by the trigeminal nerve. Second, cutting the trigeminal nerve prevents pigeons from learning to behaviourally discriminate between the presence or absence of a magnetic anomaly.
The second theory proposes that light sensitive molecules known as cryptochromes in the eye are sensitive to magnetic fields. A molecular transformation sensitive to magnetic fields is then transmitted to a specialized visual area in the forebrain called Cluster N.
To distinguish between these two alternatives, Zapka and the group studied how the European robins performed a compass orientation behaviour when either the trigeminal or the visual ‘magnetic’ pathway (Cluster N) was destroyed. What they found was quite straightforward. Cutting the connections of the trigeminal nerve between the beak and the brain, did not have any effect in the behaviour. But when they destroyed Cluster N, then the birds were no longer able to show magnetic compass orientation. This rules out the need to use the trigeminal pathway for this orienting behaviour, and instead suggests that the responsibility of the neural processing is on the visual pathway. (Although it would be nice to see some electrophysiology to rule out this is not just a result of the lesion itself that may not be specific to cluster N).
So how can this be reconciled with the trigeminal data?
It comes down to what question the experiment is actually asking. This particular study looks at a very simple question: do the birds use one or the other system to do “compass orientation”. The studies in pigeons asked “is the trigeminal system necessary to learn a magnetic discrimination task”. They are not incompatible results (even less when you think they are different types of birds).
The magnetic vector provides a compass; magnetic intensity and/or inclination play a role as a component of the navigational map.
The question that still remains to be answered is whether pigeons use the trigeminal system to navigate. Although pigeons follow magnetic contours as they home, cutting the trigeminal nerve does not prevent them from homing. This means that the trigeminal system may not be necessary for that aspect of navigation per se, and perhaps (as the Mora and collaborator’s paper suggests) it may be involved in some aspect of learning associated with magnetic fields. Hopefully, more funding will be available to these groups so they can sort this one out.
Zapka M, Heyers D, Hein CM, Engels S, Schneider NL, Hans J, Weiler S, Dreyer D, Kishkinev D, Wild JM, & Mouritsen H (2009). Visual but not trigeminal mediation of magnetic compass information in a migratory bird. Nature, 461 (7268), 1274-7 PMID: 19865170doi:10.1038/nature08528.
- Wolfgang Wiltschko and Roswitha Wiltschko. Magnetorececption in birds: two receptors for two different tasks. Journal of Ornithilogy 148:S61-S76 (2007)
- Cordula V Mora, Michael Davison, J Martin Wild and Michael M Walker. Magnetoreception and its trigeminal mediation in the homing pigeon. Nature 432, 508-511 (25 November 2004)
- A Gagliardo, P Ioale, M Savini and JM Wild. Having the nerve to home: trigeminal magnetoreceptor versus olfactory mediation of homing in pigeons. The Journal of Experimental Biology 209, 2888-2892 (2006)
- Miriam Liedvogel, Kiinori Maeda, Erik Schleicher, Thomas Simon, Christiane R Timmel, PJ Hore and Henrik Mouritsen. Chemical magnetoreception: Bird Crytochorome 1a is excited by blue light and forms long-lived radical-pairs. PLoS One 2(10) e1106 [Open Access]
- Dominik Heyers, Martina Manns, Harald Luksch, Onur Gunturkin, Henrik Mouritsen. A visual pathway links brain structures active during magnetic compass orientation in migratory birds. PLoS One 2(9) e937.[Open Access]
- Kasper Thorup and Richard A. Holland. The bird GPS – long-range navigation in migrants Journal of Experimental Biology 212, 3597-3604 (2009) doi: 10.1242/jeb.021238
Disclaimer: Martin Wild, an author on the recent Nature study is my collaborator and my Head of Department. Some of his work on this area was funded by the Marsden Fund.