The Magnetite Maps of Microchiropterans

There is a fascinating paper out in PLoS this week, describing a study that suggest that bats possess sensory cells containing magnetite, which allow them to navigate using the earth's magnetic field.

The question of how bats navigate has been an issue of considerable speculation for some time now. The authors point out that there are two alternative hypotheses: a light dependent method, or a biological mechanism incorporating magnetism. It is notable that the only other mammals known to use magnetic navigation have lifestyles that are almost the polar (no pun intended) opposite of bats: naked mole rats (Heterocephalus glaber) have been shown to use internal magnetite "compasses" to find their way around underground.

In 2006, Holland et al reported their findings that the big brown bat (Eptesicus fuscus) does in fact orient its flight using magnetic fields, but exactly how they accomplished this was still a mystery. Holland investigated the issue further, however, and the new paper suggests a fascinating mechanism that allows bats to take perceive the earth's magnetic field and use it to orient their flight.

So how do you test a chiropteran compass, anyway? The researchers used a method called a "Kalmijn-Blakemore remagnetization experiment," which has previously been used to study ferromagnetic bacteria. This basically involves exposing the animals to magnetic pulses which are either parallel or antiparallel to the "biasing field" (that would be the earth's magnetic field, in this case). Bats exposed to the antiparallel pulses would have their "compasses" reversed, and would be expected to fly the opposite direction of what they would need to in order to return to their roosts. As Holland et al note, this has been tried in birds, but the evidence did not support magnetite as the mechanism for their magnetoreception.

What did the results show when bats were tested? The anti-parallel exposed bats did indeed go the "wrong way", while the controls and the parallel pulsed bats took off in the right direction (back to roost) right away. There were a few outlier anti-parallel bats that did head directly, meaning they weren't "fooled" by the anti-parallel pulse, which the authors attribute to familiarity with the release site: if they already knew the way home, that could sometimes override their magnetic "map" (similar to human drivers that ignore their dashboard GPS units when they already know a quicker or more familiar route to the grocery store, or realizing if they are directed onto railroad tracks...). These exceptions are also instructive: it shows that the bats integrate different navigation strategies that are capable of over-riding each other in different situations.

Thus, we have strong evidence that the bats are navigating using the earth's magnetic fields. But we knew that from the 2006 paper...what we're interested in here is how. What is happening physically that is allowing bats to orient using the earth's magnetic field?

We still don't know the precise structural details of the magneto-receptor cells, but the authors have some extremely intriguing suggestions based upon their results and upon earlier studies of ferromagnetic perception in animals. The most beautifully simple model is just to couple a magnetosome (a cluster of magnetite crystals packaged into a membrane) chain to an ion channel, similar to the type found in the organ of Corti, which allows us to use the movement of cilia in our ears to hear sounds (see here for a review of how mechanically gated ion channels allow sensory information to be transmitted). By sending pulses through these chains, the polarity of the magnetite crystals would be reversed, causing them to "twist" and face the opposite direction. This would basically turn the bats' maps upside down and point them in the wrong direction, as appears to have happened in this study.

So, what is the takeaway message of this paper, what do we know now that we didn't know before? Holland et al (2008) have shown strong support for the presence of a specialized sensory cells containing magnetite, which allow bats to orient themselves and navigate using the Earth's magnetic field. We already knew from previous studies that bats do have magnetite-containing tissues (Buchler and Wasilewski 1985), but this experiment confirms that the magnetite is used for orientation by disrupting the bat's navigation capabilities after being exposed to polarity-reversing pulses.

This is just a slight aside, but what impressed me almost as much as the findings of this study were the methods. Seriously, make sure to read them. They used Eptesicus fuscus (pictured above), which we have met before in the Fieldwork Focus series (which I am going to revive as soon as mid-terms are behind me!). These guys have attitude, you have never seen a bat throw a temper tantrum until you've caught one of these beasties. I literally laughed out loud when I came to the methods section at the end of the paper: their apparatus consisted of Helmholtz coils, which they had to stuff the bats into in order to administer the pulses. This is the species of bat that sliced through one of my gloves while I was trying to get it out of a net (see picture at right, taken right before that happened, notice how many people it takes to managejust one E. fuscus in the net), I can't imagine the process of maneuvering one into a wire coil...this would be a great study to see posted on the Journal of Visualized Experiments (JoVE)... ;)

Anyway, it's always great to see bats in the news. Now that there is data strongly supporting a magnetite-based orientation system, hopefully future studies will examine the exact structure of the sensory apparatus being used, how the information is processed in the brain, and whether the mechanism varies between species.


(Note: I am not at all implying that the bats were treated cruelly, having dealt with this tough species before I have no doubt that if anyone came off worse for the wear, it was the human technicians!)

Buchler, E. R. and P. J. Wasilewski. 1985. Magnetic remanence in bats. in: Kirschvink, J. L., D. S. Jones, B. J. MacFadden, eds. Magnetite biomineralization and magnetoreception in organisms: a new biomagnetism. New Yorok: Plenum Press.

Holland, Richard A., Joseph L. Kirschvink, Thomas G. Doak, and Martin Wikelski. 2008. Bats use magnetite to detect the Earth's magentic field. PLoS ONE 3(2): e1676 doi:10.1371/journal.pone.0001676

Holland, R. A., K. Thorup, M. J. Vonhof, W. W. Cochran, and M. Wikelski. 2006. Bat orientation using Earth's magnetic field. Nature 444: 653.

(Image credits for compass, coil, and ion channel )