Loggerhead turtles use the earth's magnetic field to navigate. Click to visit the Lohmann Lab at UNC to learn more.
When I first heard the term “biomagnetism” it captured an obscure fantastical corner of my imagination. But the reality of magnetic sensing in all sorts of animals — from butterflies to sea turtles to whales — and the ways they use it is the sort of science that rivals the most brightly colored dreams of sci-fi writers. About two weeks back, I mused on the mechanism driving the earth’s magnetic field and what causes magnetic field reversals. When these flipping events occur, the earth’s magnetic field wans in intensity. Periods of declining magnetism are born out for hundreds to thousands of years, with sometimes wild fluctuations in the magnetic field lines. But what effect does declining or fluctuating magnetism have on species that rely on the earth’s magnetosphere like a global positioning system device? And how do they sense magentic fields anyways?
I first started wondering about these things when a series of quirky news items caught my eye, all involving animals and our earth’s magnetic field. Last August, NPR reported on a study that purported evidence for bovine magnetism. Researchers using Google Earth found that the majority of the time, more than happenstance anyways, cows will orient themselves on a north-south axis when laying down in a field or when grazing. The German research team, led by Hynek Burda of the University of Duisburg-Essen, also found this to be true in deer. The big question is, Why would cows and deer need this, or is it a remnant mechanism from an ancestor? Then I read a few reports about whale beaching events in the Atlantic Ocean and researchers theorizing that perhaps whales follow magnetic lines in the seafloor that run roughly north-south. But there are sometimes fractures where magnetic lines run perpendicular to the main path, and one hypothesis is that if whales are following these magnetic highways, perhaps they sometimes take a wrong turn onto a perpendicular magnetic side-street — or perhaps solar flares scramble the magnetic field, causing them to get off course — and follow it right up to the point where they beach themselves. (For an overview on magnetic sensing in elasmobranchs and cetaceans, this sensory biology page may be useful, with the whale discussion at the very end.) Even though researchers have not been able to prove for sure that whales do this (or how), they have documented bacteria that live in seawater orienting and swimming along geomagnetic field lines, a trait called “magnetotaxis.” I’ve only read the abstract, but in a recent paper in Applied and Environmental Microbiology, Christopher T. Lefevre et al. show that some of these microbes swim in an axial alignment in both directions toward either the north or south pole, while others swim only uni-directionally, with a clear preference for one pole or the other depending upon which hemisphere they live in. {1} They also conclude “oxygen is a determinant factor that controls magnetotactic behavior.”
Looking to animals that researchers know use magnetoreception* actively, for migration and moving around, there are lots of examples in birds, bats, moles, salmon, butterflies, turtles and whales. (*I learned this is what the cool kids call magnetic detection and orienteering… apparently “biomagnetism” was hijacked by new-agers hawking healing crystals.) Wired magazine very recently ran an article covering new research on magnetoreception in birds, that delves into the mechanisms of how their inner compasses work (Klaus Schulten, June 2009, Biophysical Journal, “Magnetoreception through Cryptochrome May Involve Superoxide”) {2}. According to this idea, birds have a light-sensitive protein in their eye that works in tangent with a specific kind of oxygen molecule called a superoxide “to form an in-eye compass,” (article by Brandon Keim). This is really a fascinating little piece, and it skims research seeking to prove the “superoxide theory” which predicts that a biochemical reaction occurs in birds’ eyes, which allows them to literally see a visual directional cue based on the geomagnetic field. For more on the history of scientific observation of magnetic sensing in birds, read the introduction to this paper by Klaus Shluten, et al., “A Model for Photoreceptor-Based Magnetoreception in Birds.” {3} Also, check out this Neurophilosophy blog post, “Homing in on magnetoreception” which covers some experimental work examining the flight patterns of GPS-tagged homing pigeons flying through a natural magnetic anomaly. As if that wasn’t neat enough, another paper describes the basis of two different types of magnetoreceptors in birds — one in the eyes, as covered above, and “one based on magnetite near the beak.” {4} Magnetite is a mineral that, like it sounds, has magnetic properties. But it is also characterized as being directionally-dependent, with the directionality being imposed by an external magnetic field. Magnetite was also the main mechanism in the marine bacteria’s sensing, they synthesize it and it is located along their axis. Biogenic magnetite crystals have been shown to swing like a needle due to external magnetic fields, and may help to trigger position sensors. Magnetite is also found in many other animals including… us. It was first documented in human brain tissue in 1992 by Kirschvink et al., in an article published in the Proceedings of the National Academies of Sciences. {5}
All of this is fascinating, but what happens when the earth’s magnetic field declines, or when it flips entirely? As I tried to ply the scientific literature for an answer, I ran into several abstracts from studies in the 70s and 80s seeking to find the relationship between mass extinctions and magnetic reversals, but there does not appear to be a unified opinion about this. Seems the scientific jury is still out. (Anyone know of more recent work?) I did find a few papers that found strong correlations between the extinction of one or more species and magnetic reversals, so it appears there is some effect whether direct or indirect. And even though I thought this would be a two-post series, it may need to stretch to a third so I can do some more searching in the literature. Stay tuned…
NOTES:
{1} Lefèvre, Christopher T. et al. 2009. “Characterization of Bacterial Magnetotactic Behaviors by Using a Magnetospectrophotometry Assay.” Applied and Environmental Microbiology. (75):12, pg. 3835-3841
{2} Schulten, Klaus. 2009. Biophysical Journal. “Magnetoreception through Cryptochrome May Involve Superoxide” (96):12, pg. 4804-4813
{3} Ritz, Thorsten, Salih Adema and Klaus Schulten. 2000. “A Model for Photoreceptor-Based Magnetoreception in Birds.” Biophysical Journal. (78):2, pg. 707-718
{4} Mouritsen, Henrik and Thorsten Ritz. 2005.”Magnetoreception and its use in bird navigation.” Current Opinion in Neurobiology. (15):4, pg.406-414
{5} Kirschvink, J.L., A Kobayashi-Kirschvink, and B J Woodford. 1992. “Magnetite biomineralization in the human brain.” Proceedings of the National Academies of Science. 89(16): 7683–7687
Perhaps, gives new potential meaning to the incantation, “may the force be with you?”
Interesting. I once belonged to a group that met monthly, called “lunch with a genius”, that allowed a USF professor a venue to present something that was being intellectually investigated at the time. Sadly, the program was discontinued, but I miss it. Lots of things to be interested in.
Hi Paul, Funny! Thx for stopping by … that *is* too bad the program ended, always neat to hear what’s going on at universities (at least, I think so!).