Foxes use the Earth’s magnetic field as a targeting system

By Ed Yong | January 11, 2011 7:00 pm

It’s winter on a British meadow, and a red fox is on the prowl. The snow-covered ground masks the sight of its prey but the fox can still hear the telltale rustle of a mouse. It creeps forward slowly, listening intently with erect ears. Once it pinpoints the mouse’s location, it leaps into the air to surprise its prey with a strike from above. This pounce, known as ‘mousing’, is a common sight but there’s more to it than meets the eye. Jaroslav Červený has found that when red foxes pounce, they mostly jump in a north-easterly direction. He thinks that they’re using the Earth’s magnetic field to hunt.

Červený spent over two years studying wild red foxes in the Czech Republic, with the help of a 23-strong team of wildlife biologists and experienced hunters. The team recorded almost 600 mousing jumps, performed by 84 foxes at a wide variety of locations and times.

They found that foxes strongly prefer to jump in a north-easterly direction, around 20 degrees off from magnetic north. This fixed heading was important for their success as hunters. They were more likely to make a kill if they jumped along their preferred axis, particularly if their prey was hidden by high cover or snow. If they pounced to the north-east, they killed on 73% of their attacks; if they jumped in the opposite direction, they success rate stayed at 60%. In all other directions, only 18% of their pounces were successful.

Could the foxes be taking their direction from the environment? Červený thinks not. He found that the animals leapt in the same direction regardless of the time of day, season of year, cloud cover, or wind direction. Červený thinks that the only remaining explanation is that foxes align their pounces to the Earth’s magnetic field.

John Philips, who studies magnetic senses at Virginia Tech, says “The challenge (and the fun) for anyone interested is to come up with [another] explanation that can explain the data. The strength of the authors’ conclusions are only as good as the inability of anyone else to come up with an alternative hypothesis.”
Roswitha Wiltschko, one of the pioneers who deciphered the magnetic sense of birds, says, “The findings are really astonishing.” But she also thinks that they’re speculative. “This explanation… has plausibility only because there’s hardly any other mechanism that indicates directions.”

Červený suggests that a red fox could use the Earth’s magnetic field as a “rangefinder”, to estimate the distance to its prey and make a more accurate pounce. This targeting system works because the Earth’s magnetic field tilts downward in the northern hemisphere, at an angle of 60-70 degrees below the horizontal. As the fox creeps forward, it listens for the sound of a mouse. It’s searching for that sweet spot where the angle of the sound hitting its ears matches the slope of the Earth’s magnetic field. At that spot, the fox knows that it’s a fixed distance away from its prey, and it knows exactly how far to jump to land upon it.

Philips explains it very well. He says, “Imagine you had a flashlight attached to your belt that was pointed down at the ground at a fixed angle of say 60 degrees. The beam of the flashlight would hit the ground at a fixed distance in front of you.  If you were trying to determine the exact location of a sound source coming from the ground in front, you could approach until the beam was exactly superimposed on the sound source. This would place you at a fixed distance from the source. As you attacked such targets again and again, you could perfect a highly stereotyped leap that precisely lands you accurately on the target.”

This would explain why the direction of the pounce matters most when the prey is hidden. If the fox can see its quarry, it can easily estimate distance using its eyes. But if its view is obscured by grass or snow, it needs other senses. If Červený is correct, then the red fox is unique in many ways. It would be the first animal known to use a magnetic sense to hunt, and the first to use magnetic fields to estimate distance rather than direction or position.

Many living things can sense magnetic fields. I wrote a feature for New Scientist last month which explores how birds – the best-studied magnetic detectors – do it. Other animals have a magnetic sense too, such as sharks and rays, turtles, ants, lobsters, beetles, bats and mole rats.

The list also includes cow and deer. In 2008, Červený’s group, led by Hynek Burda, found that herds of cow and deer also tend to align in a north-south line like living compass needles. Spying on the animals with Google Earth satellites, Burda’s team showed that they tend to face magnetic north regardless of wind strength, time of day, or the position of the sun. A year later, they found more evidence that these animals are influenced by a magnetic sense: their neat lines could be disrupted by high-voltage power lines, which produce strong magnetic fields. The nearer the herds get to the lines, the more chaotic their positions.

In all of these cases – be they cows of birds – it’s not entirely clear what the point of having a magnetic sense is. For example, it’s reasonable to think that magnetic compasses and maps could help migrating animals to find their way, especially when visibility is poor or landmarks aren’t obvious. That makes sense, but there’s little hard data to back it up. Červený’s study is one of the first to demonstrate a clear benefit – red foxes hunt more successfully if they jump in the right direction.

How might a fox’s magnetic sense work? No one knows and that’s the killer piece of evidence that Červený lacks. It’s not an easy question to answer either. Magnetic senses have been studied most thoroughly in birds, and after decades of research, scientists are only starting to uncover its inner workings. It is, perhaps, the most fiendish of all senses to study.

You can’t deliberately switch off the Earth’s magnetic field or tune it down. Other senses are linked to openings in the body like eyes, ears and mouths, but there’s no obvious place to look for a magnetic sense organ because magnetic fields are pervasive and pass through the body. And a magnetic sense typically works together with others such as vision or (in the case of the fox) hearing; isolating it is at best difficult and at worst irrelevant.

Nonetheless, we can make guesses about how a fox’s magnetic sense might work. Aside from sharks, animals sense magnetic fields using one of two basic methods. The first involves clustered crystals of magnetite, an iron mineral that line up according to magnetic fields. Depending on their direction, the crystals either repel or attract one another, creating tiny forces that could be picked up by proteins. The moving crystals could even open or close molecular gates on the surface of nerve cells. Either way, the crystals convert a magnetic field into a nervous signal.

The second method is used by birds and involves a molecule called cryptochrome, which is found in the retina. When light strikes cryptochrome, it shunts an electron over to a partner molecule called FAD. The result is a pair of ‘radicals’ – molecules with a solo electron. These unpaired electrons have a property called “spin” and they can either spin together, or in opposite directions. The two states can flip from one to another, and they lead to different chemical outcomes. This is where the Earth’s magnetic field comes in: it acts like a switch that influences the flips. In doing so, it can affect the outcome of the radical pair’s chemical reactions.

All of this happens in the eyes of common birds, such as robins or warblers. This is why you can deactivate a robin’s internal compass by blindfolding it. In fact, you could make it lose its bearings by blindfolding just its right eye, or covering it with a frosted goggle. Some scientists have suggested that robins and other birds can literally see magnetic fields, as a sort of heads-up display. The fields could appear as light or dark patches (or even colours) that lay on top of what the bird normally sees.

It’s tantalising to imagine foxes hunting with just such a display, and Červený mentions the possibility. However, that’s just speculation. As Wiltschko points out, we only know that the radical pairs contribute to the magnetic sense of birds – they’ve never been found in mammals. But at least now, we have an obvious place to look.

Ken Lohmann, who studies the magnetic senses of turtles and lobsters, thinks that Červený’s next move should be to disrupt the magnetic field around the fox to see if that interferes with their ability to catch mice. “If this is the case, then it would greatly strengthen the conclusions,” he says.  “But doing such a study will obviously be very challenging, given that it would involve altering the magnetic field over a large, outdoor area!”

http://www.youtube.com/v/fF2H2i4b6Io

Reference: Červený, Begall, Koubek, Novakova & Burda. 2010. Directional preference may enhance hunting accuracy in foraging foxes. Biology Letters http://dx.doi.org/10.1098/rsbl.2010.1145

Image by Eric Magnuson

More on magnetic senses:

Comments (34)

  1. Sam

    Has anyone done a study into how birds escape prey in fight/flight responses? Would it be possible that birds use a magnetic field as a innate direction tool when escaping a prey, the fox might find it useful to adopt such a mechanism as a fast bird could take off in that direction, increasing the likelihood of a successful kill.

    Sam

  2. Derek.H

    Is anybody combing the fields and forests looking for confused/dead foxes due to the recent pole shifts? Just like the birds in the center of the country, and the fish in other parts of the world. Why has no one published anything connecting the pole shifts to the recent mass animal deaths?

  3. Scott

    It would be an interesting follow-up to compare with the introduced European fox population in Australia. Do foxes align to field strength or field direction?

  4. Shade

    I think it would depend on strength of their sense.
    Could have to do with a large number of things, but I think you could rule out a lot if the strength of the sense is determined.

  5. Conor Barnes on Twitter suggested the much better headline of Fleet Foxes and Magnetic Fields Mash-up ;-)

  6. They’ll be renumbering their runways, soon :)

    “As you attacked such targets again and again, you could perfect a highly stereotyped leap that precisely lands you accurately on the target.”
    As a movement scientist, this caught my eye; the obvious question is, how stereotyped are the jumps? Trying to test the use of a magnetic sense should be like testing the use of other perceptual information: see how it’s contributing to action.

    You could also perturb the magnetic fields, in the lab anyway – but this would be a better test, because you could leave sound and vision unperturbed but push the magnetic field around. If behaviour followed that perturbation, you’d have strong evidence.

    Very cool, though!

  7. Andrea

    Could it be instead that mice sense the magnetic field and preferentially move in the north-easterly direction? And foxes simply pounce in that direction out of experience?
    Does the presence of high voltage lines nearby disrupt the ability of foxes to hunt?

  8. @Andrew – Re: perturbing magnetic fields, yes you could, and that was one of the key experiments that demonstrated an internal compass in birds. If you shove them in the middle of oscillating magnetic fields, they stop trying to migrate in a constant direction. That’s fairly straightforward to check though – you put them in a featureless room and see which direction they try to fly towards. Hunting’s more complicated though and involves more variables, not least of which is the presence of another animal.

    @Andrea – Ooh I like that idea. I thought of power lines too, but again, it boils down to a problem with experimental difficulty. Looking at the effect of power lines on cow/deer was only possible because they’re quite large animals that move in conspicuous herds, so you can reasonably track their orientation through satellite images. Not so easy for a much smaller fox.

  9. Georg

    3. Scott
    It would be an interesting follow-up to compare with the introduced European fox population in Australia.

    This would mean that You had to import that
    thing named “snow” as well :=)
    Georg

  10. Andrea

    @Ed

    Maybe they could make a couple of grad students carry around an NMR magnet during the observations ;)

  11. Haven’t they done previous studies in the laboratory where they study the use of magnetism in birds by disrupting magnetic north with a stronger local magnet? They could try this in fox pouncing in the lab and see if it really does disrupt the pounce, all they’d have to do would be to shift it by a few degrees to make their point.

  12. Sam

    @Scicurious

    I’m not sure do you have a link to this?

    20 degrees off could provide some relevance into the situation, would this angle be more likely to cross the path of an escaping target? Just because I got curious I started drawing predator and prey circles on a bit of paper and and projecting 20degree variance’s in the fox with a birds more accurate escape route it could help the foxs lag overcome the quick bird without much degree of sophisticated thinking from the fox.

    Sam

  13. SteveF

    [pedantrymode] The reference should be 2011, not 2010 [/pedantrymode]

  14. @SciCurious – Yeah I mentioned this in comment #8. This was done by the Wiltschkos (I interviewed Roswitha for the piece). They first used oscillating magnetic fields to disrupt the magnetic compass of robins, followed later by chickens and zebra finches.

  15. kharris

    The experimental difficulty is not all that difficult. Instead of observing lots of foxes to find a similar behavior across bunches of them, you’d now be satisfied with a smaller number of observations, as long as they showed a significant difference from the norm. That means simply observing near high-tension wires. One could test for masking of the earth’s magnetic field to delimit the search area. As long as you are in a high-fox area, you are as likely to observe a fox sitting still in the magnetic mess area as elsewhere. Unless foxes have learned that mice are harder to catch there and so hunt elsewhere. Learning that foxes don’t like to hunt near high-tension wires would take lots more observation, but would certainly be interesting to know.

    Does “mousing” take place in grass cover as well as in snow cover, I wonder? There may be better visual information in grass than snow but far from perfect, I’d guess.

  16. @SciCurious the problem with controlling magnetic fields is that it takes a relatively very large setup to affect the fields in a small area. And then the question is: will foxes even try hunting if you stick them into a helmholtz coil anyway?

    It sounds like foxes only resort to magnetic field sensing under pretty limited conditions, which might be hard to find under existing power lines. It should be possible to simply bury a set of long wires in a field, let some mice nest there, and control the varying current of the wires. Won’t be able to cancel earth’s field that way, though.

    Testing the mice in the lab would definitely be an interesting first step, if only because they are that much smaller.

  17. @kharris – yes, when I talked about “high cover”, I meant high grass.

  18. Doing a REALLY short search (obs this is not my expertise) I did find a study which strapped magnets to birds and got some disruption, but not a lot:
    http://www.ncbi.nlm.nih.gov/pubmed/17021386

    So rather than attempting to disrupt the foxes in the lab, you could attach a backpack or do an implant or something and then let the fox go and see what happens?

  19. Trond Engen

    Andrea: Could it be instead that mice sense the magnetic field and preferentially move in the north-easterly direction? And foxes simply pounce in that direction out of experience?

    Since mice live in tunnels under the snow, wouldn’t they simply be following the tunnel in one or other direction? I’d think that the fox (or its instinct) would know that. The fox may e.g. be jumping in some preferred angle to the tunnel after locating it by listening. The angle would be determined by how the fox uses its body to encroach on its prey. It wouldn’t eplain why foxes miss when the tunnel is differently oriented, though. Unless it’s happening when the mouse is sitting still. Or perhaps such cases are just short turns around natural obstacles. And, of course, this wouldn’t be less puzzling, since it would mean that mice dig tunnels mainly in one particular angle to the magnetic field.

    But probably not — surely somebody studying mice would have noticed.

  20. Jim

    Just curious – has there been any experimentation on human interactions with magnetic fields? I’m just wondering if that might be a practical explanation for feng shui and the human tendency not just to order things, but to put them in a certain order.

  21. Aleksandar Kuktin

    @ Jim (#20): We know humans have magnetite crystals in their brains. What are they for, however, is still mystery.

    http://www.ncbi.nlm.nih.gov/pmc/articles/PMC49775/

  22. @Jim – In the late 1970s, Robin Baker from the University of Manchester found that blindfolded students could point their way ‘home’ after being driven to far-off locations, but not if they wore magnets on their journey. Baker attributed this to a magnetic sense and he even claimed to have found magnetic bones in human sinuses. Unfortunately, other researchers repeatedly failed to replicate the results. Baker argued that these studies had been foiled by local magnetic anomalies, but to no avail. The idea of human magnetoreception eventually fell out of favour but a couple of people I spoke to said they’d be interested in testing for it again, perhaps among people like Pacific Islanders who have to navigate long distances without landmarks.

  23. Ed, in your interviews and studies on animal/magnetic field interactions, what have you found out regarding the resolution with which magnetic fields are sensed? With sharks, from what I remember, they are able to sense magnetic fields in hunting with extreme precision.

    But in the realm of birds/mammals I run into a logic leap when you try to connect generalized direction-finding (the cow example, bird migration paths), which allow considerable distance errors (tens of meters, say), and the precision hunting method of mousing. The fox would only be allowed centimeters or less of error.

    I guess I am curious if that’s a realistic scale for a mammal to sense using magnetism. I understand that it’s difficult to distill away from hearing, smell, but it’s a glaring question mark for me.

  24. Nathan Myers

    It should be easy to add to the dataset by moving foxes to a different latitudes than they matured in, and seeing if they consistently overshoot or undershoot prey. Moving them across the equator ought to have predictable effects too.

  25. @Joe H – no not really. I don’t think that’s been looked at yet. Also, you may be confusing sharks’ ability to sense electric fields, which is indeed very precise.

    @Nathan – Different prey, though…

  26. psweet

    Red Foxes occur here in the US, as well, and a different latitude should produce a different angle on the jumps. I realize that we’re talking about a different population and a different prey species, but the proposed mechanism seems like it shouldn’t be affected by those things. If you found a similar bias here, that would be a pretty nice confirmation.

  27. Phyllograptus

    Large ore bodies can cause anomalies in the earths local magnetic field, which can effect a large geographic area. There are also areas of unexplained magnetic anomalies. Both of these anomalies can be quite large and the magnetic field anomaly causes incorrect whole field magnetic north bearings. Theoretically foxes living and hunting in areas with known magnetic anomalies would be impacted and their pounce direction would vary as they moved around the anomaly area. Also as a really out there correlation – sunspot activity can alter magnetic north bearing for a period of time, compare the variance in pouncing direction to sunspot activity.

  28. This entire comment thread makes me smile. It’s joyous seeing people discussing possible follow-up experiments. SCIENCE!

  29. Eleanor

    How about sunlight as an alternative suggestion? They might learn that if they pounce facing into the sun or where their shadow obscure the target they are less likely to be successful. Doesn’t explain why they don’t pounce southwards though. Hmmm. There must be another explanation!

    From lots of trapping I’ve never noticed mice or voles having a preferred tunnel direction, which normally correlate to things like grass tussocks and hole locations.

  30. Gingerbaker

    I wouldn’t be surprised if mice align to the Northeast naturally if deer do the same thing. It would be interesting to see if their tunnels align that direction or 90 degrees opposite.

    I would imagine that a mouse, hearing a suspicious noise, might align his ears along the axis of his tunnel to amplify sounds, and then run for his life, either along the tunnel, or through its wall into deep snow. If the mouse tends to try to escape along a northeast axis, it certainly would behoove the fox to pounce in that direction, as the mouse can only run quickly in one direction.

    It also may well help the fox to pounce to the Northeast, if mouse tunnels tend to be in that direction, as it may increase his chances of putting his snout directly into the tunnel, all the better to smell which direction the mouse has headed. I seem to remember watching films of these foxes who would pounce, and then snap in a new direction(s) before actually capturing the prey.

  31. Brian Too

    Running statistical tests in the wild, while deliberately messing with the magnetic field, has a big problem.

    Since the foxes are reliant upon an alignment with something, and that something might the be the geomagnetic field, you risk reducing their hunting success rate from 73 to 18%. That’s a big deal to a wild animal. The test subjects could have dramatically higher mortality if your test is “successful”.

    If I’m not mistaken, mice form a major part of the foxes diet. It’s their ‘main course’, or at least one of them. Substituting food sources is plausible in summer but quite difficult in winter.

    It might be more humane and controllable to perform such tests on captive animals.

    Of course we must never forget that foxes are fairly intelligent. It’s possible that they could learn any systematic deviation to a magnetosense and compensate.

  32. Trond Engen

    I went searching for maps of tunnel systems, with no result. I did find a few photos of exposed tunnels, though, and they don’t support any alignment.

    But I couldn’t spend much time on this, so others may do better.

  33. Kevin N

    I’m not sure this explanation makes much sense. Isn’t it much easier to determine angle from the inner ear fluids than with magnetic fields? I can sense my own head angle and I don’t need to sense magnetic fields to do it.

  34. Dave Bell

    This is giving me some ideas for fiction…

    There’s a couple of obvious thoughts.

    1: The Earth’s magnetic poles are moving, and there’s nothing unusual in that. The magnetic declination changes by maybe half a degree per decade, in Europe. It depends where you are.

    2: It also varies by location. There’s a 30-degree shift between Texas and Maine.

    Looking at possible European locations with foxes and snow, a 5-degree difference seems fairly easy. 10 degrees change is a bit of a stretch. That should be enough, with a repeat of the observations in another part of Europe, to confirm whether this is linked to the geomagnetic field.

    A lack of this behavior might be done to the fox population being of a different sub-species, and that would be interesting in other ways.

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