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How do foxes know where to pounce when they can't see their prey? There's evidence they're using the Earth's magnetic field to help.

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You might be familiar with the cute pouncing technique many foxes use to hunt mice and voles. They’ll jump high into the air and come down nearly directly on top of their prey, even if that prey is hidden from sight in snow or grassy underbrush.

This is a tricky hunting technique to master, because a fox needs to know exactly where their prey is so they can successfully line up their attack and close the distance with that jump. Scientists know they’re using their big ears to help zero in on their target. But some think they’re also using another sense -- that the foxes are actually hunting using magnetic fields.

And no, it isn’t like they’ve got a built-in metal detector for mice. Rather, they may be sensing the Earth’s magnetic field. In fact, foxes might be the first animal we know of to use magnetoreception to gauge the distance to their prey -- and this sense may be crucial to their pouncing success.

In a 2011 study of red foxes in Europe, researchers noticed that when their prey was hidden by vegetation, a whopping 74% of successful hunting pounces occurred when the fox was pointed in a northeasterly direction. They combed through their data, looking for other factors that might bias the direction of attack, like the wind, or sunny versus cloudy conditions. After ruling all that out, they proposed that the foxes were using magnetoreception to orient themselves roughly toward magnetic north.

While they weren’t sure how, they did make a suggestion for why doing so would be helpful to the fox. You see, as a fox approaches its prey, it will use its other senses to gauge where its target is. If they can’t see their prey, they listen for it.

But the fox wants to know exactly where that hidden morsel is in order to jump the right distance to neatly close the gap. The researchers in this study proposed that the fox is searching for where its other sensory cues line up with the angle of the Earth’s magnetic field. That would result in a sort of overlay targeting system.

The angle of the magnetic field is fixed, meaning the fox could basically be using this stable reference point to line up where it thinks its prey is. Once it matches that angle, the fox knows the distance to its prey, and can employ the exact same pounce successfully, nearly every time. It’s almost like it has its own biological heads-up display -- basically something in its visual field pinging when it’s at the right distance.

So no matter when or where or what a fox is hunting, it pays to have learned just one move to come down right on top of dinner. Now, magnetoreception is actually thought to be fairly common. Many animals, especially migratory birds, are believed to use the sense for long-distance navigation, using it to find the right direction to travel.

But this would make foxes the only animal thought to use magnetoreception to sense distance. Scientists have long puzzled over the exact method as to how magnetic fields are sensed. Research in birds and fruit flies suggests that a chemical reaction in light-sensitive proteins could be the key.

These proteins are collectively called cryptochromes. And foxes have them in their eyes, just like birds and some other magneto-sensing species. So if these proteins are found in the eyes, and are sensitive to light, what does that have to do with sensing magnetic fields?

We’re not totally sure, but there is some evidence for how it might work. When a photon of blue light hits a cryptochrome, the cryptochrome transfers an electron to a partner molecule. This new arrangement leaves each partner with an uneven number of electrons.

One has an extra electron, and one has one fewer. And every electron has a property called spin, which can either be up or down. In the case of our unbalanced pair, one of those electrons has an up-spin and the other has a down-spin at the time of the transfer — but then they flip back and forth and even wobble.

Now, this electron-transfer reaction is reversible, meaning it can go back the other way. But only when the electron spins are opposite. But because magnetic fields influence the rates of the electrons flipping between up- and down-spin, and the wobble in that spin, a magnetic field could change the rates and products of the chemical reactions, too.

At least, this is how it works in theory. We know migratory bird eyes contain cryptochromes, and that their ability to navigate with magnetic fields may require the presence of light. Foxes, and some other mammals, also have cryptochromes in their retinas.

And they’re our strongest candidate for the job of biochemical magnetoreceptor. But we still need to show experimentally that cryptochromes form enough of these electron pairs to signal in some way that this way is north and this way isn’t. And in fact, even if cryptochromes are sensitive to magnetic fields at a biologically-relevant level, we don’t know how that signal would get passed along to the animal’s brain -- so we can’t be sure it actually senses anything.

So while the chemistry and math seem to check out, scientists are still working on the biological details of foxes’ hunting magneto-vision. Still, it’s no wonder that pounce is a mouse’s worst nightmare! Thanks for watching this episode of SciShow, and thanks as ever to our patrons for pouncing on the opportunity to help make it happen.

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