Photo by Chris Mazzarella.

Detecting small mammal prey beneath a thick covering of snow is among the more challenging skills that some predators must develop to survive New England winters. Most use exceptional hearing and an acute sense of smell. According to a researcher in the Czech Republic, red foxes also use the Earth’s magnetic field to orient themselves when hunting.

Red foxes jump high to surprise prey from above in a behavior called “mousing,” and when doing so, according to Jaroslav Červený, a wildlife biologist at the Czech University of Life Sciences, they strongly prefer to pounce in a northeasterly direction. He said this directional preference enhances the precision of their hunting by aligning themselves with the Earth’s magnetic field.

Červený recruited a team of 23 hunters and biologists who observed 84 different foxes make nearly 600 mousing jumps in a variety of locations throughout the year and in all kinds of weather. They found that the foxes were successful at capturing their prey 73 percent of the time when pouncing toward the northeast and 60 percent of the time when jumping in the exact opposite direction. But their success rate declined to 18 percent when pouncing in any other direction. (When red foxes can clearly see their prey, directional heading obviously plays an insignificant role in their hunting success.)

“Mousing red foxes may use the magnetic field as a range finder or targeting system to measure distance to its prey and thus increase the accuracy of predatory attacks,” wrote Červený in the journal Biological Letters. “A fox that approaches an unseen prey along a northward compass bearing could estimate the distance of its prey by moving forward until the sound source is in a fixed relationship to the magnetic field. This would consistently place the fox at a fixed distance from its prey, allowing it to attack using a highly stereotyped leap.”

This explanation is plausible, he says, because the Earth’s magnetic field tilts downward at a 60- to 70-degree angle in the northern hemisphere, so as a fox inches toward the sound of a mouse, it is looking for the point at which the angle of the sound matches the angle of the magnetic field. That’s when it knows precisely how far to leap.

The physiology of how a fox’s magnetic detection sense operates is uncertain, and Červený has been unable to prove it even exists, but he believes that he has ruled out all other environmental explanations. And he knows that many other creatures can detect magnetic fields, including birds, bats, sharks, and turtles. He determined in a 2008 study, for instance, that cows and deer must have the same ability, since they tend to align themselves in a north-south direction, and their alignment is disrupted by the magnetic fields coming from high-voltage power lines. But red foxes are the first animal believed to use this “magnetic sense,” as Červený calls it, to capture a hidden meal.


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