Fleet Foxes Might Hunt with Magnetic Fields

Biology Letters, doi:10.1098/rsbl.2010.1145
Biology Letters, doi:10.1098/rsbl.2010.1145 / Biology Letters, doi:10.1098/rsbl.2010.1145

When red foxes are hunting small prey, they use a tactic that biologists call “mousing”: A fox will stalk its quarry until it’s at a certain distance, leap high into the air, and come down on the animal from above.

A land animal using an aerial attack is pretty cool, but what’s really impressive is that it works even at this time of year, when there’s snow on the ground and prey is hiding underneath it. A fox will stalk, jump, dive headfirst into a foot or two of snow and still nab a meal that was invisible to it.

When they’re hunting like this, says zoologist Jaroslav Červený, foxes seem to rely heavily on auditory cues. They move slowly and deliberately with their ears erect, cocking their heads from side to side and paying close attention to the slightest sounds of their invisible prey. After more than two years in the field watching foxes hunt, though, Červený thinks there’s more to mousing in the snow than a discerning ear, and that foxes may have a secret sense that helps them target what they cannot see. 

Červený and his team enlisted 23 wildlife biologists and hunters to help them document fox hunting behavior. Between them, they recorded 84 foxes performing almost 600 mousing jumps in various parts of the Czech Republic over two years. When the researchers compared everyone’s notes, they found a pattern. When prey was out in the open or in low cover and easily seen, the foxes approached and leapt at it from all different directions. When prey was deep in some vegetation or hiding underneath snow, though, the foxes tended to jump towards the north-east to pounce on it. The majority of the successful attacks on hidden prey that were recorded were “confined to a cluster centered about 20 degrees clockwise of magnetic north,” the researchers say. When the foxes made those north-east-pointing attacks, they were successful around 75 percent of the time. Attacks in almost any other direction, though, ended in a kill less than 20 percent of the time.

The foxes’ preference for north-east leaps, and the advantage they brought, held up across different locations, seasons, times of day and weather conditions, and the researchers couldn’t find any environmental cues that might have influenced it. The only explanation left, they thought, was that the foxes could sense the Earth’s magnetic field and were lining their attacks up with it. 

It’s not unheard of for animals to have a magnetic sense. Birds, sharks, lobsters and a handful of other species have all been shown to perceive the planet’s magnetic field. Červený and some of the other scientists working on the fox study, in fact, had previously demonstrated that cows and deer tend to align themselves with magnetic north while grazing, suggesting that they also have some sense of “magnetoreception.” In most of these cases, though, animals use the magnetic field to aid navigation. Foxes, if they can detect it, would be the first animal scientists know of that use it to hunt. 

So how does the magnetic field help a fox find a mouse? The researchers think that field acts like a rangefinder for the foxes, telling them how far away prey is when they can’t see it and making their blind jumps more accurate. At a certain point during the hunt, noise coming from the prey overlaps with the slope of magnetic field, as the fox senses it. When this happens, the fox is a fixed distance away from the prey and as it keeps hunting and pouncing, it’ll eventually learn to perfect its jump to cover that distance so it lands right on the prey. 

The fox’s sense of the magnetic field, the researchers speculate, could be as obvious as a sort of “heads-up display” in which they literally see the field as a pattern of light or color superimposed on their surroundings. All the fox would need to do to find the sweet spot and fix the distance of its prey is creep up until the location of the prey’s sounds lines up with part of the pattern (given the fox's strong preference for north-east jumps, the part of the pattern/field it uses to target is probably in that direction and the most visually obvious).