Grazing cattle and sleeping deer tend to align their bodies along the North-South axis of the Earth's magnetic field, European researchers have found, giving new meaning to the phrase animal magnetism.
Herdsmen and hunters have long known that cattle and sheep tend to face the same direction when grazing, but had believed they were simply positioning themselves according to prevailing winds or the sun's rays.
Sabine Begall of the University of Duisburg-Essen in Germany and colleagues had a different idea. The researchers studied 8,510 satellite images of cattle and deer herds derived from Google Earth from around the globe, including 308 pastures and plains.
They also looked at deer beds - impressions left in the snow by resting deer - from nearly 3,000 deer in more than 225 locations in the Czech Republic.
They found that whether grazing or resting, these animals face either magnetic North or South. And since the direction of the wind and sun varied widely where the images were taken, the researchers believe the Earth's magnetic field to be the polarizing factor.
Although not seen before in large mammals, birds, turtles and salmon are known to use the Earth's magnetic field to guide their migrations, while rodents and one bat species have been found to possess an internal magnetic compass.
The researchers noted that humans and even whales are suspected of having an innate magnetic compass.
Some studies suggest humans who sleep in an East-West position have far shorter rapid eye movement or REM sleep cycles, in which dreams occur, compared with North-South sleepers who got more REM sleep.
"Our results call for an in-depth study of this phenomenon and challenge neuroscientists, biochemists and physicists to study the proximate mechanisms and biological significance of magnetic alignment," Begall and colleagues wrote in the Proceedings of the National Academy of Sciences.