For centuries, farmers have known that their livestock not only gather in large herds but also tend to face the same way when grazing. Experience and folk wisdom offer several possible reasons for this mutual alignment. They stand perpendicularly to the sun's rays in the cool morning to absorb heat through their large flanks, or they stand in the direction of strong winds to avoid being unduly buffeted and chilled.

But cows and sheep don't just line up during chilly spells or high wind. Their motivations for doing so during warm, pleasant and unremarkable weather, or indeed in the dead of night, have been a mystery until now. In a new paper, Sabine Begali from the University of Duisburg-Essen in Germany spied on aligned herds of cows and deer using satellite images from Google Earth.

The images revealed a striking behaviour that had been going unnoticed for millennia, right under the noses of herdsmen and hunters - their herds were lining up in a north-south line like a living compass needle. Influenced by a magnetic sense that has only just become apparent, their default point of reference is not the source of wind or the angle of the sun, but the Earth's magnetic poles.

Google Herds

With Google Earth's images at their disposal, Begali's team spied on a massive sample of cattle across six continents, from South Africa to India to the UK. They recorded the positions of over 8,500 individuals at more than 300 sites, including a range of different breeds, altitudes and times. Their global cattle census showed that the animals oriented themselves along a north-south axis so consistently that the odds of them doing so by chance was less than one in a hundred thousand.

The animals don't quite point towards the north pole, but instead face slightly off it in the direction of magnetic north. As this position, known as magnetic declination, changes across the face of the planet, so too does the direction that local herds prefer to face.

Deer aligned themselves to the magnetic poles even more strongly than cattle. Begali's team travelled to more than 200 locations in the Czech Republic and observed the positions of almost 3,000 roe deer and red deer in the field. When the animals had moved, the team also recorded the alignment of the body prints that the resting animals left behind in snow. Again, their bodies faced magnetic north. Even their heads tended to gaze in that direction (although these, predictably, were more variable due to their need to scan for lynxes).

Sun and wind

Begali says that the study's large sample size has helped the team to rule out other explanations for this uncanny alignment. Certainly, the typical answers of sun and wind seem less applicable in the face of such standard behaviour across the globe. Unfortunately, Google Earth images aren't time-stamped with enough accuracy for the researchers to be able to compare the shots with weather data. But the fact that the photos exist at all suggest that they were taken on cloudless days, and the shapes and positions of shadows suggest that the sun was not too bright.

If cattle were lining up primarily in response to gusty winds, the majority of the 308 locations that the team sampled must have been experiencing high winds at the time and specifically in a northerly-southerly direction. That's very unlikely, given that westerly breezes dominate the Northern hemisphere and south-eastern trade winds rule the Southern. If that weren't enough, wind atlases show that the prevailing winds in the countries in question vary throughout the year and if there's any directional consensus, it's a westerly one.

So much for the wind; Begali's data rules out a major influence from the sun too. The satellite images also recorded the position of the animals' shadows and these revealed that by and large, they weren't getting their bearings from the angle of the sun. And the fact that deer still faced the same way at night also argues against the sun's involvement.

There's a third alternative - some animals including insects and migratory birds can sense polarised light from the sun and use it to navigate. There's no evidence that cows or deer have the right retinas for picking up polarised light and given that they are partially active at night, they don't fit the typical profile of polarised light-users, who are usually only active by day.

Magnetic attraction

With these alternatives effectively discarded, only one explanation remains, and it's the simplest one - the cows and deer were using the Earth's magnetic field as their guide. They are not alone; many other animals, including flies, bees and goldfish, naturally line up like little compasses when there isn't anything around to disturb them.

So why do it? That's still a mystery. One slightly leftfield possibility is that keeping magnetic fields in symmetry about the axis of your body could affect certain bodily processes. There are tantalising bits of evidence to back this up - in humans, the time it takes to drift into REM sleep and the electrical activity in the brain differs depending on whether we're facing a north-south direction or an east-west one.

Alternatively, it could just be that orienting yourself in a constant direction makes it easier to get your bearings when travelling long distances or making quick getaways. Indeed, both cattle and deer are naturally social animals that travel large distances across landscapes that are often bereft of landmarks. A magnetic sense could be good for navigation (as turtles and birds could attest to) although Begali cautions that it's not clear if the animals are even aware of their internal compasses. It's possible for creatures to have the capacity to detect magnetic fields ("magnetoreception"), without consciously sensing them ("magnetoperception").

Magnetic senses are one of the most enigmatic of animal abilities. Among our fellow mammals, only a few rodents and one species of bat are known to use internal compasses. Some believe that horses, dolphins and whales use the same trick but that's been very hard to prove. Large groups of these animals don't lend themselves to careful laboratory experiments. Begali's innovation was in using satellites to turn the entire planet into a natural laboratory. Now, it's time for others to follow up on her results.

Reference: PNAS doi.10.1073/pnas.0803650105

Images: red deer by Traoth, wildebeest by Vincenzo Gianferrari Pini

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