In 2008, a team of scientists tried a new way to study animal behavior. Instead of walking through farm fields, they used Google Earth satellite images to view cattle from above. They examined 8,510 cows standing in 308 pastures on several continents. A strange pattern stood out. Grazing and resting cows tended to line up their bodies along a north-south axis, no matter which country they were in.
The team also studied thousands of red deer and roe deer. Observers watched the deer in the field, and in winter they measured deer beds, the body shaped prints left in snow where deer had rested. The deer lined up north and south too. The alignment did not follow the sun. It matched magnetic north, the direction a compass needle points. The pattern held during the day and at night, in clear skies and under heavy clouds. Sun position and prevailing wind were both examined, and neither one could explain the pattern.
Earth acts like a giant magnet. Its magnetic field spreads out through the air and the ground, even though you cannot see or feel it. A compass needle is a tiny magnet, and it swings to point north because the field of Earth pulls on it from far away. Nothing touches the needle, yet it still turns. Cows and deer seem to act like living compasses. Some part of their body senses the same invisible field and they turn to line up with it, north and south.
A compass needle turns without being touched, and so do the cows.
In 2009 the scientists published a follow-up study. This time they looked at cattle in pastures directly beneath high-voltage power lines. Under the lines, the north-south pattern vanished. The cows faced random directions, as if their invisible guide had been switched off. Power lines carry electric current, and moving current produces a magnetic field of its own. This extremely low frequency field distorts the Earth's magnetic field in the area around the lines.
Hold a small magnet next to a compass and watch the needle swing away from north. The small magnet is close and strong, so its field mixes with the field of Earth and the needle points the wrong way. A power line does the same thing to a cow. Electric current flowing through the wire makes its own magnetic field. Near the wire, that field mixes with the field of Earth, so the direction signal gets jumbled. The cow no longer gets a clear north-south cue, so it stands any which way.
A power line is like a magnet held next to a compass. It scrambles the signal.
The disruption was strongest right under the lines. As the scientists checked cows standing farther and farther away, the scrambling faded and the north-south pattern returned. Scientists do not yet know how a cow detects a magnetic field. Still, the data points to something that reaches across empty space, without touching the animals, and shapes the way they stand.
A side view of one long pasture with a high-voltage power line running across the middle. Label 1 points to the power line tower and wires at the center, with the note: Electric current flows here and makes its own magnetic field. Label 2 points to a group of cows directly under the wires. Their bodies face many different directions, drawn as arrows pointing every which way, with the note: Random. Label 3 points to cows a medium distance from the wires. Some face north-south and some do not, with the note: Partly lined up. Label 4 points to cows at the far ends of the pasture, all drawn facing north-south, with the note: Lined up north-south. Faint wavy lines drawn around the wires fade out as they get farther from the tower, showing that the disturbance gets weaker with distance. A compass rose in the corner marks magnetic north.
Close to the power line the cows face random directions, and farther away they line up north-south again.
