Among the light-laden evergreen trees, the handicraft-filled huts and the mitten-wrapped mugs of steaming spiced wine lies the most traditional part of the Czech Christmas market: carp. Round tubs teem with live fish for sale since carp is the traditional centerpiece of the Christmas feast in much of central Europe. Oddly enough, when contained in these round tubs, the carp tend to align themselves with an invisible north-south line. One might assume this is because the fish are seeking Santa in his North Pole home, but scientists argue the behavior is the result of a previously unknown capacity to perceive geomagnetic fields.
Researchers photographed the tubs of carp from above. On each photograph they assigned a directional arrow to each fish based on the line of its dorsal fin. The fish images were then removed and the bearings averaged. This detection of the Earth's magnetic poles is a well-studied phenomenon in birds and other migratory species. These animals travel long distances north and south every year based on a compass that scientists typically explain with the pull of the planet's magnetic poles. But no one had thought to explore this phenomenon in carp until the researchers noticed the curious behavior of fish at the market. The large numbers of carp in their pre-Christmas tubs provided a perfect set-up for a larger scientific analysis, which looked at over 14,000 carp from 25 different markets around the Czech Republic. The researchers found that factors such as light, noise, and onlookers didn't seem to have an effect on the carp's orientation---regardless of those, the fish tended to line up as if on cue. Geomagnetism was the most likely force, they concluded, and this is the first time such a response has been recorded in carp.
Images courtesy of visivastudio via Shutterstock (left) and Vlastimil Hart et al. (right).