Flight compass

WHETHER it is a herd of gazelle, a court of kangaroos or a crash of rhinos, the sight of a group of animals turning from a predator and bolting in unison in the same direction is one of the most majestic in nature. It is also one of the least understood. Now a group of researchers think they have come up with the reason why such animals seem instantly to know which way to run and not crash chaotically into one another: they use a sort of compass.

To investigate this, Petr Obleser, a PhD student at the Czech University of Life Sciences in Prague, and his supervisor, Hynek Burda, studied roe deer commonly found on hunting grounds in South Bohemia and West Moravia in the Czech Republic. That some animals have an innate awareness of Earth’s magnetic field is well known: many use it to guide their migration. Herdsmen and hunters have also long observed that grazing animals often tend to align themselves facing either north or south, which the researchers suspected was in readiness to escape in either of those directions should a threat emerge.

And that is largely what the roe deer did, Mr Obleser, Dr Burda and their colleagues report in Behavioral Ecology and Sociobiology. For their experiment, Mr Obleser began the arduous task of slowly walking through the wilderness areas at set times and in set locations for 46 days between April and August 2014.

Whenever he saw a roe deer, he immediately stopped and assessed whether the deer had seen him first by studying where its ears and eyes were directed. If the deer had not yet noticed him, he began recording information on the weather, position of the sun, visibility, light levels, surrounding habitat and if the deer was on its own or in a group. He also used a laser rangefinder to measure the distance between the animal and its nearest source of cover, as well as a compass to determine its geomagnetic orientation. Then he crept closer and closer. When the deer ultimately fled, he measured the direction of its flight. If the monitored deer was with others, which was often the case, the rest of the deer were ignored, so that accurate measurements of just an individual were obtained. In this way he accumulated data on 188 deer.

Mr Obleser found the deer were more likely to orient themselves in a northern or southern direction while standing and that they have a strong tendency to bolt in those directions as well. More specifically, it was found that when the deer were approached from the south, 52% of those studies ran north, 17% to the south and 31% either east or west. The results from northern approaches were not much different, with 67% racing to the south, 12% north and 21% east or west. Yet what proved fascinating were the results from eastern and western approaches. The researchers speculated that coming from the west ought to drive the majority of the deer eastward, but only 42% went that way while 50% headed north or south. An eastern approach caused 40% to go to the west and 43% north or south.

Intriguingly, the tendency to run north or south did not appear to be related to weather, the position of the sun or any other environmental conditions. Even when the nearest cover was to the east or the west, the deer still preferred to initially bolt north or south before curving around and heading for the safe haven. Moreover, when deer were in a group, their tendency to move along the north-south axis grew stronger. Deer, the researchers argue, are not only sensitive to Earth’s magnetic field but also make use of it when it is time to run away.