When a group of grazing deer are startled by a predator, they bolt. That’s multiple deer fleeing at the same time. How do they not run into each other and cause a massive leggy tangle? According to new findings published in Behavioral Ecology and Sociobiology, an internal compass helps them follow a certain direction in order to make collision-free escapes.
Escape behavior is well studied in animals. Once danger has been detected in the distance, the prey monitors the approaching threat – assessing the risk and evaluating the direction and speed of its approach. The animal eventually flees once a certain distance is reached. This so-called flight distance as well as flight trajectory (relative to the predator) has been studied in many animals. But one aspect has been largely ignored: How do animals in groups coordinate and synchronize their escape direction once frightened to avoid collisions?
Petr Obleser from the Czech University of Life Sciences and colleagues wanted to see if magnetic alignment plays a role. That’s the tendency for animals to align their body axis with geomagnetic field lines. The team observed free-ranging roe deer (Capreolus capreolus), a common grazer in flat, open agricultural land in Europe. Two men dressed in olive green separately patrolled 60 fields and meadows in South Bohemia and West Moravia in the Czech Republic at different times of day for 46 days between April and August of 2014. They took note of various factors – like sun position, light levels, and wind direction – each time they encountered roe deer. If the animals became startled, the observers recorded their direction of escape and the location of their next shelter.
After analyzing 188 of these observations, the team found that undisturbed roe deer tend to align their bodies along the north-south axis as they graze. Then, when making their getaway, they didn’t just run in the opposite direction of the observers: They consistently fled north or southwards. This preference was more pronounced in deer groups than in single animals. "This suggests that an important function of this behavior is to coordinate the movement in the group, to keep the common course of escape when frightened and to maintain the cohesion of the group," Obleser said in a statement.
The team thinks that this north-south tendency means deer can sense the magnetic field. Being magnetosensitive allows the deer to create mental maps of their landscape. Not only does this help them coordinate their predator avoidance movements, it could also help them get back to the same place later, which is important because females have been known to hide their fawns in tall grasses.
