Migratory Bats Can Detect Earth's Magnetic Field--Study Indicates that these Animals, Like Birds, Are Sensitive to the Angle of Magnetic Inclination
The soprano pipistrelle (Pipistrellus pygmaeus) weighs only a few grams, but it is estimated that members of this small bat species cover thousands of kilometres every year on their nocturnal migrations from north-eastern to south-western Europe. Precisely how they find their way across such long distances in the dark remains unclear. However, an international team led by biologist Dr Oliver Lindecke from the University of Oldenburg has found evidence suggesting that a magnetic sense may play a role in the bats' navigation. In behavioural experiments, the team discovered that two different components of the Earth's magnetic field influence the animals' orientation. Lindecke has already been studying the migratory behaviour of the small mammals for ten years.
To test whether the bats have a magnetic sense and use it for navigation, the researchers trapped 65 soprano pipistrelles. A number of these bats were exposed to a manipulated magnetic field created using a device called a Helmholtz coil. The horizontal component was rotated 120 degrees clockwise in relation to the Earth's magnetic field, such that a compass needle would point south-east rather than north. With a second group of bats, in addition to the horizontal shift the research team also reversed the inclination of the magnetic field so that it corresponded to the natural values measured in the Earth's southern hemisphere. A third group was used as a control group and exposed only to the natural geomagnetic field in the dunes on Pape beach.
The results: around half of the bats in the control group flew southwards, while the other half flew northwards. The two groups exposed to manipulated magnetic fields behaved differently from each other: the bats that had been exposed to a horizontal shift of the magnetic field generally took off in a north-westerly direction. In the group where, in addition to the horizontal shift, the inclination was also reversed, however, no discernible preferred direction of take-off was observed. These results demonstrate one thing in particular: The bats are sensitive to both the magnetic field's horizontal component, known as polarity, as well as its inclination at sunset -- and this still influences their take-off behaviour several hours later.
