Soprano pipistrelles can sense the polarity and inclination of magnetic field lines, and use the position of the setting sun to calibrate their internal compass.
Migratory bats use a magnetic sense to navigate long distances, calibrating their internal compass based on the position of the setting sun each evening.
 |
| Soprano pipistrelles migrate long distances Wildchromes/Alamy |
Many animals may use Earth’s magnetic field either to orient themselves or to navigate, including turtles, birds and possibly even humans. Until now, there has been no direct evidence that migratory mammals also use this sense, called magnetoreception.
Migratory bats travel hundreds or even thousands of kilometres each year. As nocturnal voyagers, they can’t rely on visual cues, such as landmarks, and echolocation only works over tens of metres. Night-flying migratory birds are known to navigate via the magnetic field, so William Schneider at Bangor University, UK, and his colleagues wondered whether bats were doing the same.
Previous research has suggested that soprano pipistrelles (Pipistrellus pygmaeus) use the angle of the setting sun to adjust their routes each day. Schneider’s team wanted to see whether these migratory bats also use a magnetic sense, and whether they can sense the inclination of the magnetic field as well as its polarity.
Earth’s magnetic field lines curve around the planet like a bar magnet, and the changing angle of these lines relative to the surface is the inclination. Polarity, the field’s horizontal axis, lets animals orient themselves towards the poles.
The researchers trapped 65 soprano pipistrelles on their migration route at Pape Ornithological Station in Latvia. Just before sunset, they manipulated the magnetic field around two groups of bats using a Helmholtz coil, a device that generates magnetic fields. One group had a horizontal shift in polarity 120o clockwise; the other had the same horizontal treatment and a completely reversed vertical shift in inclination, as if they were in the southern hemisphere. A third group had no magnetic manipulation.
A few hours later, the researchers released the bats into the night and recorded the direction they went. Bats that weren’t subjected to altered magnetic fields unexpectedly flew in two directions, either to the north or to the south – a split possibly due to poor weather nearby.
Those exposed to the horizontal shift only took off to the north, supporting the idea they can detect the magnetic field. The bats exposed to the horizontal and vertical shifts took off in all directions, indicating for the first time that they are sensitive to the field’s vertical inclination too – and that they were probably very confused.
This extra information gives a more accurate position, like a GPS system tapping into a number of satellites, says Juan Tomás Alcalde at the Spanish Association for the Study and Conservation of Bats. “These abilities may allow bats not only to migrate confidently in the path they are flying, but also to adapt their magnetic compass to the Earth’s ever-changing magnetic field.”
The mechanisms involved in the magnetic sense aren’t fully understood, but current evidence suggests two sensory pathways are necessary to detect both variables: a light-dependent pathway to detect inclination, and another that probably relies on a mineral known as magnetite to sense polarity.
“This new study is an important one because it will spark further interest in how these two magnetoreceptive mechanisms are realised in mammals and how they might support each other to enable efficient navigation,” says Kai Caspar at Heinrich Heine University Düsseldorf in Germany.
Journal reference: