Bats use sunset cues to calibrate their magnetic compass

Bats typically emerge from their roosts at dusk to forage for food. They use echolocation (pulses of sound and returning echos) to orient themselves over short distances, avoid objects, and detect prey. Over longer distances, echolocation is essentially useless and its unclear how the bats navigate back to their roosts each night.

Pigeon and migratory birds use the Earth’s magnetic field to navigate - perhaps bats do as well. A group of European mammalogists and ornithologists have team up to try and answer this question (Holland et al., 2010).

In an earlier study Holland and colleagues showed that disrupting the magnetic field around bats interfered with their ability to navigate. The researchers reasoned that the bats might rely on a geomagnetic sense of some kind for long distance navigation. More recently, these authors demonstrated experimentally that at least greater mouse-eared bats (Myotis myotis) use the light from sunset to calibrate their internal compass.


Figure 1. A whiskered bat Myotis mystacinus roosting in a cave. (Photo by Gilles San Martin)

Greater mouse-eared bats were captured, placed inside containers, and the containers placed inside a pair of Helmholtz coils that shifted the magnetic field 90 degrees to the east. An equal number of control bats were placed inside the magnetic coils but the magnets were not turned on. The bats were taken 15 miles from their roost cave, released, and tracked with radiotelemetry.

Bats that were magnetically disoriented after sunset had not problem returning to their home roost. However, the bats that were magnetically disoriented at twilight could not find their way home, suggesting that these bats use the light still available just after sunset to set their compass for the rest of the night.

Just how the bat compass works is not known. Some birds have tiny magnetic-sensing molecules in their eyes (special photoreceptors), other birds may use iron-based magnetic crystals in their upper beaks to measure the strength of the magnetic field. Holland’s team has previously shown that bats don’t have special photoreceptors in their eyes, but some of their cells appear to be laden with magnetite.

Thus, we can add a sunset-callibrated magnetic compass to the impressive array of bat sensory abilities.


References

Holland, R.A., Borissov, I., and B.M. Siemers. 2010. A nocturnal mammal, the greater mouse-eared bat, calibrates a magnetic compass by the sun. Proceedings of the National Academy of Sciences, 107:6941-6945. doi/10.1073/pnas.0912477107

Holland R., Thorup K., Vonhof M., Cochran W., and M. Wikelski. 2006. Navigation: Bat orientation using Earth’s magnetic field. Nature 444:702.

Holland R.A., Kirschvink J.L., Doak T.G., and M. Wikelski. 2008. Bats use magnetite to detect the Earth’s magnetic field. PLoS One 3:e1676.