Magnetic bats

Bats navigate by using magnetite to detect the Earth’s magnetic field. Photo courtesy of the National Science Foundation.

Navigation in animals has puzzled people for centuries and still provides challenges for scientists. How for instance can a Monarch butterfly with a brain weighing less than 0.02 grams find its way to its overwintering site more than 4,000 km away? Or how can the Arctic Tern circumnavigate the globe with a precision better than human navigators before the invention of the GPS system? We know now that one of the most widely used methods of navigation in the animal kingdom is to use the Earth’s magnetic field as a compass. This method is predominantly found in birds, fish and insects, but has also recently been demonstrated in bats. However, questions remain as to how the animals detect the magnetic field. Scientists from Princeton University and Caltech in the United States have investigated this in the Big Brown Bat (Eptesicus fuscus).

Two different mechanisms seem to be used to detect the magnetic field. Light dependent magnetic orientation involves an interaction between the magnetic field and either magnetite particles located within a photoreceptor or excited states of photopigment molecules. However, some animals can orientate in complete darkness, possibly by intracellular magnetite, a biogenic ferromagnetic mineral. The use of magnetite for orientation has conclusively been demonstrated in bacteria by getting them to align to a magnetic field – where some bacteria orient toward North and others toward South - and then applying a brief strong magnetic pulse antiparallel to the aligned direction. This method, known as the ‘Kalmijn-Blakemore’ re-magnetisation experiment, permanently converts North-seeking bacteria into South-seeking forms and vice-versa.

Some bats, like birds, make seasonal migrations spanning more than thousand kilometres. Vision seems to play an important role for this migration suggesting that bats use the light dependent method of detecting the magnetic field. However, magnetite has also been found in their bodies.

The scientists caught bats in the wild and exposed them to the ‘Kaljmin-Blakemore’ re-magnetisation experiments before releasing them again at a different site 20 km north of the capture site. The homing direction was then measured by fitting the bats with a small radio-transmitter.

Untreated bats or bats exposed to a parallel magnetic pulse correctly orientated towards the capture site, whereas bats exposed to the antiparallel magnetic did not. The results show that magnetite based receptors play an important role for the detection of the magnetic field in the big brown bat and is probably used for navigation alongside light-dependent orientation during long-distance migration.

The light independent use of magnetite to detect the Earth’s magnetic field thus seems to play a role for more advanced animals, including mammals, as well as for bacteria. 

Source
Holland RA, Kirschvink JL, Doak TG, Wikelski M (2008) Bats Use Magnetite to Detect the Earth’s Magnetic Field. PLoS ONE 3(2): e1676. doi:10.1371/journal.pone.0001676.

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