Bats are festooned with super-human abilities. Quite apart from flight and echolocation, remarkable enough in themselves, bats are able to gain useful information from the Earth's magnetic field.

It has been known for some time that many animals – the prime example being homing pigeons – are magneto-sensitive. The situation with bats is only now being revealed because they are far less convenient to work with:they tend to bite, preen vigorously, fly in the dark, and there is no bat-racing industry. In 2006, a team led by Martin Wikelski from Princeton University [Holland et al. (2006) Nature vol. 444, p. 702] followed displaced big brown bats as they struggled to find their way home during sunset, after exposing them to rotated magnetic fields. These field measurements, involving pursuing bats with light aircraft, suggest that the animals use a sunset-calibrated magnetic compass.

In order to sort out how bats use the Earth's magnetic field, a lab-based measure in which the bats can be kept in man-made magnetic fields is required. This is what Yinan Wang and colleagues in China and New Zealand have developed and recently published online in the Proceedings of the Royal SocietyB, allowing them to determine whether bats use the same compass mechanism as birds. They found the roosting behaviour of the Chinese noctule bat provided a very simple method for spotting exactly what about the magnetic field is being sensed. The bats that Wang and co-workers were studying chose to roost at the north end of an upturned bucket. The team placed the roosting bucket in a set of Helmholtz coils, effectively electromagnets, which allowed the scientists to manipulate the bucket's magnetic field. Knowing that the bats preferred to sleep at the north end of the roost-bucket, the team switched the bucket's magnetic polarity and found that the bats were fooled' into roosting at the south end of the bucket: they detected the polarity of the bucket's magnetic field and roosted accordingly.

This is completely different from the way birds sense magnetic fields:birds are insensitive to polarity, but can recognise the inclination of the field (i.e. the Earth's field lines are inclined vertical at the poles,horizontal at the equator and slowly change their angle relative to the earth as you move between the poles). Birds can calculate the direction towards the nearest pole by measuring the magnetic field's inclination. Testing whether the bats also measure inclination, the team experimentally switched the inclination, but not the polarity, of the magnetic field: yet the bats continued roosting at the north end of the bucket. Unlike birds, bats did not measure the field's inclination.

It is tempting to suggest an adaptive significance of this difference between bats and birds. As birds only detect field inclination, they cannot use magnetism to determine north when they are at the equator: after all,which way is down' on a horizontal magnetic slope? They must use other factors to navigate at the equator. Bats, on the other hand, are able to tell the difference between north and south perfectly well from the magnetic polarity, allowing them to migrate and forage large distances near to the equator.

What is perhaps more interesting is what this means in terms of evolution. In all non-mammalian vertebrates, such as birds and lizards, tested so far,direction sensing is based on inclination, whereas mammals, such as naked mole rats and some bats, are receptive to polarity. These findings raise the possibility that compasses have evolved more than once in vertebrates.

Wang, Y., Pan, Y., Parsons, S., Walker, M. and Zhang, S.(
2007
). Bats respond to polarity of a magnetic field.
Proc. R. Soc. B
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,
2901
-2905.