http://www.nature.com/nsu/010816/010816-7.html

As orientated as a newt

Newts may use the Earth's magnetic field as a map.

14 August 2001

JOHN WHITFIELD

Wandering newts know where they are by sensing variations in the Earth's
magnetic field, suggests new research1. Whether animals use magnetic maps
has long been controversial - evidence that they do is "groundbreaking",
claim the study's authors.

Armed with a compass, you can use the Earth's magnetic field to work out
which way to go. Because the field's contours and intensity vary across the
planet's surface, you can also, in theory, figure out where you are. For
example, the field is more steeply angled closer to the poles.

There is a lot of evidence that animals use magnetic compasses, but little
that they use magnetic maps. Studies of pigeons and turtles, for example,
have been inconclusive because it is hard to deny an animal all the other
information it might use to navigate.

Janette Fischer, of Indiana University, Bloomington, and her colleagues
moved adult red-spotted newts (Notophthalmus viridescens) 45 kilometres
north-northeast from their home ponds.

They then tested whether the newts knew the way home by seeing which end of
their experimental tanks the animals were drawn to. The researchers also
used electromagnets to simulate changes in the inclination of the field
equivalent to a journey of about 200 kilometres north and south.

In all three tests, newts set off homewards, or towards where home would be
from a magnetic point of view. This is "strong evidence for a magnetic
map," according to John Phillips, another member of the Indiana team.

Phillips concedes, however, that "scepticism is justified - there's no
precedent for the precision of measurement that we're asking these guys to do".

Home bodies

In the wild, newts travel only a kilometre or two, leaving their ponds in
summer and returning in winter. On these small scales, local variation in
the magnetic field swamps any global trends. Youngsters probably learn the
magnetic features of their neighbourhood as they wander about.

The fact that they can extrapolate from local maps to such unfamiliar
distances is "amazing", says Fischer. Subsequent experiments using smaller
manipulations of the field seem to bear out the newts' homing powers.

Any map that could be used on a sub-kilometre scale would have to be
incredibly sensitive, says Roswitha Wiltschko, who studies bird navigation
at the University of Frankfurt, Germany, "but that doesn't exclude the
possibility that these newts use a magnetic map. After all, the visual
system can sense a single photon."

References

1.Fischer, J. H., Freake, M. J., Borland, S. C. & Phillips, J. B.Evidence
for the use of magnetic map information by an amphibian. Animal Behaviour,
62, 1 - 10, (2001).

Ĺ  Nature News Service / Macmillan Magazines Ltd 2001