Birds can navigate by the Earth’s magnetic field. How they do it is still a mystery - Juli 13, 2013
WHERE would people be without magnetic compasses? The short
answer is: lost. By giving human beings a sixth sense—an ability to detect the
hitherto invisible magnetic field of the Earth—the compass proved one of the
most important inventions ever. It let sailors navigate without sight of the
night sky. And that led to the voyages of discovery, trade and conquest which
created the political geography of the modern world.
Imagine, then, what animals which had their own, built-in
compasses could achieve. They might spend their summers doing the English
Season in Glyndebourne or Henley, and then overwinter in the warmth of Mombasa.
They might strike out, like intrepid pioneers, from Angola to Anchorage. They
might even, if truly gripped by wanderlust and a hatred of the darkness, live
in near-perpetual daylight by migrating from Pole to Pole.
And that is just what some birds do. Swallows travel between
Europe and Africa. Northern wheatears fly from Africa to Alaska, and back.
Arctic terns each year make the journey from one end of the planet to the other.
And they can do it, at least in part, because they do have a magnetic sense
denied to humans.
The most familiar avian navigation trick is that pulled off
by homing pigeons. As a consequence pigeons have often found themselves at the
sharp end of investigations about how bird navigation in general, and magnetic
sense in particular, actually work. That pigeons have such a sense was shown
more than 40 years ago, by William Keeton of Cornell University, in upstate New
York, who attached magnets to pigeons to see if they could still home. They
could not, though birds fitted with non-magnetic dummies managed perfectly
well. Since then, experiments on other species have shown magnetic sensitivity
is common among birds. What these experiments have not shown, however, is how
the birds manage it.
See it? Hear it? Smell it?
There are two theories. One is that the magnetic sensors are
grains of magnetite, a form of iron oxide which, as its name suggests, is
easily magnetised. The other is that the Earth’s magnetic field affects a
particular chemical reaction in the retina in a way that reaches into the
arcane depths of quantum mechanics.
The magnetite hypothesis concentrates on birds’ beaks.
Magnetite grains are common in living things, and are known to be involved in
magnetic sensing in bacteria. In birds they are particularly abundant in the
beak. So last year David Keays of the Institute of Molecular Pathology, in
Vienna, dissected the beaks of nearly 200 unfortunate pigeons, to find out
What he discovered was not encouraging. There were, indeed,
lots of magnetite grains. But he had expected they would congregate in some
sort of specialised sensory cell akin to the taste buds of the tongue or the
hair cells of the ear. Instead, he found that the beak’s magnetite is mostly in
macrophages. These are cells whose job is to wander around amoeba-like, chewing
up bacteria and debris from other body cells as they go. Not, then, likely
candidates as magnetic sensors.
Other experiments, though, do suggest the beak is involved.
The nerve that connects it to the brain is known as the trigeminal. When
Dominik Heyers and Henrik Mouritsen of Oldenburg University, in Germany, cut
the trigeminals of reed warblers the birds’ ability to detect which way was
north remained intact. They did, however, lose their sense of magnetic dip (the
angle the Earth’s field makes with the ground). Dip indicates latitude, another
important part of navigation.
To confuse matters further, some people accept Dr Keays’s
interpretation of what is going on in the beak, but think that the relevant
magnetite grains are elsewhere—in the hair cells of the ear, which are also
rich in iron oxide. If they are right, then from the birds’ point of view they are
probably “hearing” the magnetic signal.
The main alternative to the nasal-magnetite hypothesis,
though, is not that birds hear magnetic fields, but that they see them. One
line of evidence for this is that part of a bird’s brain, called cluster N,
which gets its input directly from the eyes, seems to be involved in magnetic
sensing. Experiments Dr Mouritsen’s team conducted on robins showed that
destroying cluster N destroys a bird’s north-detecting sense (they did not look
at the question of dip), and other experiments, on meadow pipits, show that
cells in cluster N are far more active when the birds are using their magnetic
sense than when they are not.
The problem with this idea is that birds’ eyes do not have
magnetite in them. If they do house magnetism detectors, those detectors must
be something else.
That something, according to a hypothesis advanced by Klaus
Schulten, who works at the University of Illinois at Urbana-Champaign, is a
type of retinal protein called a cryptochrome. When hit by light, a
cryptochrome produces pairs of molecules called free radicals that are
electrically neutral but have unpaired electrons in them. Electrons are tiny
magnets, so they tend to attract each other and pair up in a way that
neutralises their joint magnetic fields. Unpaired electrons, however, remain
magnetic, and thus sensitive to the Earth’s field.
Moreover, because the unpaired electrons in the free
radicals were originally paired in the molecule that split to form the
radicals, quantum mechanics dictates that these electrons remain “entangled”.
This means that however far apart they move, what happens to one affects the
other’s behaviour. Calculations suggest the different ways the two radicals
feel the Earth’s field as they separate is enough to change the way they will
react with other chemicals—including ones that trigger nerve impulses, and
that, via entanglement, they can “transmit” this information to each other, and
thus affect each other’s reactions.
This, the calculations indicate, would be enough for a
bird’s brain to interpret the magnetic field. It would probably see a pattern
of spots before its eyes, which would remain stationary as it scanned its head
from side to side. And some birds do, indeed, scan their heads this way when
assessing the direction of magnetic north.
It is possible, of course, that both hypotheses are right,
and that birds have two magnetic senses, with one perhaps concentrated on north
detection and the other on detecting dip. But there is something particularly
poetic about the idea that even part of this mysterious sixth sense depends on
a still-more-mysterious quantum effect—one that Einstein himself described as
“spooky action at a distance”.