My latest Mind and Matter column is on the
esoteric topic of insect navigation:
A friend who once studied courtship in dung beetles alerted me
last week to a discovery. On moonless nights, African scarab
beetles, which roll balls of dung, can use the Milky Way to
navigate in fairly straight lines away from dung piles, thus
avoiding other dung beetles keen to steal their dung balls. "Now
this is real science, simple, fascinating and completely
wonderful," enthused my friend.
Marie Dacke of Lund University in Sweden and her colleagues put
dung beetles inside a planetarium at Wits University in South
Africa with a pile of dung, and with or without little caps over
their eyes. The results of the beetles' peregrinations clearly showed that being able to see the
stars keeps the beetles relatively straight, even if just the Milky
Way is projected overhead without other stars. This is the first
demonstration of star navigation by insects and of Milky Way
navigation by any animal.
As my friend implies, insect navigation is about as ivory-tower
as science gets-practical uses seem far-fetched in the extreme. But
as I delved further into the topic, I soon began to bump into
things that might eventually be, if not of use, then perhaps of
relevance to human beings. At least one of the molecular mechanisms
used by insects for navigation is shared with people, hinting that
we may have at least a vestigial capability to sense direction.
Far more spectacular than the short-distance scrambles of dung
beetles are the migrations of monarch butterflies, which home in on
one small region of Mexico for the winter then return as far north
as Canada in a flight of thousands of miles that takes more than
one generation. Clearly the insects have an inherited "map" of
where to go, but what compass do they use?
It seems they have at least two compasses. One is a
"time-compensated sun compass," located in their antennae, which
calculates bearings from the angle of the sun corrected for the
time of day. Steven M. Reppert of the University of Massachusetts
Medical School and colleagues found that removing one antenna does not
disrupt navigation, but painting one black does, because it messes
up the clock mechanism in the animal's brain.
But butterflies can also use the Earth's magnetic field to
navigate. The butterfly antennae contain a protein molecule called
cryptochrome, which can apparently act as a magnetic compass when
exposed to blue or violet light. Human beings and other mammals
also have a cryptochrome in their retinas, albeit in slightly
different form, but until recently it was thought not to have
magnetic directional properties.
Recently Dr. Reppert and his colleagues took the human version of the gene that's the
recipe for cryptochrome and genetically engineered it into flies,
replacing the flies' own version. They then showed that, presented
with two routes in a maze, the flies could choose a magnetic
direction they had been trained to associate with a sugar reward,
and they did so just as well with the "human" cryptochrome as with
If it is at least possible to use our cryptochrome molecules to
sense direction from the Earth's magnetic field, do we? Birds do.
Night-migrating songbirds, when they cannot see the stars, use the
Earth's magnetic field as a cue. From recent research, it appears that they "see" it,
using cryptochrome-rich neurons in the retina of the eye.
To us that sensation, seeing a magnetic field, sounds unfamiliar
to say the least, though it could be unconscious. So far the
evidence that people can navigate magnetically is bedeviled by
unreliable experiments and extravagant claims. Chances are that our
navigational instinct was either not used for long-distance
migration or has atrophied; stories of people returning to the same
spot over thousands of miles are hardly common. Without a compass,
travel agent or GPS, that is.
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