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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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Insects that put Google maps to shame

Dung beetles, monarch butterflies and the role of cryptochrome

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 their own.

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.