An ant's-eye view

I amamazed at the ability of some animals to find their way. In the southern Indian Ocean, wandering albatrosses leave their nests and go on foraging flights of hundreds or even thousands of kilometres, yet they can reliably return to their home island, which to us would be but a tiny speck in the vast expanse of sea.

The idea that birds use the Sun, the stars and the Earth's magnetic field to guide them in their wide-ranging migrations is well known. But Rudiger Wehner, a zoologist in Zurich, is uncomfortable with such grand theories, and thinks that other factors are involved. He also believes that one needs to look at the details.

He has focussed not on birds, but on a long-legged desert ant, Cataglyphis fortis. Cataglyphis lives in the Saharan desert, and hibernates underground in winter. In the heat of summer, all other insects forage only at night, but Cataglyphis comes out when the temperature is 53C. No other animal can survive such a high body temperature, and it has special mechanisms to reduce water loss by expelling carbon dioxide discontinuously in short bursts, while oxygen is taken in continuously.

Wehner still remains astonished at the pathways taken by these ants when hunting for food. They separately leave their tiny hole, which leads to underground colonies, and travel several hundred metres to capture prey. They move over the surface of the ground, across sand dunes and gravel, wandering in a variety of directions. Having found food, they have the ability to return directly, in an almost straight line, to their home starting-point.

To do this, they must have in their "cockpit" a compass for determining direction, a gauge for measuring distance, and some record of the pathway they have taken, as well as an integrator to tell them the direct path home. The latest research into this tiny creature shows that when it moves over little hills, it can calculate the distance back along the flat.

The compass used by this ant is based on sunlight, which is scattered as it meets air molecules in the earth's atmosphere. The result is that the light is polarised in a way that we humans are not able to see. But the ant has cells in its eyes that are sensitive to this polarised light, and the information is processed so that the ant has a sense of the direction in which it is facing.

It thus obtains a generalised map of the sky; but the process is even more complicated than it appears, as the scattering of the sunlight varies with the time of day and the elevation of the sun. So Cataglyphis must recalibrate its compass each time it ventures forth, and must not stay out too long. When it leaves its colony it does a little dance, presumably to set its compass correctly.

But it does not only use this system, together with how far it has travelled, to get back home. It also uses landmarks along its horizon to correct any errors it might make in its calculations. Experiments have shown that, as it approaches its home, the ant continually compares what it sees with a memorised snapshot that it took when it set out, and moves so as to reduce any difference between the two. If the terrain is flat with no landmarks, the ant adopts a systematic search strategy, looping around the area where the home base is most likely to be.

Cataglyphis has used a number of interlinked special mechanisms to find its way home. One can only be in awe of how evolution has exploited what is available, like scattered light, and linked it to other processes. It is also amazing that so small a brain can compute so much.

It turns out that the US Navy uses a similar looping strategy to that of our ant for searching for missiles lost by the US Air Force. And robots are now being built that roam the desert using the mechanisms first identified in the ant. Surely, no engineer working at the drawing board could ever have come up with such imaginative solutions.

Lewis Wolpert is professor of biology as applied to medicine at University College London