Free You Are Here by Colin Ellard Page A

keep track of self-motion. Ants don’t have a vestibular system, but they have nicely developed eyes and a sophisticated visual system that is sensitive to a property of light called polarization. Because the human eye is not sensitive to polarization, we cannot know exactly how the world looks to an ant, but it is not hard tounderstand polarization. Imagine that you have just thrown a stone into a still pond. You will see a series of waves in the shape of concentric circles moving from the place where the stone entered the water. If you were to look closely at one of these waves, you would notice that though the wave is moving outward across the water, the water itself is moving up and down. What makes the shape of the wave is the
vertical
motion of water, but the wave is propagated
outward
, across the surface of the pond.
    Exactly the same thing is true of light. Light waves are propagated from objects to our eyes, but there are other aspects of wave motion in light just like the vertical motion of water in the pond. Natural sunlight is said to be unpolarized because all directions of wave motion are mixed up and present in equal proportions in light. The earth’s atmosphere acts as a kind of filter for sunlight such that some wave motions are strengthened while others are weakened, producing partially polarized light. How much polarization is present in the light from the sky depends on the position of the sun. If we were able to see the pattern of polarization of light across the sky, we could estimate the position of the sun. (Interestingly enough, this would be true even if the sun were behind the clouds.)
    Ants, along with many other insects and a few other animals, can see such patterns. Wehner proved that ants used polarization patterns to find their way home. He designed a cart with a large window fitted with a special filter, much like Polaroid sunglasses, that affected the polarization of light on its way to the eyes of the ant. By following ants across the desert floor with this cart so that the window was always between the sun and the insects, Wehner was able to cause the ants to make errors in homing. In a way, Wehner was using a simple variant of the method pioneered by Tinbergen: the most powerful way to prove that something is being usedas a source of information is to manipulate the information in such a way that you can predict errors and then see if the errors conform to prediction.
    The compass in the sky provided by patterns of light polarization can tell the ant about the directions of its turns, but it cannot tell it how far it has ranged from home. Ants may solve the distance conundrum in several ways. One idea is that ants measure distance in units of effort. Just like us, ants may know when they’ve taken a long walk because they feel tired. To test this, Wehner’s group trained ants to retrieve food from a feeder while wearing tiny backpacks containing heavy weights (up to four times their own body weight) during the outward-bound parts of foraging expeditions. If effort is measured to estimate distance, then the extra effort required to carry the weight would be expected to produce errors in distance estimation on the (backpack-free) way home. Wehner showed that the weights had no effect. 5
    Another possibility is that ants use optic flow to compute distance. Just as airplane pilots use optic flow to judge their distance from the ground, it might be that ants can measure these fields of flowing movement to calculate how far they have walked. Experiments with honeybees have shown that optic flow can be a powerful source of information about distance traveled, but results with ants have been more equivocal. In one experiment, ants were trained to run along alleyways to obtain food. The alleyways were marked with black and white stripes that would produce nice patterns of optic flow as the ants traveled along them. Having grabbed the food, they were removed from the alleyway and placed on an