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Given our clumsiness in the deeply familiar spaces of our darkened homes, it is not surprising that we human beings find many other ways to lose ourselves quickly, and sometimes with disastrous effects. Some telling statistics come from studies of “lost person behavior” that are designed to help guide search-and-rescue workers to those who wander off in forested parks. A first glance at the numbers suggests that becoming lost in wilderness is rare. One account of lost-person incidents in a large wilderness park in western Canada records that there are about 26 reports of lost persons each year in the park. Of those 26 reports, about two-thirds of lost people find their own way back to their points of origin, and many of them argue that they were never lost in the first place (though, if my own experience is any guide, there is probably some denial).
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Given that the total number of visitors to the park annually numbers in the hundreds of thousands, 26 might seem like an extraordinarily small number. Most park visits, though, consist of picnics, short visits for swimming, or the occasional hike down a short trail. If the numbers were to include only visits that consisted of extensive walking through wilderness, especially where such walking took people away from established trails, the proportion of lost-person incidents would be considerably higher.

My own brief experience as a lost person in Algonquin Park pales in comparison with those of the truly lost. The line between a dramatic family anecdote and a life-ending experience is thin and easily crossed. The scientific study of how such victims act, and how to go about finding them, is serious business.

Dwight McCarter, an experienced forest ranger and tracker, has documented accounts of several such cases in the gripping book
Lost! A Ranger’s Journal of Search and Rescue.
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Some common themes appear in the stories he tells. Otherwise sensible people wander off marked trails for many reasons. They can be looking for a shortcut (as in my own case), confused because of bad weather, or reacting to emotional disturbance (teenage angst or rebellion, or a disagreement with other members of the group). Commonly, losses occur when individual group members have differing abilities and one member lags behind the rest. Once out of sight and earshot, the temptation to try to shortcut, turn back, or do some other unwise thing can be irresistible. Once truly lost, the prognosis becomes grim with astonishing speed, especially when cold temperatures are involved. As time goes by, the likelihood of the lost person engaging in tragically misguided behavior increases quickly.

Many lost individuals are discovered with much of their clothing missing. Hypothermia can produce a burning sensation that the victim tries to ameliorate by disrobing. Many lost persons, especially after lacking human contact for several days, will begin to avoid any searchers deliberately. Children fail to respond to shouts and may even hide from would-be rescuers. Even adults, when searchers make close approaches, may respond with abject terror, presumably because the anxieties that have welled up over the hours and days of isolation spill over into panic at the sound of a rescuer crashing through the woods. Such factors conspire to make what is already a daunting task even more difficult.

Despite the finding that most victims are eventually discovered within a mile or two of their point of disappearance, the amount of ground that is covered by a circle of one or two miles’ radius, especially if the terrain is complex, is such that it may take hundreds of searchers many days to complete a comprehensive search.

Elaborate mathematical formulas, based on the nature of the person who has become lost (age, background, reason for being in wilderness) and the type of terrain (changes in altitude, bodies of water), are used to constrain an intensive search to the areas most likely to yield success. Nevertheless, the best chance for survival of a lost person comes at the initial “hasty search,” in which a few fit, experienced searchers are dispatched in the area immediately surrounding the point of last contact. If the hasty search fails, the odds against survival steepen dramatically. Unless conditions are very mild, those lost in the wilderness for more than 24 hours are in real peril of losing everything.

Rüdiger Wehner has spent most of his professional life wandering the Sahara Desert in search of ants. One ant in particular, the
Cataglyphis fortis
, a bit less than a centimeter in length and weighing about 10 milligrams, has occupied most of Wehner’s scientific curiosity for a career spanning many decades.
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It isn’t unusual for scientists to devote staggering amounts of time and attention to seemingly arcane subjects, but there are few cases where this attention has been as richly rewarded as it has been in Wehner’s case.

Desert ants are scavengers searching for insects that have succumbed to the rigors of life in a harsh environment. When such victims are found, the ants collect the carcasses and then return to the nest. Though this pattern of behavior sounds simple enough, Wehner noticed something remarkable about the movements of the ant. Ants that are looking for food meander in seemingly random and circuitous paths that can carry them far from their nests (up to about 200 meters). Wehner’s surprising discovery was that, on their return to the nest with the food, the ants strike out on a direct, straight-line path for the nest.

Unlike wandering human bush travelers, these ants maintain a seemingly iron grip on their location. How do they do it? One possibility is that the nest emits some kind of signal, such as a smell, that the ants can easily pinpoint. It is well known that ants sometimes follow each other’s paths using odor trails, so this seems like an obvious possibility. But using a simple but clever technique, Wehner proved that the ants were not following a scent. When foraging ants reached food sources in the desert, he picked up the ants and moved them to a new location. The ants responded to this displacement by running immediately in a direct course to where the nest
would
have been located if they had not been displaced. This proved that the ants were keeping track of the location of the nest by means of a continually updated estimate of the location and distance to it. The ability to keep such a careful record of one’s movements, and to extract an estimate of one’s current position from this record, is referred to as path integration. Path integration is one of the chief navigational tools possessed by many different types of animals, such as Wehner’s ants, which so excel at it that they can be considered to possess a biological version of Ariadne’s thread.

Having shown that ants were making use of path integration rather than a signal that was continuously emitted by the nest, Wehner set out to determine how their knowledge of their own position was established and maintained. There are two separate requirements for the ant: to keep track of the direction of the nest, and to keep track of its own distance from the nest.

We saw earlier that the vestibular system, at least theoretically, can be used to 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 to
understand 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 used
as 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.
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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 adjacent alleyway that also contained stripes, but of a different width. When the ants were released in the second alleyway, they would attempt to run back toward their nests. If they were using optic flow, manipulating stripe width should have caused them to make errors. Thinner stripes on
the homeward journey should have made them stop running too soon and thicker stripes should have made them run too far.

Although this was exactly the result that Wehner obtained, it was not the end of the story. Ants that had the lower parts of their eyes covered with black paint so that they couldn’t see the ground still ran accurately to the nest. So it looks as though optic flow
can
influence perception of distance in ants, but they can calculate distance even when they can’t see optic flow. In a way this makes good sense: in their natural setting, running along salt pans in the desert, there would probably be little visual texture on the ground, and so optic flow information might not be prominent.

A third possibility is that ants count their own steps. Wehner tested the “ant odometer” hypothesis by both lengthening ants’ legs by gluing tiny stilts to them (made of pig hair, in case you’re wondering) and shortening them using, well, scissors! Ants whose leg lengths were altered in this way made predictable errors in nest homing, suggesting that these tiny creatures do, indeed, count their steps to find their way home.

If ants count steps, they do so in intelligent ways. In another experiment, Wehner trained ants to run over a steep hill to reach a source of food. On the homeward trip, the hill was removed. If ants simply counted steps, one would predict that they would run much too far on the homeward leg of the trip, but this is not what happened. Somehow, the ants were able to correct for the change in altitude, arriving safely home after having run just the right distance to the nest. How do the ants keep track of ground distances while traveling over hills? As Rüdiger Wehner says, this is a mystery whose solution “remains to be unraveled.”
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Before we move on to consider the path-integrating abilities of beings less capable than ants, we should get an idea of the precision of path integration in ants. When the ant completes a foraging run
and makes a dash for home, how accurate is it? A typical foraging run carries an ant to a distance of about 200 meters from home. It travels over a meandering course of at least twice that distance before it finds food. At this point, it turns toward home in a path that intersects the outbound path several times but for the most part takes the ant across territory that it has never encountered before. Though the path is not perfectly straight, there is no suggestion that the ant pauses to search for the nest entrance until it is within about 1 meter of home. Using a generous estimate of 1 centimeter for the body length of an ant, this suggests that path integration as used by ants can yield accurate fixes on the nest from a distance of at least 20,000 times its own body length. Translating into human dimensions, using an estimate of 1.8 meters for the height of the average human, an equivalent feat would be to conduct accurate path integration from a distance of about 36 kilometers, a bit less than the length of a marathon. Try to imagine being able to walk in a meandering course, changing directions randomly, for a distance of about 70 kilometers (at a brisk pace this would probably take you about ten hours, not including time to rehydrate and nurse blistered feet), perhaps tackling a few craggy 400-meter peaks along the way. At the end of the walk, without using visible landmarks and without being able to see the point at which you began your walk, imagine being able to turn toward home with a precision of less than 10 degrees. To equal the performance of the ant, you should also be able to estimate the distance to home to within about 200 meters.