Probably one of the most fundamental abilities of animals is to know places. Many operate from nests and shelters, but also know and repeatedly visit other places of significance, especially places where they can find food and water. On a local scale, the most potent cue to the identity of a location in the world is its unique visual appearance. The fact that insects can utilize this property of the world in order to pinpoint their nest was shown in the first half of the previous century in an elegant and simple series of experiments by Tinbergen and Kruyt (Figure 3c): they displaced objects around the nest of a ground-nesting wasp just a few tens of centimeters to the left or the right of the nest entrance, while the wasp was out foraging. The returning wasp searched not at its true location for her nest, but at a location defined by the displaced landmarks. These visual features, not the true geographical location, or olfactory cues, thus defined the goal location for this insect. Many experiments of a similar kind have been carried out since then, aimed at unraveling the detailed mechanisms underlying this ability of view-based homing not only in insects, but also in vertebrates. Homing insects behave as if they had stored snapshots of the world as viewed from the vicinity of the goal and upon returning then move to minimize the difference between these stored views and what they currently see. These stored views seem to contain the apparent sizes of salient objects, their shape, and possibly also their color. However, insects also seem to use salient objects as beacons, not just as part of a panoramic scene, and they can acquire information on the absolute distance between objects and the goal, not just on how large they ought to appear. View-based homing is so potent, because places in the natural world are uniquely defined by the view taken from them. The reason is that there are very few repetitive structures in nature that would make different places appear alike. On the other hand, the appearance of natural scenes change, because of changes in illumination and because objects are displaced by wind, water, and large animals. Ground-nesting insects, therefore, have to update their memory of the nest environment regularly, especially first thing every morning. To what extent our knowledge of view-based homing in insects can also explain the homing abilities of, for instance, birds which operate over distances of tens to hundreds of kilometres, is not entirely clear. It is only recently that the paths of birds at these scales can be accurately recorded. Like insects, birds visit the same food and nest sites over years -often after long migrations - and are attracted to and follow route landmarks. Both the pinpoint homing accuracy and the use of guiding landscape features, such as roads, rivers, or coastlines in the case of birds, are likely to be based on remembered views.
In barren landscapes, like featureless deserts, or the open and the deep ocean, view-based navigation fails. Yet, even there, animals are known to keep to idiosyncratic routes, to find tiny islands, and to navigate on a global scale. One suggestion is that animals may have something akin to a global positioning system and the only cue they could access for that purpose would be the structure of the Earth's magnetic field. By now, animals as diverse as bacteria, bees, lobsters, salamanders, fish, turtles, and birds have been shown to be sensitive to at least some aspects of the magnetic field. Experiments with turtles in artificial magnetic fields that mimicked the situation in geographical locations that were hundreds of kilometres apart indicate that experienced animals actually recognized these locations by their magnetic field properties (although swimming on a tether in a backyard swimming pool) and moved in appropriate directions that would bring them back to their home grounds (Figure 4). The magnetic field does not only vary along latitudes and longitudes across the Earth, but also has local features, called magnetic anomalities, that could serve the same purpose as visual landmarks: something that uniquely specifies a place. Indeed, birds have been shown to change their behavior in the vicinity of such 'magnetic landmarks'. Yet it is still unclear whether any animal actually knows where it is at any point on the Earth by the particular property of the magnetic field that specifies that place, something that has been called the ability of 'true navigation'. The best evidence so far has been the behavior of turtles and lobsters in artificially created magnetic fields that mimic the situation in specific places on Earth (Figures 4b and 4c). It is only very recently that the tools have become available that allow us to track animal movements over global scales. Knowing exactly where animals go, how they move, and where they make navigational decisions is a prerequisite for understanding the knowledge base of their navigational abilities (Figure 5).
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