There is an abundance of polarized light in natural environments, but there are only two main sources from which such light arises: the scattering of sunlight within the atmosphere (Fig. 1) and hydrosphere, and the reflection of light by water surfaces (Fig. 2) and other shiny, non-metallic, dielectric surfaces such as soil, rocks and vegetation. Recently, it has become apparent that animals can make use of these rich sources of information in a multitude of ways. Hence, it seems pertinent to ask whether all these ways of exploiting polarized-light information are based on one common neural polarization-vision system designed to process information about polarized light and employed by different animal species. For example, does such a common neural polarization-vision channel unambiguously determine, in a first step, the angle of polarization (the orientation of the plane in which the electric vector, or e-vector, of light oscillates) in any particular point of the animal���s polarized visual world, and is it this unambiguous e-vector information that is later, in a second step, used to fulfil whatever the particular ecological situation requires? Of course, any system using polarized light as a source of environmental information must have some kind of sensor that is differentially activated by different states of polarization, but it might be at this peripheral stage that the common characteristics of such systems end, let alone the possibility that even the sensory devices ��� the polarization analyzers ��� might differ depending on the particular task the animal must accomplish and also on the animal���s evolutionary history. What is the behavioural task that the animal must accomplish by using polarized-light information? It is manifold. A large number of behavioural studies have been performed by various authors under various conditions and experimental paradigms in various groups of animals. These studies have led to a variety of results and, in turn, to considerable debate about how to define ���polarization vision���. These definitions range widely from any general ability to respond to polarized light to what could be called the most sophisticated e-vector-detecting system, namely one that is able to determine the angle of polarization (the e-vector orientation, ��) independently of variations in intensity (I), degree of polarization (d) and spectral content (��). The conditions such a ���true polarization-vision system��� (sensu Nilsson and Warrant, 1999) must meet were outlined nearly a quarter of a century ago (Bernard and Wehner, 1977), but whether the polarization-sensitive systems of any animal species obey these theoretical requirements has not been shown yet. Hence, let us move from definitions to observations, and consider three stimulus situations. Stimulus situation I: water/air interface Stimulus situation I is characterized by a rather simple set of polarization cues: light reflected from water surfaces is linearly polarized (Fig. 2). In reflection polarization, the degree of polarization varies with the angle of incidence, the elevation of the sun and the properties of the dielectric interface (e.g. air/water or air/glass), but for all practical purposes light polarized by reflection from water (and other shiny) surfaces is polarized predominantly parallel to the reflecting surface. As Rudolf Schwind has shown in a number 2589 The Journal of Experimental Biology 204, 2589���2596 (2001) Printed in Great Britain �� The Company of Biologists Limited 2001 JEB3386 In this concept paper, three scenarios are described in which animals make use of polarized light: the underwater world, the water surface and the terrestrial habitat vaulted by the pattern of polarized light in the sky. Within these various visual environments, polarized light is used in a number of ways that make quite different demands on the neural circuitries mediating these different types of behaviour. Apart from some common receptor and pre-processing mechanisms, the underlying neural mechanisms may differ accordingly. Often, information about �� (the angle of polarization), d (the degree of polarization) and �� (the spectral content) might not ��� and need not ��� be disentangled. Hence, the hypothesis entertained in this account is that polarization vision comes in various guises, and that the answer to the question posed in the title is most probably no. Key words: polarized light, underwater vision, e-vector compass, contrast enhancement, optical signalling, Cataglyphis spp. Summary Introduction POLARIZATION VISION ��� A UNIFORM SENSORY CAPACITY? R��DIGER WEHNER* Zoologisches Institut der Universit��t Z��rich, Winterthurerstrasse 190, CH-8057 Z��rich, Switzerland *e-mail: rwehner@zool.unizh.ch Accepted 19 April 2001

2590 of painstaking studies (e.g. Schwind, 1984 Schwind, 1991 see also Kriska et al., 1998), many water beetles and bugs flying on dispersal in search of bodies of water are attracted by horizontally polarized reflections from the ground. Unpolarized light is ineffective, even if its intensity is several times higher. Note, however, that polarized reflections can also give rise to incoherent and erratic polarization cues, which can invade, so to speak, any colour vision system if the latter receives its input from photoreceptors that ��� as in arthropods ��� are inherently sensitive to polarized light. This problem is avoided by destroying the polarization sensitivity of photoreceptors in those parts of the eye that are involved in colour vision (Wehner and Bernard, 1993). Butterflies, however, at least the Australian orchid butterflies of the species Papilio aegeus, do not seem to disentangle the ambiguity between colour and polarization (Kelber, 1999 Kelber et al., 2001). Rather than being a failure, the use of polarization-induced ���false��� colours might help the butterflies to detect appropriate oviposition sites, horizontally oriented green leaves. It appears that, whenever the polarized reflections create predictable arrays of stimuli indicative of biologically meaningful parts of the environment, some species of animal have evolved sensory filters that are matched to perceive them. The backswimmer Notonecta glauca is a prime example. It possesses a set of specialized photoreceptors that are most sensitive to horizontally polarized light. Let us term the part of the eye that contains these specialized photoreceptors the POL area of the eye. Each visual unit (ommatidium) of this POL area contains two sets of photoreceptors that have their e- vector tuning axes oriented in mutually perpendicular ways: one axis horizontal, the other vertical (Schwind, 1983). As we shall see below, such an orthogonal arrangement of polarization analyzers enhances polarization contrast if appropriately combined and renders the resulting signal invariant against fluctuations in radiant intensity. Thus, the structural peculiarities of the waterbug���s POL channel are adapted to the stimulus characteristics prevailing at the surface of its aquatic world. Stimulus situation II: water Let us now move right into the aquatic world. Of course, there is the polarization pattern of the sky (see stimulus situation III) that can be seen within Snell���s window just above the observer, especially if the observer is close to the water surface. However, outside this aerial window, light is polarized by scattering within the water itself. The angle of this water-induced polarization is almost always horizontal, but the degree of polarization increases the further one moves away from the shore towards the open water. Again, these consistent environmental stimulus characteristics have been exploited by polarization-sensitive animals. Small branchiopod crustaceans such as Daphnia species swim consistently towards the light with a higher degree of polarization if the e-vector is oriented horizontally and do so regardless of light intensity (Schwind, 1999). Outdoors in a pond, this behaviour will lead to the well-known ecological phenomenon of ���shore flight���, i.e. the horizontal swimming movements of small pelagic crustaceans away from the shoreline towards deeper waters, where the density of predators is lower than in shoreline regions. However, there is more to polarization vision under water than just using it for swimming away from the shore. Note that, in underwater vision, the scattering of light largely degrades contrast by interposing a ���veil of light��� between the observer and any object observed (Lythgoe and Hemmings, 1967 Lythgoe, 1971 Nilsson, 1996). As a result of the prevailing horizontal polarization, a vertical analyzer would reduce the amount of scattered light perceived and, hence, increase contrast. This is analogous to the effect of polaroid sunglasses, but for scattered rather than reflected light (or to the use of yellow glasses when skiing in fog). More particularly, there is much more scatter in the background spacelight than between R. WEHNER Fig. 1. Polarization arising from light scattering within the earth���s atmosphere. Unpolarized sunlight (upper left panel) remains unpolarized if it reaches the observer directly (scattering angle 0 ��, right panel), but is linearly polarized if it is scattered by atmospheric O2 and N2 molecules. Within a theoretical (Rayleigh) atmosphere, the degree of polarization reaches 100 %, if the scattering angle is 90 �� (lower left panel). Other scattering angles yield smaller degrees of polarization (lower right panel). The light is then said to be partially linearly polarized. In the real atmosphere, the degree of polarization ��� even in full blue skies ��� is almost always less than 70 % (see Horvath and Wehner, 1999). Background landscape: Naukluft gravel plain desert, north of Gobabeb, Namibia.