ABSTRACT For polarized light to inform behaviour, the typical range of degrees of polarization observable in the animal's natural environment must be above the threshold for detection and interpretation. Here, we present the first investigation of the degree of linear polarization threshold for orientation behaviour in a nocturnal species, with specific reference to the range of degrees of polarization measured in the night sky. An effect of lunar phase on the degree of polarization of skylight was found, with smaller illuminated fractions of the moon's surface corresponding to lower degrees of polarization in the night sky. We found that the South African dung beetle Escarabaeus satyrus can orient to polarized light for a range of degrees of polarization similar to that observed in diurnal insects, reaching a lower threshold between 0.04 and 0.32, possibly as low as 0.11. For degrees of polarization lower than 0.23, as measured on a crescent moon night, orientation performance was considerably weaker than that observed for completely linearly polarized stimuli, but was nonetheless stronger than in the absence of polarized light.

ABSTRACT The swallowtail butterfly Papilio xuthus can perceive the linear polarization of light. Using a novel polarization projection system, we recently demonstrated that P. xuthus can detect visual motion based on polarization contrast. In the present study, we attempt to infer via behavioural experiments the mechanism underlying this polarization-based motion vision. Papilio xuthus do not perceive contrast between unpolarized and diagonally polarized light, implying that they cannot unambiguously estimate angle and degree of polarization, at least as far as motion detection is concerned. Furthermore, they conflate brightness and polarization cues, such that bright vertically polarized light resembles dim unpolarized light. These observations are consistent with a one-channel ‘monopolatic’ detector mechanism. We extend our existing model of motion vision in P . xuthus to incorporate these polarization findings, and conclude that the photoreceptors likely to form the basis for the putative monopolatic polarization detector are R3 and R4, which respond maximally to horizontally polarized green light. R5–R8, we propose, form a polarization-insensitive secondary channel tuned to longer wavelengths of light. Consistent with this account, we see greater sensitivity to polarization for green-light stimuli than for subjectively equiluminant red ones. Somewhat counter-intuitively, our model predicts greatest sensitivity to vertically polarized light; owing to the non-linearity of photoreceptor responses, light polarized to an angle orthogonal to a monopolatic detector's orientation offers the greatest contrast with unpolarized light.

ABSTRACT Many animal species take advantage of polarization vision for vital tasks such as orientation, communication and contrast enhancement. Previous studies have suggested that decapod crustaceans use a two-channel polarization system for contrast enhancement. Here, we characterize the polarization contrast sensitivity in a grapsid crab . We estimated the polarization contrast sensitivity of the animals by quantifying both their escape response and changes in heart rate when presented with polarized motion stimuli. The motion stimulus consisted of an expanding disk with an 82 deg polarization difference between the object and the background. More than 90% of animals responded by freezing or trying to avoid the polarized stimulus. In addition, we co-rotated the electric vector (e-vector) orientation of the light from the object and background by increments of 30 deg and found that the animals' escape response varied periodically with a 90 deg period. Maximum escape responses were obtained for object and background e-vectors near the vertical and horizontal orientations. Changes in cardiac response showed parallel results but also a minimum response when e-vectors of object and background were shifted by 45 deg with respect to the maxima. These results are consistent with an orthogonal receptor arrangement for the detection of polarized light, in which two channels are aligned with the vertical and horizontal orientations. It has been hypothesized that animals with object-based polarization vision rely on a two-channel detection system analogous to that of color processing in dichromats. Our results, obtained by systematically varying the e-vectors of object and background, provide strong empirical support for this theoretical model of polarized object detection.

SUMMARY Sensitivity to polarized light is widespread among marine animals, including crustaceans, cephalopods and some fishes. They use this ability to orient and find prey, and possibly for a number of other visual tasks. Unlike the ultraviolet-sensitive polarization receptors of most insects, the polarization receptors of marine invertebrates tend to be maximally sensitive near 500nm, suggesting that polarized light in water differs from that in air. The underwater field of partially linearly polarized light has been studied for nearly 50 years, but data are still limited and sparse. We measured the submarine polarized light field from 350 to 600nm throughout the day on a coral reef in the Florida Keys at a depth of 15m using the underwater laboratory Aquarius as a research platform. Our results show that the angle of polarization as viewed along any given line of sight at this depth is a relatively simple function of solar position and that the degree of polarization is greatest 60–90° from the sun. Both e-vector angle and degree of polarization vary only slightly with wavelength, although light is sometimes less polarized in the ultraviolet. Since light is most intense at medium wavelengths and polarization is nearly maximal at these wavelengths, invertebrate polarization photoreceptors are spectrally well placed. Also, the relative spectral constancy of the angle and degree of polarization supports fish polarization sensitivity, which relies on spectrally diverse photoreceptor sets.

SUMMARY Several fish species appear to be polarization sensitive, i.e. to be able to discriminate a light source’s maximum plane of polarization from any other plane. However, the functional significance of this ability remains unclear. We tested the hypothesis that polarized light improves the prey location ability of free-swimming rainbow trout ( Oncorhynchus mykiss ) in laboratory aquaria. We found that prey location distances increased while the vertical component of prey location angle decreased under polarized compared with unpolarized (diffuse) illumination. The average frequency distribution of the horizontal component of prey location angle was more bimodal under polarized than unpolarized illumination. These results indicate that polarization sensitivity enhances prey location by juvenile rainbow trout.