Receptor noise as a determinant of colour thresholds

Sugar concentration of fruits and their detection via color in the Central American spider monkey (Ateles geoffroyi)

Although most arguments explaining the predominance of polymorphic color vision in platyrrhine monkeys are linked to the advantage of trichromacy over dichromacy for foraging for ripe fruits, little information exists on the relationship between nutritional reward and performance in fruit detection with different types of color vision. The principal reward of most fruits is sugar, and thus it seems logical to investigate whether fruit coloration provides a long-distance sensory cue to primates that correlates with sugar content. Here we test the hypothesis that fruit detection performance via trichromatic color vision phenotypes provides better information regarding sugar concentration than dichromatic phenotypes (i.e., is a color vision phenotype with sufficient red-green (RG) differentiation necessary to "reveal" the concentration of major sugars in fruits?). Accordingly, we studied the fruit foraging behavior of Ateles geoffroyi by measuring both the reflectance spectra and the concentrations of major sugars in the consumed fruits. We modeled detection performance with different color phenotypes. Our results provide some support for the hypothesis. The yellow-blue (YB) color signal, which is the only one available to dichromats, was not significantly related to sugar concentration. The RG color vision signal, which is present only in trichromats, was significantly correlated with sugar content, but only when the latter was defined by glucose. There was in fact a consistent negative relationship between fruit detection performance and sucrose concentration, although this was not significant for the 430 nm and 550 nm phenotypes. The regular trichromatic phenotypes (430 nm, 533 nm, and 565 nm) showed higher correlations between fruit performance and glucose concentration than the other two trichromatic phenotypes. Our study documents a trichromatic foraging advantage in terms of fruit quality, and suggests that trichromatic color vision is advantageous over dichromatic color vision for detecting sugar-rich fruits.

Detection of Fruit and the Selection of Primate Visual Pigments for Color Vision

Primates have X chromosome genes for cone photopigments with sensitivity maxima from 535 to 562 nm. Old World monkeys and apes (catarrhines) and the New World (platyrrhine) genus Alouatta have separate genes for 535-nm (medium wavelength; M) and 562-nm (long wavelength; L) pigments. These pigments, together with a 425-nm (short wavelength) pigment, permit trichromatic color vision. Other platyrrhines and prosimians have a single X chromosome gene but often with alleles for two or three M/L photopigments. Consequently, heterozygote females are trichromats, but males and homozygote females are dichromats. The criteria that affect the evolution of M/L alleles and maintain genetic polymorphism remain a puzzle, but selection for finding food may be important. We compare different types of color vision for detecting more than 100 plant species consumed by tamarins (Saguinus spp.) in Peru. There is evidence that both frequency-dependent selection on homozygotes and heterozygote advantage favor M/L polymorphism and that trichromatic color vision is most advantageous in dim light. Also, whereas the 562-nm allele is present in all species, the occurrence of 535- to 556-nm alleles varies between species. This variation probably arises because trichromatic color vision favors widely separated pigments and equal frequencies of 535/543- and 562-nm alleles, whereas in dichromats, long-wavelength pigment alleles are fitter.

Goldfish color vision sensitivity is high under light-adapted conditions

The wavelength discrimination threshold of three goldfish was examined in a series of behavioral experiments. Using an auto-shaping technique, detection thresholds were established for 531 and 648 nm spectral increments presented on a 6.6 cd m(-2) white background. Next, discrimination between the wavelengths was established at equal, suprathreshold, intensities. Finally, the intensities of the two stimuli were reduced to establish the intensity threshold for the wavelength discrimination. The results indicate that goldfish, like several mammalian species, can discriminate wavelength at detection threshold intensity. This finding suggests that high color sensitivity is not confined to mammals or dependent upon a very high percentage of wavelength opponent ganglion cells. Rather, high color vision sensitivity may be based upon an inherent sensitivity advantage of wavelength opponent receptive fields compared to non-wavelength opponent receptive fields and be an important selective advantage of wavelength opponency and color vision.

Effect of polymorphic colour vision for fruit detection in the spider monkey Ateles geoffroyi, and its implications for the maintenance of polymorphic colour vision in platyrrhine monkeys

Most platyrrhine monkeys have an X-linked tri-allelic polymorphism for medium and long wavelength (M/L) sensitive cone photopigments. These pigments' sensitivity maxima (lambdamax) range from 535 to 562 nm. All animals also have an autosomally coded short-wavelength-sensitive (S) cone pigment. In populations with three M/L alleles there are six different colour vision phenotypes. Heterozygous females have trichromatic colour vision, while males and homozygous females are dichromats. The selective basis for this polymorphism is not understood, but is probably affected by the costs and benefits of trichromatic compared to dichromatic colour vision. For example, it has been suggested that trichromats are better equipped than dichromats to detect fruit against a leaf background. To investigate this possibility, we modeled fruit detection by various colour vision phenotypes present in the frugivorous spider monkey, Ateles geoffroyi. Our study population is thought to have three M/L alleles with cone pigment lambdamax values close to 535, 550 and 562 nm. The model predicted that all trichromat phenotypes had an advantage over dichromats, and the 535/562 nm phenotype was best; however, the model predicted that dichromats could detect all of the fruit species consumed by spider monkeys. We conclude that the heterozygote advantage experienced by females may be the most plausible explanation for the maintenance of this polymorphism in A. geoffroyi. Nevertheless, more studies need to evaluate social foraging behaviour and the performance of different phenotypes of other New World monkeys to determine if this is a global explanation for this phenomena or more specific to A. geofforyi.

EVOLUTION OF COLOR VARIATION IN DRAGON LIZARDS: QUANTITATIVE TESTS OF THE ROLE OF CRYPSIS AND LOCAL ADAPTATION

Many animal species display striking color differences with respect to geographic location, sex, and body region. Traditional adaptive explanations for such complex patterns invoke an interaction between selection for conspicuous signals and natural selection for crypsis. Although there is now a substantial body of evidence supporting the role of sexual selection for signaling functions, quantitative studies of crypsis remain comparatively rare. Here, we combine objective measures of coloration with information on predator visual sensitivities to study the role of crypsis in the evolution of color variation in an Australian lizard species complex (Ctenophorus decresii). We apply a model that allows us to quantify crypsis in terms of the visual contrast of the lizards against their natural backgrounds, as perceived by potential avian predators. We then use these quantitative estimates of crypsis to answer the following questions. Are there significant differences in crypsis/conspicuousness among populations? Are there significant differences in crypsis conspicuousness between the sexes? Are body regions "exposed" to visual predators more cryptic than "hidden" body regions? Is there evidence for local adaptation with respect to crypsis against different substrates? In general, our results confirmed that there are real differences in crypsis/conspicuousness both between populations and between sexes; that exposed body regions were significantly more cryptic than hidden ones, particularly in females; and that females, but not males, are more cryptic against their own local background than against the background of other populations [corrected]. Body regions that varied most in contrast between the sexes and between populations were also most conspicuous and are emphasized by males during social and sexual signaling. However, results varied with respect to the aspect of coloration studied. Results based on chromatic contrast ("hue" of color) provided better support for the crypsis hypothesis than did results based on achromatic contrast ("brightness" of color). Taken together, these results support the view that crypsis plays a substantial role in the evolution of color variation and that color patterns represent a balance between the need for conspicuousness for signaling and the need for crypsis to avoid predation.

Interspecific and intraspecific views of color signals in the strawberry poison frogDendrobates pumilio

Poison frogs in the anuran family Dendrobatidae use bright colors on their bodies to advertise toxicity. The species Dendrobates pumilio Schmidt 1858, the strawberry poison frog, shows extreme polymorphism in color and pattern in Panama. It is known that females of D. pumiliopreferentially choose mates of their own color morph. Nevertheless, potential predators must clearly see and recognize all color morphs if the aposematic signaling system is to function effectively. We examined the ability of conspecifics and a model predator to discriminate a diverse selection of D. pumilio colors from each other and from background colors. Microspectrophotometry of isolated rod and cone photoreceptors of D. pumilio revealed the presence of a trichromatic photopic visual system. A typical tetrachromatic bird system was used for the model predator. Reflectance spectra of frog and background colors were obtained, and discrimination among spectra in natural illuminants was mathematically modeled. The results revealed that both D. pumilio and the model predator discriminate most colors quite well, both from each other and from typical backgrounds, with the predator generally performing somewhat better than the conspecifics. Each color morph displayed at least one color signal that is highly visible against backgrounds to both visual systems. Our results indicate that the colors displayed by the various color morphs of D. pumilio are effective signals both to conspecifics and to a model predator.

Modelling divergence in luminance and chromatic detection performance across measured divergence in surfperch (Embiotocidae) habitats

This study predicts target detection performance in species-specific habitats for six surfperch (Embiotocidae) living in optically variable California kelp forests. Using species-specific measurements of habitat irradiance and photoreceptor absorbance in a simple dichromatic model for luminance and chromatic detection, the estimated performance of species' measured photopigments was compared to the theoretical maximum for each habitat. Modelling results suggest that changes in peak photoreceptor absorbance (lambda(max)), photoreceptor optical density, and photic environment may affect detection performance. Estimated performances for luminance detection were consistently high, while chromatic detection varied by habitat and demonstrated substantial improvements with increasing optical density differences between cone classes.

Discrimination of oriented visual textures by poultry chicks

We investigated how poultry chicks discriminate textures of lines sharing a common orientation from textures of variably oriented lines. Stimulus colours along with illumination were adjusted to selectively stimulate different receptor mechanisms. Chicks could discriminate achromatic textures, but not textures isoluminant to the double cones that gave a long vs. medium wavelength chromatic signal, nor an intensity signal for the short and very-short wavelength cones. These results suggest that detection of line orientation and texture discrimination uses an achromatic signal derived either from the double cones, or summed outputs of long and medium wavelength sensitive single cones.

Differences in Visual Signal Design and Detectability between Allopatric Populations ofAnolisLizards

We tested the prediction of the sensory drive hypothesis using four allopatric populations of the lizard Anolis cristatellus from two distinct environments (i.e., mesic and xeric conditions). For each population, we measured habitat light characteristics and quantified signal design by measuring the spectral and total reflectance and transmittance of the dewlap. We used these data to calculate dewlap detectability using an empirically based model of signal detection probability. We found that populations from mesic and xeric conditions occupy two distinct habitats with respect to light intensity and spectral quality and that dewlap design has diverged between populations in a way that increases signal detectability in each habitat. The major difference in dewlap design was in total reflectance and transmittance, making dewlaps from xeric habitats darker and dewlaps from mesic habitats brighter. Furthermore, dewlap detection decreased significantly when a dewlap from a xeric habitat is detected under the spectral conditions of a mesic habitat. The converse is true for a dewlap from a mesic habitat. We propose that sensory drive has promoted divergence in dewlap design in distinct habitat light conditions, and we discuss the possibility that selection might promote early stages of reproductive isolation as a by-product of selection on dewlap design to distinct habitat light conditions.

Marginal Differentiation between the Sexual and General Carotenoid Pigmentation of Guppies (Poecilia reticulata) and a Possible Visual Explanation

We present the first detailed analysis of carotenoid pigmentation of the integument of guppies (Poecilia reticulata Peters), quantifying variation in carotenoid content and composition of wild guppies from three drainages on Trinidad (1) between the sexual and general pigmentation of males, (2) between the sexes, and (3) geographically in relation to carotenoid availability. We report that the carotenoid pigments in the integument of guppies are predominantly esters of tunaxanthin. The peak wavelength of carotenoids in the orange spots of males lay only ca. 2.8 nm higher than that of pigments outside of the orange spots, and the peak wavelength of carotenoids in the male whole integument does not differ from that in the female whole integument. Carotenoid composition of the general integument of females and the non-orange spot fraction of males, but not of the orange spot fraction of males, varied with diet, correlating with the ratio beta-carotene to lutein in the different streams. Male guppies deposit higher concentrations of carotenoids in their orange spots than in the rest of the integument (five to nine times higher), but not at the expense of the general integument, which was similarly endowed as the general integument of females, even in carotenoid-poor streams. Presumably males absorb/retain more pigments than females. Photoreceptor-based simulations suggest that tunaxanthin provides both greater brightness and chroma than would 4-keto-carotenoids such as astaxanthin.

The effect of colour vision status on the detection and selection of fruits by tamarins (Saguinus spp.)

The evolution of trichromatic colour vision by the majority of anthropoid primates has been linked to the efficient detection and selection of food, particularly ripe fruits among leaves in dappled light. Modelling of visual signals has shown that trichromats should be more efficient than dichromats at distinguishing both fruits from leaves and ripe from unripe fruits. This prediction is tested in a controlled captive setting using stimuli recreated from those actually encountered by wild tamarins (Saguinus spp.). Dietary data and reflectance spectra of Abuta fluminum fruits eaten by wild saddleback (Saguinus fuscicollis) and moustached (Saguinus mystax) tamarins and their associated leaves were collected in Peru. A. fluminum leaves, and fruits in three stages of ripeness, were reproduced and presented to captive saddleback and red-bellied tamarins (Saguinus labiatus). Trichromats were quicker to learn the task and were more efficient at selecting ripe fruits than were dichromats. This is the first time that a trichromatic foraging advantage has been demonstrated for monkeys using naturalistic stimuli with the same chromatic properties as those encountered by wild animals.

Structural colouration of avian skin: convergent evolution of coherently scattering dermal collagen arrays

Structural colours of avian skin have long been hypothesized to be produced by incoherent (Rayleigh/Tyndall) scattering. We investigated the colour, anatomy, nanostructure and biophysics of structurally coloured skin, ramphotheca and podotheca from 31 species of birds from 17 families in 10 orders from across Aves. Integumentary structural colours of birds include ultraviolet, dark blue, light blue, green and yellow hues. The discrete peaks in reflectance spectra do not conform to the inverse fourth power relationship predicted by Rayleigh scattering. The dermis of structurally coloured skin consists of a thick (100-500 micro m) layer of collagen that is usually underlain by a layer of melanin granules. Transmission electron micrographs (TEMs) of this colour-producing dermal collagen layer revealed quasi-ordered arrays of parallel collagen fibres. Two-dimensional (2-D) Fourier analysis of TEMs of the collagen arrays revealed a ring of peak spatial frequencies in the spatial variation in refractive index that are the appropriate size to make the observed ultraviolet-yellow colours by coherent scattering alone. One species, Philepitta castanea (Eurylaimidae), has exceptionally ordered, hexagonal arrays of collagen fibres that produce a hexagonal pattern of spatial frequency peaks in the power spectra. Ultraviolet, blue, green and yellow structural colours of avian skin are produced by coherent scattering (i.e. constructive interference) by arrays of collagen fibres in the dermis. Some yellow and orange skin colours are produced with a combination of structural and pigmentary mechanisms. These combined colours can have reflectance spectra with discrete peaks that are more saturated than hues produced by carotenoid pigments alone. Bluish facial skin from two species of Neotropical antbirds (Thamnophilidae) are nanostructurally too small to produce visible light by coherent scattering, and the colour production mechanism in these species remains unknown. Based on the phylogenetic distribution of structurally coloured skin in Aves, this mechanism of colour production has evolved convergently more than 50 independent times within extant birds.

Coloured oil droplets enhance colour discrimination

The eyes of most diurnal reptiles and birds contain coloured retinal filters-oil droplets. Although these filters are widespread, their adaptive advantage remains uncertain. To understand why coloured oil droplets appeared and were retained during evolution, I consider both the benefits and the costs of light filtering in the retina. Oil droplets decrease cone quantum catch and reduce the overlap in sensitivity between spectrally adjacent cones. The reduction of spectral overlap increases the volume occupied by object colours in a cone space, whereas the decrease in quantum catch increases noise, and thus reduces the discriminability of similar colours. The trade-off between these two effects determines the total benefit of oil droplets. Calculations show that coloured oil droplets increase the number of object colours that can be discriminated, and thus are beneficial for colour vision.

Independence of mechanisms tuned along cardinal and non-cardinal axes of color space: evidence from factor analysis

Many previous studies employing paradigms such as adaptation, masking and summation-near-threshold have demonstrated the existence of separate mechanisms underlying the detection of the three cardinal axes of color space: L+M, L-M and S-(L+M). In addition, some studies have demonstrated the existence of higher-order mechanisms tuned to non-cardinal axes (which are made up of combinations of the cardinal axes). In order to address the issue of separate and independent color mechanisms further, here we applied factor analysis to contrast threshold data obtained from 41 subjects for nine different axes in color space (the three cardinal axes and the six non-cardinal axes midway between). In line with previous studies, the results of a three-factor analysis performed on contrast thresholds for the cardinal axes revealed independence across the three. However, in some of our factor analyses (for example, when a two-factor analysis was performed on the cardinal axes), intercorrelation was observed between L-M and S-(L+M) stimuli. With regard to higher-order mechanisms, our factor analyses revealed mechanisms selective for non-cardinal axes within the (L-M)/(L+M) and (S-(L+M))/(L+M) color planes, but not the (L-M)/(S-(L+M)) color plane. To ensure that the intercorrelation observed between L-M and S-(L+M) cardinal axes was not due to the particular stimulus parameters or testing measures employed, in three of our subjects we performed a "summation-near-threshold" experiment using experimental conditions nearly identical to those in the factor analysis experiments. In accordance with previous findings [Vision Research 39 (1999) 733], L-M and S-(L+M) stimuli were found to be separable in this analysis. This seeming discrepancy between the results of our factor analysis and those obtained from paradigms such as summation-near-threshold can be resolved by proposing that the mechanisms underlying detection of L-M and S-(L+M) stimuli are separable (as defined by the ability to isolate activity within each mechanism using select stimuli), yet nonetheless intercorrelated. Such intercorrelation could arise if these two mechanisms are limited by the same source of variability and/or subject to the same gain control.

Colour vision: colouring the dark

Humans lose colour vision at night and it has often been assumed that this happens to other animals as well. It is not true of nocturnal moths, however: a recent study has shown that the elephant hawk moth makes use of trichromatic colour vision when seeking flowers by starlight.

The roles of receptor noise and cone oil droplets in the photopic spectral sensitivity of the budgerigar, Melopsittacus undulatus

Individual budgerigars (Melopsittacus undulatus) were taught to detect narrow bands of wavelengths under ambient illumination of known spectral composition. Because the cone pigments of this species of bird have been identified and data on carotenoid absorbance present in the cone oil droplets are available, predictions of the Vorobyev-Osorio equations can be calculated with reasonable confidence. Based on more than 27,600 individual choices made by several birds at 10 wavelengths, the photopic sensitivity (i.e., color thresholds) of these birds is found to be consistent with the hypothesis that threshold discrimination of colored targets is limited by receptor noise and that high sensitivity to near-ultraviolet wavelengths is in harmony with the relatively small number of ultraviolet cones present in the retina. The pronounced fine structure of the sensitivity spectrum is caused by the absorption of cone oil droplets. Under natural sunlight, containing more energy in the near-ultraviolet than is present in artificial indoor lighting, the birds' peak of sensitivity in the ultraviolet should be much less prominent than it is in laboratory experiments.

Animal colour vision--behavioural tests and physiological concepts

Over a century ago workers such as J. Lubbock and K. von Frisch developed behavioural criteria for establishing that non-human animals see colour. Many animals in most phyla have since then been shown to have colour vision. Colour is used for specific behaviours, such as phototaxis and object recognition, while other behaviours such as motion detection are colour blind. Having established the existence of colour vision, research focussed on the question of how many spectral types of photoreceptors are involved. Recently, data on photoreceptor spectral sensitivities have been combined with behavioural experiments and physiological models to study systematically the next logical question: 'what neural interactions underlie colour vision?' This review gives an overview of the methods used to study animal colour vision, and discusses how quantitative modelling can suggest how photoreceptor signals are combined and compared to allow for the discrimination of biologically relevant stimuli.

Topography of Photoreceptors and Retinal Ganglion Cells in the Spotted Hyena (Crocuta crocuta)

The spatial distributions of photoreceptors and retinal ganglion cells were examined in the spotted hyena (Crocuta crocuta). Two populations of cones were identified by immunocytochemical labeling. The hyena retina contains approximately 2.3 million middle- to long-wavelength sensitive (M/L) cones that reach peak densities of about 7,500/mm(2) in the vicinity of the optic nerve head. A sparser population of short-wavelength sensitive (S) cones, totaling about 0.3 million, was also detected. There is a striking disparity in the spatial distributions of the two cone types with S cones achieving peak density in a region located well below the optic nerve head. The differences in the spatial distributions of the two cone types have implications both for visual sensitivity and for color vision. Hyena rods outnumber cones by about 100:1 with rod density falling off modestly along a central-peripheral gradient. Ganglion cells were identified in retinal wholemounts by Nissl staining patterns. Their distribution defines a prominent visual streak with highest spatial packing (approx. 4,200/mm(2)) in an area centralis that is located in the temporal retina. The total number of ganglion cells is estimated at about 260,000. Using standard assumptions the maximum spatial resolution of the spotted hyena is calculated to be about 8.4 cycles/degree, a value similar to estimates obtained for other terrestrial carnivores.

Vision in the peafowl (Aves:Pavo cristatus)

The visual sense of the Indian blue-shouldered peafowl Pavo cristatus was investigated with respect to the spectral absorption characteristics of the retinal photoreceptors, the spectral transmittance of the ocular media and the topographic distribution of cells in the retinal ganglion cell layer. Microspectrophotometry revealed a single class of rod,four spectrally distinct types of single cone and a single class of double cone. In the case of the single cone types, which contained visual pigments with wavelengths of maximum absorbance (λmax) at 424, 458,505 and 567 nm, spectral filtering by the ocular media and the different cone oil droplets with which each visual pigment is associated gives predicted peak spectral sensitivities of 432, 477, 537 and 605 nm, respectively. Topographic analysis of retinal ganglion cell distribution revealed a large central area of increased cell density (at peak, 35,609 cells mm-2) with a poorly defined visual streak extending nasally. The peafowl has a calculated maximum spatial resolution (visual acuity) in the lateral visual field of 20.6 cycles degrees-1. These properties of the peafowl eye are discussed with respect to its visual ecology and are compared with those of other closely related species.

A cytological study on the development of the different types of visual cells in the chicken (Gallus domesticus)

The formation of visual cells and their intracellular organelles was studied in the embryonic chicken (Gallus domesticus) between stage 36 and hatching. Cilia formation was observed at stage 30 and by stage 42, outer segment formation from the cilia was evident. The inner segments appeared as buddings at stage 36. By stage 37, the buddings of double cones were observed clearly and such buddings elongated by stage 42. Both the single cones and rods appeared as buddings by stage 38 and elongation of the buddings was seen by stage 42. Oil droplets initially appeared by stage 39 in accessory cones and were observed in other cones by stage 42. Glycogen bodies were demonstrated firstly in rods and accessory cones at stage 43 and their development was completed by stage 45. In essence, all the essential elements of the visual cells were fully developed by hatching.

The influence of flicker rate on plasma corticosterone levels of European starlings, Sturnus vulgaris

Recent reviews have highlighted the differences between human and avian vision with regard to temporal resolution and the potential problems it may cause for avian welfare and video playback experiments. Birds tend to have much higher critical fusion frequencies than do humans (>100 Hz vs 50-60 Hz in humans), which means that they perceive light as flickering up to and over 100 Hz. This is higher than most television monitors (which have refresh rates of 50 or 60 Hz) and normal fluorescent lighting (100 or 120 Hz), and because humans find flickering light aversive, it has been suggested that birds will as well. If this were the case, then there would be welfare implications of maintaining them under such lighting and also a potential effect on their behavioral responses in video playback experiments. However, there is some behavioral evidence that indicates that birds do not appear to find flicker aversive and may even prefer flickering lighting. The authors aimed to determine whether a passerine, the European starling, found flicker aversive by measuring the corticosterone stress response in birds maintained under high- or low-frequency fluorescent lighting (35-40 kHz vs 100 Hz) for 1 or 24 h. The results suggest that low-frequency lighting is potentially more stressful because, where differences exist, birds in the low-frequency treatment always showed higher basal corticosterone. However, the evidence is not consistent because in half of the blocks, there were no significant treatment effects and, where there were, the time course of the effects was variable.

Colour categorization by domestic chicks

Spectral stimuli form a physical continuum, which humans divide into discrete non-overlapping regions or categories that are designated by colour names. Little is known about whether non-verbal animals form categories on stimulus continua, but work in psychology and artificial intelligence provides models for stimulus generalization and categorization. We compare predictions of such models to the way poultry chicks (Gallus gallus) generalize to novel stimuli following appetitive training to either one or two colours. If the two training colours are (to human eyes) red and greenish-yellow or green and blue, chicks prefer intermediates, i.e. orange rather than red or yellow and turquoise rather than green or blue. The level of preference for intermediate colours implies that the chicks interpolate between the training stimuli. However, they do not extrapolate beyond the limits set by the training stimuli, at least for red and yellow training colours. Similarly, chicks trained to red and blue generalize to purple, but they do not generalize across grey after training to the complementary colours yellow and blue. These results are consistent with a modified version of a Bayesian model of generalization from multiple examples that was proposed by Shepard and show similarities to human colour categorization.

The visual ecology of avian photoreceptors

The spectral sensitivities of avian retinal photoreceptors are examined with respect to microspectrophotometric measurements of single cells, spectrophotometric measurements of extracted or in vitro regenerated visual pigments, and molecular genetic analyses of visual pigment opsin protein sequences. Bird species from diverse orders are compared in relation to their evolution, their habitats and the multiplicity of visual tasks they must perform. Birds have five different types of visual pigment and seven different types of photoreceptor-rods, double (uneven twin) cones and four types of single cone. The spectral locations of the wavelengths of maximum absorbance (lambda(max)) of the different visual pigments, and the spectral transmittance characteristics of the intraocular spectral filters (cone oil droplets) that also determine photoreceptor spectral sensitivity, vary according to both habitat and phylogenetic relatedness. The primary influence on avian retinal design appears to be the range of wavelengths available for vision, regardless of whether that range is determined by the spectral distribution of the natural illumination or the spectral transmittance of the ocular media (cornea, aqueous humour, lens, vitreous humour). Nevertheless, other variations in spectral sensitivity exist that reflect the variability and complexity of avian visual ecology.

Is the ultraviolet waveband a special communication channel in avian mate choice?

There is growing evidence that ultraviolet (UV) wavelengths play an important role in avian mate choice. One of the first experiments to support this idea showed that female zebra finches (Taeniopygia guttata) prefer UV-reflecting males to males whose ultraviolet reflection has been removed. The effect was very strong despite little or no UV reflection from several plumage areas. However, it is not clear how the importance of the UV waveband compares to other regions of the bird-visible spectrum. We tested whether the response of female zebra finches to the removal of male UV reflection is greater than to the removal of other wavebands. We presented females with a choice of males whose appearance was manipulated using coloured filters. The filters removed single blocks of the avian visible spectrum corresponding closely to the spectral sensitivities of each of the zebra finch’s single cone classes. This resulted in males that effectively had no UV (UV−), no short-wave (SW−), no medium-wave (MW−) or no long-wave (LW−) plumage reflection. Females preferred UV− and SW− males. LW− and MW− males were least preferred, suggesting that female zebra finches show the greatest response to the removal of longer wavelengths. Quantal catches of the single cone types viewing body areas of the male zebra finch are presented for each treatment. Our study suggests it is important to consider the role of the UV waveband in avian mate choice in conjunction with the rest of the avian visible spectrum.

The effects of the light environment on prey choice by zebra finches

Recent research has highlighted the extent to which birds utilise ultraviolet vision in mate choice and foraging. However, neither the importance of the ultraviolet compared with other regions of the visual spectrum nor the use of wavelength cues in other visual tasks have been explored. We assessed the individual choices of zebra finches (Taeniopygia guttata) for different-coloured seeds (red and white millet) under lighting conditions in which filters selectively removed blocks of the avian-visible spectrum corresponding to the spectral sensitivity of the four retinal cone types that subserve colour vision in this species. The effects corresponded to those predicted from the calculated distances between seed types, and between each seed type and the background, in a simple model of tetrachromatic colour space. As predicted for this foraging task, the removal of long-wavelength information had a greater influence than the removal of shorter wavelengths, including ultraviolet wavelengths. These results have important implications for predator–prey interactions and suggest that future studies of natural foraging should consider variations in the light environment.

Avian ultraviolet vision and frequency-dependent seed preferences

It is well established that ultraviolet sensitivity plays an important role in the visually guided behaviour of birds. From a foraging perspective, evidence now exists that ultraviolet wavelengths are used by birds when foraging for insects, berries, seeds and mammals. Here, we present the results of two laboratory experiments that test the effect of removing (i) ultraviolet wavelengths and (ii) wavebands in the human-visible region on the frequency-dependent seed preferences of zebra finches (Taeniopygia guttata). Although the seeds and backgrounds used in our experiments reflected mainly at long wavelengths, we found that removal of ultraviolet wavelengths significantly changed the strength and direction of frequency dependence compared with full-spectrum illumination. We also found that the removal of ultraviolet wavelengths (300–400nm) did not affect the strength of frequency dependence compared with the removal of short wavelengths (approximately 400–500nm), medium wavelengths (approximately 500–600nm) or long wavelengths (approximately 600–700nm). Since frequency-dependent selection has direct consequences for the stability of prey populations and the spectral quality of ambient light is known to vary considerably with climate, time of day and local habitat geometry, our results suggest that ultraviolet wavelengths might play an important role in the dynamics of plant populations. However, we urge caution about overestimating the importance of ultraviolet wavelengths compared with wavelengths in the human-visible spectrum.

The influence of stimulus and background colour on signal visibility in the lizard Anolis cristatellus

Anoline lizards communicate with visual displays in which they open and close a colourful throat fan called the dewlap. We used a visual fixation reflex as an assay to test the effects of stimulus versus background chromatic and brightness contrast on the probability of detecting a moving coloured (i.e. dewlap-like) stimulus in Anolis cristatellus. The probability of stimulus detection depended on two additive visual-system channels, one responding to brightness contrast and one responding to chromatic contrast, independent of brightness. The brightness channel was influenced only by wavelengths longer than 450nm and probably received input only from middle- and/or long-wavelength photoreceptors. The chromatic contrast channel appeared to receive input from three, or possibly four, different classes of cone in the anoline retina, including one with peak sensitivity in the ultraviolet. We developed a multi-linear regression equation that described most of the results of this study to a reasonable degree of accuracy. In the future, this equation could be used to predict the relative visibility of different-coloured stimuli in different habitat light conditions, which should be very useful for testing hypotheses that attempt to relate habitat light conditions and visual-system response to the evolution of signal design.

Colour thresholds and receptor noise: behaviour and physiology compared

Photoreceptor noise sets an absolute limit for the accuracy of colour discrimination. We compared colour thresholds in the honeybee (Apis mellifera) with this limit. Bees were trained to discriminate an achromatic stimulus from monochromatic lights of various wavelengths as a function of their intensity. Signal-to-noise ratios were measured by intracellular recordings in the three spectral types of photoreceptor cells. To model thresholds we assumed that discrimination was mediated by opponent mechanisms whose performance was limited by receptor noise. Most of the behavioural thresholds were close to those predicted from receptor signal-to-noise ratios, suggesting that colour discrimination in honeybees is affected by photoreceptor noise. Some of the thresholds were lower than this theoretical limit, which indicates summation of photoreceptor cell signals.

Spectral tuning of dichromats to natural scenes

Multispectral images of natural scenes were collected from both forests and coral reefs. We varied the wavelength position of receptors in hypothetical dichromatic visual systems and, for each receptor pair estimated the percentage of discriminable points in natural scenes. The optimal spectral tuning predicted by this model results in photoreceptor pairs very like those of forest dwelling, dichromatic mammals and of coral reef fishes. Variations of the natural illuminants in forests have little or no effect on optimal spectral tuning, but variations of depth in coral reefs have moderate effects on the spectral placement of S and L cones. The ratio of S and L cones typically found in dichromatic mammals reduces the discriminability of forest scenes; in contrast, the typical ratio of S and L cones in coral reef fishes achieves nearly the optimal discrimination in coral reef scenes.

Spectral and ultraviolet-polarisation sensitivity in juvenile salmonids: a comparative analysis using electrophysiology

Spectral and polarisation sensitivity were compared among juvenile (parr) rainbow trout (Oncorhynchus mykiss), steelhead (O. mykiss), cutthroat trout (O. clarki clarki), kokanee (O. nerka) and brook char (Salvelinus fontinalis) using multi-unit recording from the optic nerve. Although reared under the same conditions, differences in photopic spectral sensitivity were evident. Specifically, ON-responses were co-dominated by L- and M-cone mechanisms in all fish except O. nerka, consistent with an M-cone mechanism sensitivity. The sensitivity of OFF-responses was dominated by the M-cone mechanism for all fish, but O. mykiss appeared to show an additional contribution from the L-cone mechanism. Using chromatic adaptation, an independent ultraviolet-sensitive mechanism is described for the first time for the salmonid genus Salvelinus. In addition, this ultraviolet-cone mechanism was present in the members of the genus Oncorhynchus that were examined. Thus, ultraviolet sensitivity appears to be common to the major extant clades of the subfamily Salmoninae. All species showed differential sensitivity to both vertical and horizontal linearly polarised light. This sensitivity differed between ON- and OFF-responses. The ON-responses were maximally sensitive to both vertically and horizontally polarised light, whereas the OFF-responses displayed maximal sensitivity to horizontally polarised light in all species, with reduced sensitivity to vertically polarised light compared with ON-responses. Because of the similarity in the physiological characteristics of polarisation sensitivity among the salmonid species examined, no relationship between the degree of migratory tendency and the ability to detect polarised light could be identified.

Retinal asymmetry in birds

Vertebrate sensory systems are generally based on bilaterally symmetrical sense organs. It is evident, nevertheless, that birds preferentially use either their left or right eye for viewing novel or familiar stimuli [1], and perform visual discrimination tasks under monocular viewing conditions better with one eye than with the other [2] [3]. Because of the nearly complete contralateral decussation of the optic nerves in birds [4], it has been assumed that this division of labour is due solely to cerebral hemispheric specialisation, generated as a result of uneven photostimulation of the eyes of the developing embryo during the last three or four days before hatching [5] [6]. Here, however, we present evidence that in the European starling, Sturnus vulgaris, even the retinae are morphologically asymmetrical in terms of photoreceptor distribution. This is the first evidence for such asymmetry in any bird and suggests that retinal photoreceptor composition should be assessed during studies involving the lateralisation of visually mediated behaviours.

Colour vision of domestic chicks

The colour vision of domestic chicks (Gallus gallus) was investigated by training them to small food containers decorated with tilings of grey and coloured rectangles. Chicks learn to recognise the colour quickly and accurately. Chicks have four types of single-cone photoreceptor sensitive to ultraviolet, short-, medium- or long-wavelength light. To establish how these receptors are used for colour vision, stimuli were designed to be distinguished only by specific combinations of receptors. We infer (1) that all four single cones are used, and (2) that their outputs are encoded by at least three opponency mechanisms: one comparing the outputs of ultraviolet- and short-wavelength-sensitive receptors, one comparing the outputs of medium- and long-wavelength receptors and a third comparing of the outputs of short- and long- and/or medium-wavelength receptors. Thus, the chicks have tetrachromatic colour vision. These experiments do not exclude a role for the fifth cone type, double cones, but other evidence suggests that these cones serve luminance-based tasks, such as motion detection, and not colour recognition.

Accurate memory for colour but not pattern contrast in chicks

The visual displays of animals and plants often look dramatic and colourful to us, but what information do they convey to their intended, non-human, audience [1] [2]? One possibility is that stimulus values are judged accurately - so, for example, a female might choose a suitor if he displays a specific colour [3]. Alternatively, as for human advertising, displays may attract attention without giving information, perhaps by exploiting innate preferences for bright colours or symmetry [2] [4] [5]. To address this issue experimentally, we investigated chicks' memories of visual patterns. Food was placed in patterned paper containers which, like seed pods or insect prey, must be manipulated to extract food and their patterns learnt. To establish what was learnt, birds were tested on familiar stimuli and on alternative stimuli of differing colour or contrast. For colour, birds selected the trained stimulus; for contrast, they preferred high contrast patterns over the familiar. These differing responses to colour and contrast show how separate components of display patterns could serve different roles, with colour being judged accurately whereas pattern contrast attracts attention.