Evolution of vertebrate colour vision

Spectral inference reveals principal cone-integration rules of the zebrafish inner retina

Retinal bipolar cells integrate cone signals at dendritic and axonal sites. The axonal route, involving amacrine cells, remains largely uncharted. However, because cone types differ in their spectral sensitivities, insights into bipolar cells' cone integration might be gained based on their spectral tunings. We therefore recorded in vivo responses of bipolar cell presynaptic terminals in larval zebrafish to widefield but spectrally resolved flashes of light and mapped the results onto spectral responses of the four cones. This "spectral circuit mapping" allowed explaining ∼95% of the spectral and temporal variance of bipolar cell responses in a simple linear model, thereby revealing several notable integration rules of the inner retina. Bipolar cells were dominated by red-cone inputs, often alongside equal sign inputs from blue and green cones. In contrast, UV-cone inputs were uncorrelated with those of the remaining cones. This led to a new axis of spectral opponency where red-, green-, and blue-cone "Off" circuits connect to "natively-On" UV-cone circuits in the outermost fraction of the inner plexiform layer-much as how key color opponent circuits are established in mammals. Beyond this, and despite substantial temporal diversity that was not present in the cones, bipolar cell spectral tunings were surprisingly simple. They either approximately resembled both opponent and non-opponent spectral motifs already present in the cones or exhibited a stereotyped non-opponent broadband response. In this way, bipolar cells not only preserved the efficient spectral representations in the cones but also diversified them to set up a total of six dominant spectral motifs, which included three axes of spectral opponency.

The Impact of the Spectral Radiation Environment on the Maximum Absorption Wavelengths of Human Vision and Other Species

Since the earliest development of the eye (and vision) around 530 million years ago (Mya), it has evolved, adapting to different habitats, species, and changing environmental conditions on Earth. We argue that a radiation environment determined by the atmosphere played a determining role in the evolution of vision, specifically on the human eye, which has three vision regimes (photopic-, scotopic-, and mesopic vision) for different illumination conditions. An analysis of the irradiance spectra, reaching the shallow ocean depths, revealed that the available radiation could have determined the bandwidth of the precursor to vision systems, including human vision. We used the radiative transfer model to test the existing hypotheses on human vision. We argue that, once on the surface, the human photopic (daytime) and scotopic (night-time) vision followed different evolutionary directions, maximum total energy, and optimum information, respectively. Our analysis also suggests that solar radiation reflected from the moon had little or no influence on the evolution of scotopic vision. Our results indicate that, apart from human vision, the vision of only a few birds, rodents, and deep-sea fish are strongly correlated to the available radiation within their respective habitats.

Antipredator behavior of newts (Cynops pyrrhogaster) against snakes

Newts and salamanders show remarkable diversity in antipredator behavior, developed to enhance their chemical defenses and/or aposematism. The present study reports on the antipredator behavior of newts (Cynops pyrrhogaster) in response to snakes. Newts displayed a significant amount of tail-wagging and tail-undulation in response to a contact stimulus from the snake’s tongue, which is a snake-specific predator stimulus, as compared to a control stimulus (behavioral scores: tongue, 1.05 ± 0.41; control, 0.15 ± 0.15). Newts that were kept in warm temperature conditions, 20°C (at which snakes are active in nature), performed tail displays more frequently than newts kept in low-temperature conditions, 4°C (at which snakes are inactive in nature). Our results suggest that the tail displays of C. pyrrhogaster could function as an antipredator defense; they direct a snake’s attention to its tail to prevent the snake from attacking more vulnerable body parts. We also discussed the reason for inter-populational variation in the tendency of newts to perform tail displays.

Retinal characterization in the eyes of two bats endemic in the Egyptian fauna, the Egyptian fruit bat (Rousettus aegyptiacus) and insectivorous bat (Pipistrellus kuhlii), using the light microscope and transmission electron microscope

Bats are the only mammals that can fly in the dark without eye usage. This study was conducted to describe the structural and functional adaptations of the retina of two bats very common in the Egyptian fauna having a different lifestyle: the Egyptian fruit bat (Rousettus aegyptiacus) and insectivorous bat (Pipistrellus kuhlii). Seven eyes were collected from adult individuals of each species. Examination of the retina using a light microscope and a transmission electron microscope was carried out. The retina of P. kuhlii was thicker than that of R. aegyptiacus, which had many projections extended from the choroid layer into retina forming papillae. Despite rods being dominant in retinae of both species, cone photoreceptors were encountered in both retinae. The outer plexiform layer of R. aegyptiacus was arranged into islets between the outer nuclear layer produced differences in its thickness. However, the retina of P. kuhlii showed a normal arrangement of retinal structure. The retinal pigment epithelium of both bat species consists of a single layer of the cuboidal cells with a round to oval vesicular nuclei, which showed a lack of pigmentation in R. aegyptiacus and poor pigmentation in the P. kuhlii. In conclusion, our investigation detected many structural and ultrastructural differences between the two bat species. The presence of many projections protruded from the choroid layer of R. aegyptiacus retina is considered the most characteristic difference between the retinae of R. aegyptiacus and P. kuhlii.

The historical distribution of megaherbivores does not determine the distribution of megafaunal fruit in southern Africa

Large specialized fruit (megafaunal fruit) have evolved alongside megaherbivores to take advantage of their unparalleled seed dispersal service. Megaherbivores were widespread and abundant in the Pleistocene but due to multiple extinction events have been extirpated from all continents except Africa and small pockets of South East Asia. In Africa, we are in the unique position of being able to study megafaunal fruits where there are still areas with a largely intact megaherbivore community. The megafaunal fruits of the African forests have been examined but those of the African savannas have been largely overlooked. We use an operational definition of megafaunal fruit developed in the Neotropics to identify megafaunal fruit in the South African tree flora. Thirty-one species were identified as megafaunal fruit-bearers, representing only 3% of the tree flora. Megafaunal tree species are well represented in the families Mimosoideae, Arecaceae, Strychnaceae and Caesalpinoideae. We explored the factors underlying the distribution of these megafaunal tree species. We found that the historical distribution of megaherbivores in South Africa does not explain the distribution of these fruit. Megaherbivores have historically been found throughout South Africa while megafaunal fruit tree species occur almost exclusively in the northern tropical reaches of the country. Abiotic factors such as precipitation and temperature appear to best explain the distribution of megafaunal fruit species in the region. We conclude that megafaunal fruit are a tropical phenomenon and their tropical origins now limit their distribution.

Diurnal variation in opsin expression and common housekeeping genes necessitates comprehensive normalization methods for quantitative real-time PCR analyses

To determine the visual sensitivities of an organism of interest, quantitative reverse transcription-polymerase chain reaction (qRT-PCR) is often used to quantify expression of the light-sensitive opsins in the retina. While qRT-PCR is an affordable, high-throughput method for measuring expression, it comes with inherent normalization issues that affect the interpretation of results, especially as opsin expression can vary greatly based on developmental stage, light environment or diurnal cycles. We tested for diurnal cycles of opsin expression over a period of 24 hr at 1-hr increments and examined how normalization affects a data set with fluctuating expression levels using qRT-PCR and transcriptome data from the retinae of the cichlid Pelmatolapia mariae. We compared five methods of normalizing opsin expression relative to (a) the average of three stably expressed housekeeping genes (Ube2z, EF1-α and β-actin), (b) total RNA concentration, (c) GNAT2, (the cone-specific subunit of transducin), (d) total opsin expression and (e) only opsins expressed in the same cone type. Normalizing by proportion of cone type produced the least variation and would be best for removing time-of-day variation. In contrast, normalizing by housekeeping genes produced the highest daily variation in expression and demonstrated that the peak of cone opsin expression was in the late afternoon. A weighted correlation network analysis showed that the expression of different cone opsins follows a very similar daily cycle. With the knowledge of how these normalization methods affect opsin expression data, we make recommendations for designing sampling approaches and quantification methods based upon the scientific question being examined.

Colour vision of green turtle (Chelonia mydas) hatchlings: do they still prefer blue under water?

Background

Several anatomical studies provide evidence that green turtles (Chelonia mydas) possess the necessary anatomy for colour vision. Behavioural experiments have previously been conducted with newly emerged hatchlings, concluding that they are attracted to shorter wavelengths compared to longer wavelengths within a terrestrial environment, suggesting a possible attraction towards blue. This paper assessed the colour vision of hatchlings within an aquatic environment, and investigated whether the attraction for shorter wavelengths remains consistent within water, whether the colour saturation of the chromatic stimuli was an important factor, and whether rearing and testing individual animals in different coloured housing tanks has an impact on their visual choices.

Methods

Forty-one hatchling green turtles were presented with a three-choice experiment where food was attached to three different coloured plates. The plates (blue, yellow, and red) were randomly arranged in the turtle’s tank and four different colour saturations were tested (100, 75, 50, and 25%). Turtles were individually placed into their housing tanks (coloured either red, white, blue or grey) with three different colour plates in front of them, from the same saturation level. The colour of the plate with food first approached and bitten by the turtle was recorded.

Results

The colour of the tank in which an individual was reared, and where experiments were conducted, significantly influenced which food item was selected on the different coloured plates. While individual turtles preferred to select the food items associated with blue plates across the entire experiment (66.1% of the time compared to 18.2% and 15.7% for yellow and red plates respectively), the preference for blue plates was influenced by the colour of the rearing/experimental tank. Individuals raised in red, white or blue tanks appeared to consistently prefer food on blue plates, but there appeared to be no plate colour preference by turtles in grey tanks. There was no significant effect of either colour saturation or the spatial arrangement of the three colours within an experimental tank on colour choice, and no significant interaction between tank colour and colour saturation.

Discussion

Thesefindings confirm that the terrestrial preference towards shorter wavelength colours, such as blue, compared to longer wavelength colours remains consistent within an aquatic environment. This preference for blue continues even as the colour saturation reduces from 100% down to 25%, and the colours become darker. Thus, it is suggested that green turtle hatchlings have a strong attraction towards blue. This attraction, however, is influenced by the colour of the tank the turtles were raised in. While this supports the notion that environmental colour may influence individual turtle visual capabilities, it suggests that this relationship is more complicated, and requires further investigation.

The evolution of eyes: major steps. The Keeler lecture 2017: centenary of Keeler Ltd

Ocular evolution is an immense topic, and I do not expect to cover all the details of this process in this manuscript. I will present some concepts about some of the major steps in the evolutionary process to stimulate your thinking about this interesting and complex topic. In the prebiotic soup, vision was not inevitable. Eyes were not preordained. Nor were their shapes, sizes, or current physiology. Sight is an evolutionary gift but it was not ineluctable. The existence of eyes is so basic to our profession that we often do not consider how and why vision appeared or evolved on earth at all. Although vision is a principal sensory modality for at least three major phyla and is present in three or four more phyla, there are other sensory mechanisms that could have been and were occasionally selected instead. Some animals rely on other sensory mechanisms such as audition, echolocation, or olfaction that are much more effective in their particular niche than would be vision. We may not believe those sensory mechanisms to be as robust as vision, but the creatures using those skills would argue otherwise. Why does vision exist at all? And why is it so dominant at least in the number of species that rely upon it for their principal sensory mechanism? How did vision begin? What were the important steps in the evolution of eyes? How did eyes differentiate along their various paths, and why?

Medium/Long wavelength sensitive opsin diversity in Pitheciidae

New World primates feature a complex colour vision system. Most species have polymorphic colour vision where males have a dichromatic colour perception and females can be either dichromatic or trichromatic. The adaptive value of high allelic diversity of opsins, a light sensitive protein, found in primates' eyes remains unknown. Studies revealing the allelic diversity are important as they shed light on our understanding of the adaptive value of differences in the colouration of species and their ecologies. Here we investigate the allelic types found in Pitheciidae, an understudied New World primate family, revealing the diversity of medium/long wavelength sensitive opsins both in cryptic and conspicuous species of this primate family. We found five alleles in Cacajao, six in Callicebinae (i.e. Plecturocebus, Cheracebus, and Callicebus), four in Chiropotes, and three in Pithecia, some of them reported for the first time. Both cryptic and conspicuous species in this group presented high allelic diversity.

Optical influence of oil droplets on cone photoreceptor sensitivity

Oil droplets are spherical organelles found in the cone photoreceptors of vertebrates. They are generally assumed to focus incident light into the outer segment, and thereby improve light catch because of the droplets' spherical lens-like shape. However, using full-wave optical simulations of physiologically realistic cone photoreceptors from birds, frogs and turtles, we find that pigmented oil droplets actually drastically reduce the transmission of light into the outer segment integrated across the full visible wavelength range of each species. Only transparent oil droplets improve light catch into the outer segments, and any enhancement is critically dependent on the refractive index, diameter of the oil droplet, and diameter and length of the outer segment. Furthermore, oil droplets are not the only optical elements found in cone inner segments. The ellipsoid, a dense aggregation of mitochondria situated immediately prior to the oil droplet, mitigates the loss of light at the oil droplet surface. We describe a framework for integrating these optical phenomena into simple models of receptor sensitivity, and the relevance of these observations to evolutionary appearance and loss of oil droplets is discussed.

Behavioral color vision in a cichlid fish: Metriaclima benetos

Color vision is the capacity to discriminate color regardless of brightness. It is essential for many fish species as they rely on color discrimination for numerous ecological tasks. The study of color vision is important because it can unveil the mechanisms that shape coloration patterns, visual system sensitivities and, hence, visual signals. In order to better understand the mechanisms underlying color vision, an integrative approach is necessary. This usually requires combining behavioral, physiological and genetic experiments with quantitative modeling, resulting in a distinctive characterization of the visual system. Here, we provide new data on the color vision of a rock-dwelling cichlid from Lake Malawi: Metriaclima benetos. For this study we used a behavioral approach to demonstrate color vision through classical conditioning, complemented with modeling of color vision to estimate color contrast. For our experiments we took into account opsin coexpression and considered whether cichlids exhibit a dichromatic or a trichromatic visual system. Behavioral experiments confirmed color vision in M. benetos; most fish were significantly more likely to choose the trained over the distracter stimuli, irrespective of brightness. Our results are supported by visual modeling that suggests that cichlids are trichromats and achieve color vision through color opponency mechanisms, which are a result of three different photoreceptor channels. Our analyses also suggest that opsin coexpression can negatively affect perceived color contrast. This study is particularly relevant for research on the cichlid lineage because cichlid visual capabilities and coloration patterns are implicated in their adaptive radiation.

Howler monkey foraging ecology suggests convergent evolution of routine trichromacy as an adaptation for folivory

Primates possess remarkably variable color vision, and the ecological and social factors shaping this variation remain heavily debated. Here, we test whether central tenants of the folivory hypothesis of routine trichromacy hold for the foraging ecology of howler monkeys. Howler monkeys (genus Alouatta) and paleotropical primates (Parvorder: Catarrhini) have independently acquired routine trichromacy through fixation of distinct mid- to long-wavelength-sensitive (M/LWS) opsin genes on the X-chromosome. The presence of routine trichromacy in howlers, while other diurnal neotropical monkeys (Platyrrhini) possess polymorphic trichromacy, is poorly understood. A selective force proposed to explain the evolution of routine trichromacy in catarrhines-reliance on young, red leaves-has received scant attention in howlers, a gap we fill in this study. We recorded diet, sequenced M/LWS opsin genes in four social groups of Alouatta palliata, and conducted colorimetric analysis of leaves consumed in Sector Santa Rosa, Costa Rica. For a majority of food species, including Ficus trees, an important resource year-round, young leaves were more chromatically conspicuous from mature leaves to trichromatic than to hypothetical dichromatic phenotypes. We found that 18% of opsin genes were MWS/LWS hybrids; when combined with previous research, the incidence of hybrid M/LWS opsins in this species is 13%. In visual models of food discrimination ability, the hybrid trichromatic phenotype performed slightly poorer than normal trichromacy, but substantially better than dichromacy. Our results provide support for the folivory hypothesis of routine trichromacy. Similar ecological pressures, that is, the search for young, reddish leaves, may have driven the independent evolution of routine trichromacy in primates on separate continents. We discuss our results in the context of balancing selection acting on New World monkey opsin genes and hypothesize that howlers experience stronger selection against dichromatic phenotypes than other sympatric species, which rely more heavily on cryptic foods.

Allopatry, competitor recognition and heterospecific aggression in crater lake cichlids

Background

Aggressive behaviour can have significant evolutionary consequences–not only within species, but also in the context of heterospecific interactions. Here, we carried out an experimental field study to investigate the importance of phenotypic similarity on levels of aggression between species whilst controlling for familiarity effects using manipulated allopatric stimuli. Specifically, we investigated aggressive responses of territory holding males and females in two species of Neotropical cichlid fish, Amphilophus sagittae and Hypsophrys nicaraguensis, that differ in their phenotypic similarity to our allopatric stimulus species, Amphilophus astorquii.

Results

We found that, independent of phenotypic similarity (and correlated phylogenetic proximity) between the territory holders and intruder, territorial aggression was not adjusted in relation to allopatric intruder colour markings that are associated with different levels of threat and known to provoke different responses in a sympatric setting. We also found that males and females did not differ in their overall patterns of aggression adjustment towards intruder cues. Nevertheless, the two focal species, which share the same breeding grounds and external threats, exhibited different sex roles in breeding territory defence.

Conclusion

Together with earlier studies assessing hetrospecific aggression in sympatry, our current results highlight the importance of coevolution and learning in species interactions.

Color and the Cone Mosaic

Vision is limited by the measurements taken by the cone photoreceptors. To provide useful perceptual representations, the brain must go beyond the measurements and make inferences about the scene being viewed. This article considers the first stages of spatiochromatic vision. We show how spatial and chromatic information become intertwined by the optics of the eye and because of the structure of the retinal cone mosaic, and we consider the consequent implications for perception. Because there is at most one cone at each retinal location, the standard treatment of human trichromacy does not apply at fine spatial scales. Rather, trichromacy results from a perceptual inference based on measurements from cones of different classes at different locations. Our treatment emphasizes linking physics, biology, and computation with the goal of providing a framework for a larger understanding of how the brain interprets photoreceptor excitations to see objects and their properties.

Nocturnality in synapsids predates the origin of mammals by over 100 million years

Nocturnality is widespread among extant mammals and often considered the ancestral behavioural pattern for all mammals. However, mammals are nested within a larger clade, Synapsida, and non-mammalian synapsids comprise a rich phylogenetic, morphological and ecological diversity. Even though non-mammalian synapsids potentially could elucidate the early evolution of diel activity patterns and enrich the understanding of synapsid palaeobiology, data on their diel activity are currently unavailable. Using scleral ring and orbit dimensions, we demonstrate that nocturnal activity was not an innovation unique to mammals but a character that appeared much earlier in synapsid history, possibly several times independently. The 24 Carboniferous to Jurassic non-mammalian synapsid species in our sample featured eye morphologies consistent with all major diel activity patterns, with examples of nocturnality as old as the Late Carboniferous (ca 300 Ma). Carnivores such as Sphenacodon ferox and Dimetrodon milleri, but also the herbivorous cynodont Tritylodon longaevus were likely nocturnal, whereas most of the anomodont herbivores are reconstructed as diurnal. Recognizing the complexity of diel activity patterns in non-mammalian synapsids is an important step towards a more nuanced picture of the evolutionary history of behaviour in the synapsid clade.

Correlation between dichromatic colour vision and jumping performance in horses

There is general agreement that horses have dichromatic colour vision with similar capabilities to human beings with red-green colour deficiencies. However, whether colour perception has an impact on equine jumping performance and how pronounced the colour stimulus might be for a horse is unknown. The present study investigated the relationship between the colour of the fences (blue or green) and the show jumping performance of 20 horses ridden by two riders using an indoor and outdoor set of green and blue fences. In the indoor arena, significantly more touches and faults were made on blue fences in comparison to green fences (median difference of 2.5 bars). When only touched bars were included, a significant median difference of one bar was found. Mares (n = 4) demonstrated more faults and had a significantly greater difference in touches and faults between the two colours than male horses (n = 16). Repeating the same experiment with eight horses in an outdoor grass arena revealed no significant differences between the two colours. In order to draw any definite conclusions, more research concerning the colour perception, influence of contrast with the arena surface and sex of horse is required.

Divergent selection for opsin gene variation in guppy (Poecilia reticulata) populations of Trinidad and Tobago

The guppy is known to exhibit remarkable interindividual variations in spectral sensitivity of middle to long wavelength-sensitive (M/LWS) cone photoreceptor cells. The guppy has four M/LWS-type opsin genes (LWS-1, LWS-2, LWS-3 and LWS-4) that are considered to be responsible for this sensory variation. However, the allelic variation of the opsin genes, particularly in terms of their absorption spectrum, has not been explored in wild populations. Thus, we examined nucleotide variations in the four M/LWS opsin genes as well as blue-sensitive SWS2-B and ultraviolet-sensitive SWS1 opsin genes for comparison and seven non-opsin nuclear loci as reference genes in 10 guppy populations from various light environments in Trinidad and Tobago. For the first time, we discovered a potential spectral variation (180 Ser/Ala) in LWS-1 that differed at an amino acid site known to affect the absorption spectra of opsins. Based on a coalescent simulation of the nucleotide variation of the reference genes, we showed that the interpopulation genetic differentiation of two opsin genes was significantly larger than the neutral expectation. Furthermore, this genetic differentiation was significantly related to differences in dissolved oxygen (DO) level, and it was not explained by the spatial distance between populations. The DO levels are correlated with eutrophication that possibly affects the color of aquatic environments. These results suggest that the population diversity of opsin genes is significantly driven by natural selection and that the guppy could adapt to various light environments through color vision changes.

Ultraviolet vision in lacertid lizards: evidence from retinal structure, eye transmittance, SWS1 visual pigment genes, and behaviour

Ultraviolet (UV) vision and UV colour patches have been reported in a wide range of taxa and are increasingly appreciated as an integral part of vertebrate visual perception and communication systems. Previous studies with Lacertidae, a lizard family with diverse and complex coloration, have revealed the existence of UV-reflecting patches that may function as social signals. However, confirmation of the signalling role of UV coloration requires demonstrating that the lizards are capable of vision in the UV waveband. Here we use a multidisciplinary approach to characterize the visual sensitivity of a diverse sample of lacertid species. Spectral transmission measurements of the ocular media show that wavelengths down to 300 nm are transmitted in all the species sampled. Four retinal oil droplet types can be identified in the lacertid retina. Two types are pigmented and two are colourless. Fluorescence microscopy reveals that a type of colourless droplet is UV-transmitting and may thus be associated with UV-sensitive cones. DNA sequencing shows that lacertids have a functional SWS1 opsin, very similar at 13 critical sites to that in the presumed ancestral vertebrate (which was UV-sensitive) and other UV-sensitive lizards. Finally, males of Podarcis muralis are capable of discriminating between two views of the same stimulus that differ only in the presence/absence of UV radiance. Taken together, these results provide convergent evidence of UV vision in lacertids, very likely by means of an independent photopigment. Moreover, the presence of four oil droplet types suggests that lacertids have a four-cone colour vision system.

Imaging light responses of retinal ganglion cells in the living mouse eye

This study reports development of a novel method for high-resolution in vivo imaging of the function of individual mouse retinal ganglion cells (RGCs) that overcomes many limitations of available methods for recording RGC physiology. The technique combines insertion of a genetically encoded calcium indicator into RGCs with imaging of calcium responses over many days with FACILE (functional adaptive optics cellular imaging in the living eye). FACILE extends the most common method for RGC physiology, in vitro physiology, by allowing repeated imaging of the function of each cell over many sessions and by avoiding damage to the retina during removal from the eye. This makes it possible to track changes in the response of individual cells during morphological development or degeneration. FACILE also overcomes limitations of existing in vivo imaging methods, providing fine spatial and temporal detail, structure-function comparison, and simultaneous analysis of multiple cells.

Losses of functional opsin genes, short-wavelength cone photopigments, and color vision--a significant trend in the evolution of mammalian vision

All mammalian cone photopigments are derived from the operation of representatives from two opsin gene families (SWS1 and LWS in marsupial and eutherian mammals; SWS2 and LWS in monotremes), a process that produces cone pigments with respective peak sensitivities in the short and middle-to-long wavelengths. With the exception of a number of primate taxa, the modal pattern for mammals is to have two types of cone photopigment, one drawn from each of the gene families. In recent years, it has been discovered that the SWS1 opsin genes of a widely divergent collection of eutherian mammals have accumulated mutational changes that render them nonfunctional. This alteration reduces the retinal complements of these species to a single cone type, thus rendering ordinary color vision impossible. At present, several dozen species from five mammalian orders have been identified as falling into this category, but the total number of mammalian species that have lost short-wavelength cones in this way is certain to be much larger, perhaps reaching as high as 10% of all species. A number of circumstances that might be used to explain this widespread cone loss can be identified. Among these, the single consistent fact is that the species so affected are nocturnal or, if they are not technically nocturnal, they at least feature retinal organizations that are typically associated with that lifestyle. At the same time, however, there are many nocturnal mammals that retain functional short-wavelength cones. Nocturnality thus appears to set the stage for loss of functional SWS1 opsin genes in mammals, but it cannot be the sole circumstance.

Nocturnal light environments and species ecology: implications for nocturnal color vision in forests

Although variation in the color of light in terrestrial diurnal and twilight environments has been well documented, relatively little work has examined the color of light in nocturnal habitats. Understanding the range and sources of variation in nocturnal light environments has important implications for nocturnal vision, particularly following recent discoveries of nocturnal color vision. In this study, we measured nocturnal irradiance in a dry forest/woodland and a rainforest in Madagascar over 34 nights. We found that a simple linear model including the additive effects of lunar altitude, lunar phase and canopy openness successfully predicted total irradiance flux measurements across 242 clear sky measurements (r=0.85, P<0.0001). However, the relationship between these variables and spectral irradiance was more complex, as interactions between lunar altitude, lunar phase and canopy openness were also important predictors of spectral variation. Further, in contrast to diurnal conditions, nocturnal forests and woodlands share a yellow-green-dominant light environment with peak flux at 560 nm. To explore how nocturnal light environments influence nocturnal vision, we compared photoreceptor spectral tuning, habitat preference and diet in 32 nocturnal mammals. In many species, long-wavelength-sensitive cone spectral sensitivity matched the peak flux present in nocturnal forests and woodlands, suggesting a possible adaptation to maximize photon absorption at night. Further, controlling for phylogeny, we found that fruit/flower consumption significantly predicted short-wavelength-sensitive cone spectral tuning in nocturnal mammals (P=0.002). These results suggest that variation in nocturnal light environments and species ecology together influence cone spectral tuning and color vision in nocturnal mammals.

Visual wavelength discrimination by the loggerhead turtle, Caretta caretta

Marine turtles are visual animals, yet we know remarkably little about how they use this sensory capacity. In this study, our purpose was to determine whether loggerhead turtles could discriminate between objects on the basis of color. We used light-adapted hatchlings to determine the minimum intensity of blue (450 nm), green (500 nm), and yellow (580 nm) visual stimuli that evoked a positive phototaxis (the phototaxis "threshold" [pt]). Juvenile turtles were later trained to associate each color (presented at 1 log unit above that color's pt) with food, then to discriminate between two colors (the original rewarded stimulus plus one of the other colors, not rewarded) when both were presented at 1 log unit above their pt. In the crucial test, turtles were trained to choose between the rewarded and unrewarded color when the colors varied in intensity. All turtles learned that task, demonstrating color discrimination. An association between blue and food was acquired in fewer trials than between yellow and food, perhaps because some prey of juvenile loggerheads in oceanic surface waters (jellyfishes, polyps, and pelagic gastropods) are blue or violet in color.

Differential expression patterns of striate cortex-enriched genes among Old World, New World, and prosimian primates

A group of 5 genes, OCC1, testican-1, testican-2, 5-HT1B, and 5-HT2A, are selectively expressed in layer 4 (4C of Brodmann) of striate cortex (visual area V1) of both Old World macaques and New World marmoset monkeys. The expression of these genes is activity dependent, as expression is reduced after blocking retinal activity. Surprisingly, the pronounced expression pattern has not been found in rodents or carnivores. Thus, these genes may be highly expressed in V1 of some but perhaps not all primates. Here, we compared the gene expression in members of 3 major branches of primate evolution: prosimians, New World monkeys, and Old World monkeys. Although the expression pattern of 5-HT1B was well conserved, those of the other genes varied from the least distinct in prosimian galagos to successively more in New World owl monkeys, marmosets, squirrel monkeys, and Old World macaque monkeys. In owl monkeys, the expression of 5-HT2A was significantly reduced by monocular tetrodotoxin injection, while those of OCC1 and 5-HT1B were not. Thus, we propose that early primates had low levels of expression and higher levels emerged with anthropoid primates and became further enhanced in the Old World catarrhine monkeys that are more closely related to humans.

Retinal photoreceptors of two subterranean tuco-tuco species (Rodentia, Ctenomys): morphology, topography, and spectral sensitivity

Traditionally, vision was thought to be useless for animals living in dark underground habitats, but recent studies in a range of subterranean rodent species have shown a large diversity of eye features, from small subcutaneous eyes to normal-sized functional eyes. We analyzed the retinal photoreceptors in the subterranean hystricomorph rodents Ctenomys talarum and Ctenomys magellanicus to elucidate whether adaptation was to their near-lightless burrows or rather to their occasional diurnal surface activity. Both species had normally developed eyes. Overall photoreceptor densities were comparatively low (95,000-150,000/mm(2) in C. magellanicus, 110,000-200,000/mm(2) in C. talarum), and cone proportions were rather high (10-31% and 14-31%, respectively). The majority of cones expressed the middle-to-longwave-sensitive (L) opsin, and a 6-16% minority expressed the shortwave-sensitive (S) opsin. In both species the densities of L and S cones were higher in ventral than in dorsal retina. In both species the tuning-relevant amino acids of the S opsin indicate sensitivity in the near UV rather than the blue/violet range. Photopic spectral electroretinograms were recorded. Unexpectedly, their sensitivity profiles were best fitted by the linear summation of three visual pigment templates with lambda(max) at 370 nm (S pigment, UV), at 510 nm (L pigment), and at 450 nm (an as-yet unexplained mechanism). Avoiding predators and selecting food during the brief aboveground excursions may have exerted pressure to retain robust cone-based vision in Ctenomys. UV tuning of the S cone pigment is shared with a number of other hystricomorphs.

Retinal photoreceptor arrangement, SWS1 and LWS opsin sequence, and electroretinography in the South American marsupial Thylamys elegans (Waterhouse, 1839)

We studied the retinal photoreceptors in the mouse opossum Thylamys elegans, a nocturnal South American marsupial. A variety of photoreceptor properties and color vision capabilities have been documented in Australian marsupials, and we were interested to establish what similarities and differences this American marsupial showed. Thylamys opsin gene sequencing revealed two cone opsins, a longwave-sensitive (LWS) opsin and a shortwave-sensitive (SWS1) opsin with deduced peak sensitivities at 560 nm and 360 nm (ultraviolet), respectively. Immunocytochemistry located these opsins to separate cone populations, a majority of LWS cones (density range 1,600-5,600/mm(2)) and a minority of SWS1 cones (density range 100-690/mm(2)). With rod densities of 440,000-590,000/mm(2), the cones constituted 0.4-1.2% of the photoreceptors. This is a suitable adaptation to nocturnal vision. Cone densities peaked in a horizontally elongated region ventral to the optic nerve head. In ventral-but not dorsal-retina, roughly 40% of the LWS opsin-expressing cones occurred as close pairs (double cones), and one member of each double cone contained a colorless oil droplet. The corneal electroretinogram (ERG) showed a high scotopic sensitivity with a rod peak sensitivity at 505 nm. At mesopic light levels, the spectral ERG revealed the contributions of a UV-sensitive SWS1 cone mechanism and an LWS cone mechanism with peak sensitivities at 365 nm and 555 nm, respectively, confirming the tuning predictions from the cone opsin sequences. The two spectral cone types provide the basis for dichromatic color vision, or trichromacy if the rods contribute to color processing at mesopic light levels.

Evolution of colour vision in mammals

Colour vision allows animals to reliably distinguish differences in the distributions of spectral energies reaching the eye. Although not universal, a capacity for colour vision is sufficiently widespread across the animal kingdom to provide prima facie evidence of its importance as a tool for analysing and interpreting the visual environment. The basic biological mechanisms on which vertebrate colour vision ultimately rests, the cone opsin genes and the photopigments they specify, are highly conserved. Within that constraint, however, the utilization of these basic elements varies in striking ways in that they appear, disappear and emerge in altered form during the course of evolution. These changes, along with other alterations in the visual system, have led to profound variations in the nature and salience of colour vision among the vertebrates. This article concerns the evolution of colour vision among the mammals, viewing that process in the context of relevant biological mechanisms, of variations in mammalian colour vision, and of the utility of colour vision.

The evolution of early vertebrate photoreceptors

Meeting the challenge of sampling an ancient aquatic landscape by the early vertebrates was crucial to their survival and would establish a retinal bauplan to be used by all subsequent vertebrate descendents. Image-forming eyes were under tremendous selection pressure and the ability to identify suitable prey and detect potential predators was thought to be one of the major drivers of speciation in the Early Cambrian. Based on the fossil record, we know that hagfishes, lampreys, holocephalans, elasmobranchs and lungfishes occupy critical stages in vertebrate evolution, having remained relatively unchanged over hundreds of millions of years. Now using extant representatives of these 'living fossils', we are able to piece together the evolution of vertebrate photoreception. While photoreception in hagfishes appears to be based on light detection and controlling circadian rhythms, rather than image formation, the photoreceptors of lampreys fall into five distinct classes and represent a critical stage in the dichotomy of rods and cones. At least four types of retinal cones sample the visual environment in lampreys mediating photopic (and potentially colour) vision, a sampling strategy retained by lungfishes, some modern teleosts, reptiles and birds. Trichromacy is retained in cartilaginous fishes (at least in batoids and holocephalans), where it is predicted that true scotopic (dim light) vision evolved in the common ancestor of all living gnathostomes. The capacity to discriminate colour and balance the tradeoff between resolution and sensitivity in the early vertebrates was an important driver of eye evolution, where many of the ocular features evolved were retained as vertebrates progressed on to land.

Opsin gene polymorphism predicts trichromacy in a cathemeral lemur

Recent research has identified polymorphic trichromacy in three diurnal strepsirrhines: Coquerel's sifaka (Propithecus coquereli), black and white ruffed lemurs (Varecia variegata), and red ruffed lemurs (V. rubra). Current hypotheses suggest that the transitions to diurnality experienced by Propithecus and Varecia were necessary precursors to their independent acquisitions of trichromacy. Accordingly, cathemeral lemurs are thought to lack the M/L opsin gene polymorphism necessary for trichromacy. In this study, the M/L opsin gene was sequenced in ten cathemeral blue-eyed black lemurs (Eulemur macaco flavifrons). This analysis identified a polymorphism identical to that of other trichromatic strepsirrhines at the critical amino acid position 285 in exon 5 of the M/L opsin gene. Thus, polymorphic trichromacy is likely present in at least one cathemeral Eulemur species, suggesting that strict diurnality is not necessary for trichromacy. The presence of trichromacy in E. m. flavifrons suggests that a re-evaluation of current hypotheses regarding the evolution of strepsirrhine trichromacy may be necessary. Although the M/L opsin polymorphism may have been independently acquired three times in the lemurid-indriid clade, the distribution of opsin alleles in lemurids and indriids may also be consistent with a common origin of trichromacy in the last common ancestor of either the lemurids or the lemurid-indriid clade.

Optimal design of photoreceptor mosaics: why we do not see color at night

While color vision mediated by rod photoreceptors in dim light is possible (Kelber & Roth, 2006), most animals, including humans, do not see in color at night. This is because their retinas contain only a single class of rod photoreceptors. Many of these same animals have daylight color vision, mediated by multiple classes of cone photoreceptors. We develop a general formulation, based on Bayesian decision theory, to evaluate the efficacy of various retinal photoreceptor mosaics. The formulation evaluates each mosaic under the assumption that its output is processed to optimally estimate the image. It also explicitly takes into account the statistics of the environmental image ensemble. Using the general formulation, we consider the trade-off between monochromatic and dichromatic retinal designs as a function of overall illuminant intensity. We are able to demonstrate a set of assumptions under which the prevalent biological pattern represents optimal processing. These assumptions include an image ensemble characterized by high correlations between image intensities at nearby locations, as well as high correlations between intensities in different wavelength bands. They also include a constraint on receptor photopigment biophysics and/or the information carried by different wavelengths that produces an asymmetry in the signal-to-noise ratio of the output of different receptor classes. Our results thus provide an optimality explanation for the evolution of color vision for daylight conditions and monochromatic vision for nighttime conditions. An additional result from our calculations is that regular spatial interleaving of two receptor classes in a dichromatic retina yields performance superior to that of a retina where receptors of the same class are clumped together.

Retinal parallel pathways: seeing with our inner fish

The general organization of the vertebrate retina is highly conserved, in spite of structural variations that occur in different animal classes. The retinas of cyprinid fish, for example, differ in many aspects from those of primates. However, these differences are in the same order of magnitude as those found among mammalian species. Therefore, it is important to consider whether these changes are minor variations on the same theme or whether they lead to fundamentally different functions. In this light, we compare the retinal organization of teleost fish and mammals as regards parallel processing and discuss their many similarities.

Cone visual pigments of monotremes: Filling the phylogenetic gap

We have determined the sequence and genomic organization of the genes encoding the cone visual pigment of the platypus (Ornithorhynchus anatinus) and the echidna (Tachyglossus aculeatus), and inferred their spectral properties and evolutionary pathways. We prepared platypus and echidna retinal RNA and used primers of the middle-wave-sensitive (MWS), long-wave-sensitive (LWS), and short-wave sensitive (SWS1) pigments corresponding to coding sequences that are highly conserved among mammals; to PCR amplify the corresponding pigment sequences. Amplification from the retinal RNA revealed the expression of LWS pigment mRNA that is homologous in sequence and spectral properties to the primate LWS visual pigments. However, we were unable to amplify the mammalian SWS1 pigment from these two species, indicating this gene was lost prior to the echidna-platypus divergence (∼21 MYA). Subsequently, when the platypus genome sequence became available, we found an LWS pigment gene in a conserved genomic arrangement that resembles the primate pigment, but, surprisingly we found an adjacent (∼20 kb) SWS2 pigment gene within this conserved genomic arrangement. We obtained the same result after sequencing the echidna genes. The encoded SWS2 pigment is predicted to have a wavelength of maximal absorption of about 440 nm, and is paralogous to SWS pigments typically found in reptiles, birds, and fish but not in mammals. This study suggests the locus control region (LCR) has played an important role in the conservation of photo receptor gene arrays and the control of their spatial and temporal expression in the retina in all mammals. In conclusion, a duplication event of an ancestral cone visual pigment gene, followed by sequence divergence and selection gave rise to the LWS and SWS2 visual pigments. So far, the echidna and platypus are the only mammals that share the gene structure of the LWS-SWS2 pigment gene complex with reptiles, birds and fishes.

Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup

Charles Darwin appreciated the conceptual difficulty in accepting that an organ as wonderful as the vertebrate eye could have evolved through natural selection. He reasoned that if appropriate gradations could be found that were useful to the animal and were inherited, then the apparent difficulty would be overcome. Here, we review a wide range of findings that capture glimpses of the gradations that appear to have occurred during eye evolution, and provide a scenario for the unseen steps that have led to the emergence of the vertebrate eye.

Colour perception in a dichromat

Most mammals have dichromatic colour vision based on two different types of cones: a short-wavelength-sensitive cone and a long-wavelength-sensitive cone. Comparing the signal from two cone types gives rise to a one-dimensional chromatic space when brightness is excluded. The so-called ;neutral point' refers to the wavelength that the animal cannot distinguish from achromatic light such as white or grey because it stimulates both cone types equally. The question is: how do dichromats perceive their chromatic space? Do they experience a continuous scale of colours or does the neutral point divide their chromatic space into two colour categories, i.e. into colours of either short or long wavelengths? We trained horses to different colour combinations in a two-choice behavioural experiment and tested their responses to the training and test colours. The horses chose colours according to their similarity/relationship to rewarded and unrewarded training colours. There was no evidence for a categorical boundary at the neutral point or elsewhere. This study suggests that dichromats perceive their chromatic space as a continuous scale of colours, treating the colour at the neutral point as any other colour they can distinguish.

The machinery of colour vision

Some fundamental principles of colour vision, deduced from perceptual studies, have been understood for a long time. Physiological studies have confirmed the existence of three classes of cone photoreceptors, and of colour-opponent neurons that compare the signals from cones, but modern work has drawn attention to unexpected complexities of early organization: the proportions of cones of different types vary widely among individuals, without great effect on colour vision; the arrangement of different types of cones in the mosaic seems to be random, making it hard to optimize the connections to colour-opponent mechanisms; and new forms of colour-opponent mechanisms have recently been discovered. At a higher level, in the primary visual cortex, recent studies have revealed a simpler organization than had earlier been supposed, and in some respects have made it easier to reconcile physiological and perceptual findings.

The functional and anatomical organization of marsupial neocortex: evidence for parallel evolution across mammals

Marsupials are a diverse group of mammals that occupy a large range of habitats and have evolved a wide array of unique adaptations. Although they are as diverse as placental mammals, our understanding of marsupial brain organization is more limited. Like placental mammals, marsupials have striking similarities in neocortical organization, such as a constellation of cortical fields including S1, S2, V1, V2, and A1, that are functionally, architectonically, and connectionally distinct. In this review, we describe the general lifestyle and morphological characteristics of all marsupials and the organization of somatosensory, motor, visual, and auditory cortex. For each sensory system, we compare the functional organization and the corticocortical and thalamocortical connections of the neocortex across species. Differences between placental and marsupial species are discussed and the theories on neocortical evolution that have been derived from studying marsupials, particularly the idea of a sensorimotor amalgam, are evaluated. Overall, marsupials inhabit a variety of niches and assume many different lifestyles. For example, marsupials occupy terrestrial, arboreal, burrowing, and aquatic environments; some animals are highly social while others are solitary; different species are carnivorous, herbivorous, or omnivorous. For each of these adaptations, marsupials have evolved an array of morphological, behavioral, and cortical specializations that are strikingly similar to those observed in placental mammals occupying similar habitats, which indicate that there are constraints imposed on evolving nervous systems that result in recurrent solutions to similar environmental challenges.

Computation of color and brightness differences by neurons in the lateral geniculate nucleus of the rabbit

The responses of 51 neurons in the lateral geniculate nucleus of the rabbit to substitution of colored stimuli different brightness and stimuli differing only in intensity were studied. Neurons in the geniculate nucleus, like neurons in the visual cortex, were found to respond with initial phasic discharges at 50-90 msec after stimulus substitution, the magnitudes of these responses correlating with the interstimulus differences; neurons also showed prolonged tonic responses in which the spike frequency depended on the intensity of the stimulus presented. Analysis of phasic responses allowed two groups of neurons to be identified: some were specialized to discriminate stimulus intensity only, while others were specialized to discriminate both the intensity and the color tone of the stimulus. Use of the magnitude of the early phasic discharge as a measure of the difference between stimuli yielded a sensory space for lateral geniculate nucleus neurons. The responses of neurons in the first group (44 cells, 86%) produced a two-dimensional achromatic space with two axes--brightness and darkness; this structure appeared independently of whether stimuli were of the same or different color tones. The phasic responses of neurons in the second group (seven of 51, 14%) generated a four-dimensional space with two color and two achromatic axes. The color and achromatic spaces of lateral geniculate nucleus neurons were analogous to the spaces previously identified for neurons in the rabbit visual cortex using the same stimulation conditions. The sensory spaces reconstructed on the basis of neuron phasic discharges essentially coincided with the spaces obtained from analysis of the N85 component of visual evoked potentials in rabbits, which provides support for the vector information coding principle in the visual analyzer. The tonic discharges of most lateral geniculate nucleus neurons correlated linearly with changes in stimulus intensity and can be regarded as reflecting a pre-detector function for the visual cortex detector neurons.

Perceptual considerations in the use of colored photographic and video stimuli to study nonhuman primate behavior

The use of photographs, slides, computerized images, and video to study behavior is increasingly being employed in nonhuman primates. However, since these mediums have been designed to simulate natural coloration for normal trichromatic human vision, they can fail to reproduce color in meaningful and accurate ways for viewers with different visual systems. Given the range of color perception that exists both across and within different species, it is necessary to consider this variation in order to discern the suitability of these mediums for experimental use. Because of the high degree of visual similarity among humans, Old World monkeys, and apes, the use of photographic and video stimuli should be acceptable in terms of replicating naturalistic coloration and making noticeable color manipulations. However, among New World primates and prosimians, there exists a considerable degree of variation in color perceptual abilities depending on the species, sex, and allelic combination of the animals involved. Therefore, the use of these mediums to study behavior is problematic for these species, and should be done with caution.

Evolution of melanopsin photoreceptors: discovery and characterization of a new melanopsin in nonmammalian vertebrates

In mammals, the melanopsin gene (Opn4) encodes a sensory photopigment that underpins newly discovered inner retinal photoreceptors. Since its first discovery in Xenopus laevis and subsequent description in humans and mice, melanopsin genes have been described in all vertebrate classes. Until now, all of these sequences have been considered representatives of a single orthologous gene (albeit with duplications in the teleost fish). Here, we describe the discovery and functional characterisation of a new melanopsin gene in fish, bird, and amphibian genomes, demonstrating that, in fact, the vertebrates have evolved two quite separate melanopsins. On the basis of sequence similarity, chromosomal localisation, and phylogeny, we identify our new melanopsins as the true orthologs of the melanopsin gene previously described in mammals and term this grouping Opn4m. By contrast, the previously published melanopsin genes in nonmammalian vertebrates represent a separate branch of the melanopsin family which we term Opn4x. RT-PCR analysis in chicken, zebrafish, and Xenopus identifies expression of both Opn4m and Opn4x genes in tissues known to be photosensitive (eye, brain, and skin). In the day-14 chicken eye, Opn4m mRNA is found in a subset of cells in the outer nuclear, inner nuclear, and ganglion cell layers, the vast majority of which also express Opn4x. Importantly, we show that a representative of the new melanopsins (chicken Opn4m) encodes a photosensory pigment capable of activating G protein signalling cascades in a light- and retinaldehyde-dependent manner under heterologous expression in Neuro-2a cells. A comprehensive in silico analysis of vertebrate genomes indicates that while most vertebrate species have both Opn4m and Opn4x genes, the latter is absent from eutherian and, possibly, marsupial mammals, lost in the course of their evolution as a result of chromosomal reorganisation. Thus, our findings show for the first time that nonmammalian vertebrates retain two quite separate melanopsin genes, while mammals have just one. These data raise important questions regarding the functional differences between Opn4x and Opn4m pigments, the associated adaptive advantages for most vertebrate species in retaining both melanopsins, and the implications for mammalian biology of lacking Opn4x.

A new melanopsin gene, identified in fish, bird, and amphibian genomes, is the true ortholog of the melanopsin gene previously described in mammals.

Diversity of mammalian photoreceptor properties: adaptations to habitat and lifestyle?

All mammalian retinae contain rod photoreceptors for low-light vision and cone photoreceptors for daylight and color vision. Most nonprimate mammals have dichromatic color vision based on two cone types with spectrally different visual pigments: a short-wavelength-sensitive (S-)cone and a long-wavelength-sensitive (L-)cone. Superimposed on this basic similarity, there are remarkable differences between species. This article reviews some striking examples. The density ratio of cones to rods ranges from 1:200 in the most nocturnal to 20:1 in a few diurnal species. In some species, the proportion of the spectral cone types and their distribution across the retina deviate from the pattern found in most mammals, including a complete absence of S-cones. Depending on species, the spectral sensitivity of the L-cone pigment may peak in the green, yellow, or orange, and that of the S-cone pigment in the blue, violet, or near-ultraviolet. While exclusive expression of one pigment per cone is the rule, some species feature coexpression of the L- and S-pigment in a significant proportion of their cones. It is widely assumed that all these variations represent adaptations to specific visual needs associated with particular habitats and lifestyles. However, in many cases we have not yet identified the adaptive value of a given photoreceptor arrangement. Comparative anatomy is a fruitful approach to explore the range of possible arrangements within the blueprint of the mammalian retina and to identify species with particularly interesting or puzzling patterns that deserve further scrutiny with physiological and behavioral assays.

Color vision pigment frequencies in wild tamarins (Saguinus spp.)

The adaptive importance of polymorphic color vision found in many New World and some prosimian primates has been discussed for many years. Polymorphism is probably maintained in part through a heterozygote advantage for trichromatic females, as such individuals are observed to have greater foraging success when selecting ripe fruits against a background of forest leaves. However, recent work also suggests there are some situations in which dichromatic individuals may have an advantage, and that variation in color vision among individuals possessing different alleles may also be significant. Alleles that confer a selective advantage to individuals are expected to occur at a higher frequency in populations than those that do not. Therefore, analyzing the frequencies of color vision alleles in wild populations can add to our understanding of the selective advantages of some color vision phenotypes over others. With this aim, we used molecular techniques to determine the frequencies of color vision alleles in 12 wild tamarin groups representing three species of the genus Saguinus. Our results show that allele frequencies are not equal, possibly reflecting different selective regimes operating on different color vision phenotypes.

Recent advances in color vision research

The remarkable variation in color vision both among and within primate species is receiving increasing attention from geneticists, psychophysicists, physiologists, and behavioral ecologists. It is known that color vision ability affects foraging behavior. Color vision is also likely to have implications for predation avoidance, social behavior, mate choice, and group dynamics, and should also influence the choice of stimuli for cognitive experiments. Therefore, understanding the color vision of a study species is important and of particular significance to scientists studying species with polymorphic color vision (most platyrrhines and some strepsirrhines). The papers in this issue were inspired by a symposium held during the 20th Congress of the International Primatological Society at Turin, Italy, in August 2004. The aim of the symposium was to bring together research from a range of disciplines, using recent methodological advances in molecular, modeling, and experimental techniques, to help elucidate the evolution, ecological importance, and distribution of color vision genotypes and phenotypes. The symposium achieved its aim, and as with most research in expanding disciplines, there are surprises and many questions still to be answered. Further advances will be made using a combination of different approaches involving analyses at the level of molecu1es, types of cell and neural networks, detailed and long-term field work, modeling, and carefully controlled experimentation.

Polymorphic New World monkeys with more than three M/L cone types

Most New World (platyrrhine) monkeys have M/L cone photopigment polymorphisms that map directly into individual variations in visual sensitivity and color vision. We used electroretinogram flicker photometry to examine M/L cone photopigments in the New World monkey Callicebus moloch (the dusky Titi). Like other New World monkeys, this species has an M/L cone photopigment polymorphism that reflects the presence of X-chromosome opsin gene alleles. However, unlike other platyrrhines in which three M/L photopigments are typical, Callicebus has a total of five M/L cone photopigments. The peak sensitivity values for these pigments extend across the range from 530 to 562 nm. The result is an enhanced array of potential color vision phenotypes in this species.