Dichromatism in macaque monkeys

Comparative connectomics reveals noncanonical wiring for color vision in human foveal retina

The Old World macaque monkey and New World common marmoset provide fundamental models for human visual processing, yet the human ancestral lineage diverged from these monkey lineages over 25 Mya. We therefore asked whether fine-scale synaptic wiring in the nervous system is preserved across these three primate families, despite long periods of independent evolution. We applied connectomic electron microscopy to the specialized foveal retina where circuits for highest acuity and color vision reside. Synaptic motifs arising from the cone photoreceptor type sensitive to short (S) wavelengths and associated with "blue-yellow" (S-ON and S-OFF) color-coding circuitry were reconstructed. We found that distinctive circuitry arises from S cones for each of the three species. The S cones contacted neighboring L and M (long- and middle-wavelength sensitive) cones in humans, but such contacts were rare or absent in macaques and marmosets. We discovered a major S-OFF pathway in the human retina and established its absence in marmosets. Further, the S-ON and S-OFF chromatic pathways make excitatory-type synaptic contacts with L and M cone types in humans, but not in macaques or marmosets. Our results predict that early-stage chromatic signals are distinct in the human retina and imply that solving the human connectome at the nanoscale level of synaptic wiring will be critical for fully understanding the neural basis of human color vision.

Variation and heritability of retinal cone ratios in a free-ranging population of rhesus macaques

A defining feature of catarrhine primates is uniform trichromacy-the ability to distinguish red (long; L), green (medium; M), and blue (short; S) wavelengths of light. Although the tuning of photoreceptors is conserved, the ratio of L:M cones in the retina is variable within and between species, with human cone ratios differing from other catarrhines. Yet, the sources and structure of variation in cone ratios are poorly understood, precluding a broader understanding of color vision variability. Here, we report a large-scale study of a pedigreed population of rhesus macaques (Macaca mulatta). We collected foveal RNA and analyzed opsin gene expression using cDNA and estimated additive genetic variance of cone ratios. The average L:M ratio and standard error was 1.03:1 ± 0.02. There was no age effect, and genetic contribution to variation was negligible. We found marginal sex effects with females having larger ratios than males. S cone ratios (0.143:1 ± 0.002) had significant genetic variance with a heritability estimate of 43% but did not differ between sexes or age groups. Our results contextualize the derived human condition of L-cone dominance and provide new information about the heritability of cone ratios and variation in primate color vision.

Creation of photocyclic vertebrate rhodopsin by single amino acid substitution

Opsins are universal photoreceptive proteins in animals and can be classified into three types based on their photoreaction properties. Upon light irradiation, vertebrate rhodopsin forms a metastable active state, which cannot revert back to the original dark state via either photoreaction or thermal reaction. By contrast, after photoreception, most opsins form a stable active state which can photoconvert back to the dark state. Moreover, we recently found a novel type of opsins whose activity is regulated by photocycling. However, the molecular mechanism underlying this diversification of opsins remains unknown. In this study, we showed that vertebrate rhodopsin acquired the photocyclic and photoreversible properties upon introduction of a single mutation at position 188. This revealed that the residue at position 188 contributes to the diversification of photoreaction properties of opsins by its regulation of the recovery from the active state to the original dark state.

The effect of pelage, background, and distance on predator detection and the evolution of primate color vision

Primates' predators, such as carnivore mammals, usually rely on camouflage to increase proximity to prey and start a predatory attempt. Camouflage depends not only on the color pattern presented by a predator's pelage but also on the background scene in which the hunting takes place. Another factor that influences camouflage effectiveness is prey's color vision since a given camouflage strategy might not work for all visual phenotypes. Still, little research has been made on the effect of primate visual phenotype on predator detection. Here, we investigate the effects of natural pelages, background scenarios, visual phenotypes, and detection distances on predator detection. We used photographs of taxidermized carnivores (ocelots, cougars, and lesser grisons) as detection stimuli, taken in three different natural scenarios (forest, savanna, and grassland), and at two viewing distances (near and far). On a touchscreen monitor, sets of four images (only one containing a hidden animal) were randomly presented to 39 human males (19 dichromats and 20 trichromats). We found that trichromats, when compared to dichromats, present a lower latency and a higher accuracy of carnivore detection for some conditions tested. We also found that pelage color, background scenario, and detection distance interact to influence the effectiveness of camouflage. Our results suggest that trichromacy might be even more advantageous for carnivore detection than thought before, since it facilitates detection of mammals with diverse pelage colorations, in environments with different phytophysiognomies, and at longer distances. We also propose that the higher rates of dichromacy found in modern human societies could have resulted from a relaxation in predation.

Paradoxical impact of memory on color appearance of faces

What is color vision for? Here we compared the extent to which memory modulates color appearance of objects and faces. Participants matched the colors of stimuli illuminated by low-pressure sodium light, which renders scenes monochromatic. Matches for fruit were not predicted by stimulus identity. In contrast, matches for faces were predictable, but surprising: faces appeared green and looked sick. The paradoxical face-color percept could be explained by a Bayesian observer model constrained by efficient coding. The color-matching data suggest that the face-color prior is established by visual signals arising from the recently evolved L-M cone system, not the older S-cone channel. Taken together, the results show that when retinal mechanisms of color vision are impaired, the impact of memory on color perception is greatest for face color, supporting the idea that trichromatic color plays an important role in social communication.

What is the function of color vision? Here, the authors show that when retinal mechanisms of color are impaired, memory has a paradoxical impact on color appearance that is selective for faces, providing evidence that color contributes to face encoding and social communication.

Color Perception in Protanomalous Female Macaca fascicularis

Protanomalous females with X chromosome-linked color vision deficiency exhibit mild abnormalities, whereas dichromats show a distinct deficiency in discriminating certain color pairs. Dichromats have an advantage in detecting a textured target when it is camouflaged by red-green colors, owing to their insensitivity to these colors. However, it is not certain whether protanomalous females possess a similar advantage in breaking camouflage. Here, we introduce an animal model of dichromatic macaque monkeys and protanomalous females. We examined whether protanomalous females have the same advantage in breaking color camouflage as shown by dichromatic macaques. We also tested whether they could discriminate a certain color pair that trichromats could, where the dichromats are confused. Our experiments show that protanomalous macaques can break color camouflage, similar to dichromats, and can discriminate colors similarly to trichromats. Protanomalous females are thus thought to have the combined ecological advantages of being both trichromats and dichromats.

Germination fitness of two temperate epiphytic ferns shifts under increasing temperatures and forest fragmentation

Ferns are an important component of ecosystems around the world. Studies of the impacts that global changes may have on ferns are scarce, yet emerging studies indicate that some species may be particularly sensitive to climate change. The lack of research in this subject is much more aggravated in the case of epiphytes, and especially those that live under temperate climates. A mathematical model was developed for two temperate epiphytic ferns in order to predict potential impacts on spore germination kinetics, in response to different scenarios of global change, coming from increasing temperature and forest fragmentation. Our results show that an increasing temperature will have a negative impact over the populations of these temperate epiphytic ferns. Under unfragmented forests the germination percentage was comparatively less influenced than in fragmented patches. This study highlight that, in the long term, populations of the studied epiphytic temperate ferns may decline due to climate change. Overall, epiphytic fern communities will suffer changes in diversity, richness and dominance. Our study draws attention to the role of ferns in epiphytic communities of temperate forests, emphasizing the importance of considering these plants in any conservation strategy, specifically forest conservation. From a methodological point of view, the model we propose could be easily used to dynamically monitor the status of ecosystems, allowing the quick prediction of possible future scenarios, which is a crucial issue in biodiversity conservation decision-making.

Opn5L1 is a retinal receptor that behaves as a reverse and self-regenerating photoreceptor

Most opsins are G protein-coupled receptors that utilize retinal both as a ligand and as a chromophore. Opsins' main established mechanism is light-triggered activation through retinal 11-cis-to-all-trans photoisomerization. Here we report a vertebrate non-visual opsin that functions as a Gi-coupled retinal receptor that is deactivated by light and can thermally self-regenerate. This opsin, Opn5L1, binds exclusively to all-trans-retinal. More interestingly, the light-induced deactivation through retinal trans-to-cis isomerization is followed by formation of a covalent adduct between retinal and a nearby cysteine, which breaks the retinal-conjugated double bond system, probably at the C₁₁ position, resulting in thermal re-isomerization to all-trans-retinal. Thus, Opn5L1 acts as a reverse photoreceptor. We conclude that, like vertebrate rhodopsin, Opn5L1 is a unidirectional optical switch optimized from an ancestral bidirectional optical switch, such as invertebrate rhodopsin, to increase the S/N ratio of the signal transduction, although the direction of optimization is opposite to that of vertebrate rhodopsin.

Photopigments and the dimensionality of animal color vision

Early color-matching studies established that normal human color vision is trichromatic. Subsequent research revealed a causal link between trichromacy and the presence in the retina of three classes of cone photopigments. Over the years, measurements of the photopigment complements of other species have expanded greatly and these are frequently used to predict the dimensionality of an animal's color vision. This review provides an account of how the linkage between the number of active photopigments and the dimensions of human color vision developed, summarizes the various mechanisms that can impact photopigment spectra and number, and provides an across-species survey to examine cases where the photopigment link to the dimensionality of color vision has been claimed. The literature reveals numerous instances where the human model fails to account for the ways in which the visual systems of other animals exploit information obtained from the presence of multiple photopigments in support of their behavior.

Advantage of Dichromats over Trichromats in Discrimination of Color-Camouflaged Stimuli in Humans

This study investigated whether 12 participants with color-vision deficiency had superior visual discrimination of color-camouflaged stimuli shown on a computer screen compared with 12 participants with normal trichromatic vision. Participants were asked to distinguish a circular pattern from other patterns in which textural elements differed from the background in orientation and thickness. In one condition, stimuli were single-colored, green or red; in the other condition, stimuli were color camouflaged with a green and red mosaic overlaid onto the pattern. Color-vision deficient participants selected the correct stimuli in the color-camouflaged condition as quickly as they did in the single-colored condition. However, normal color-vision participants took longer to select the correct choice in the color-camouflaged condition than in the single-colored condition. These results suggest that participants with color-vision deficiency may have a superior visual ability to discriminate the color-camouflaged stimuli.

Color vision diversity and significance in primates inferred from genetic and field studies

Color provides a reliable cue for object detection and identification during various behaviors such as foraging, mate choice, predator avoidance and navigation. The total number of colors that a visual system can discriminate is largely dependent on the number of different spectral types of cone opsins present in the retina and the spectral separations among them. Thus, opsins provide an excellent model system to study evolutionary interconnections at the genetic, phenotypic and behavioral levels. Primates have evolved a unique ability for three-dimensional color vision (trichromacy) from the two-dimensional color vision (dichromacy) present in the majority of other mammals. This was accomplished via allelic differentiation (e.g. most New World monkeys) or gene duplication (e.g. Old World primates) of the middle to long-wavelength sensitive (M/LWS, or red-green) opsin gene. However, questions remain regarding the behavioral adaptations of primate trichromacy. Allelic differentiation of the M/LWS opsins results in extensive color vision variability in New World monkeys, where trichromats and dichromats are found in the same breeding population, enabling us to directly compare visual performances among different color vision phenotypes. Thus, New World monkeys can serve as an excellent model to understand and evaluate the adaptive significance of primate trichromacy in a behavioral context. I shall summarize recent findings on color vision evolution in primates and introduce our genetic and behavioral study of vision-behavior interrelationships in free-ranging sympatric capuchin and spider monkey populations in Costa Rica.

Sphere-formation culture of testicular germ cells in the common marmoset, a small New World monkey

Spermatogonia are specialized cells responsible for continuous spermatogenesis and the production of offspring. Because of this biological property, in vitro culture of spermatogonia provides a powerful methodology to advance reproductive biology and engineering. However, methods for culturing primate spermatogonia are poorly established. We have designed a novel method for culturing spermatogonia in the common marmoset (Callithrix jacchus), a small primate. By using our method with a suite of growth factors, adult marmoset testis-derived germ cells could be cultured in the form of a floating sphere for several weeks. Notably, this method could be applied not only to freshly isolated cells but also to cryopreserved cell stocks. The spheres enriched spermatogonia and early spermatocytes, and could be assembled from a C-KIT(+) spermatogonial population. Techniques for culturing spermatogonia could facilitate increased understanding of primate reproduction as well as the preservation of valuable biomaterials from nonhuman primates.

Color-detection thresholds in rhesus macaque monkeys and humans

Macaque monkeys are a model of human color vision. To facilitate linking physiology in monkeys with psychophysics in humans, we directly compared color-detection thresholds in humans and rhesus monkeys. Colors were defined by an equiluminant plane of cone-opponent color space. All subjects were tested on an identical apparatus with a four-alternative forced-choice task. Targets were 2° square, centered 2° from fixation, embedded in luminance noise. Across all subjects, the change in detection thresholds from initial testing to plateau performance (“learning”) was similar for +L − M (red) colors and +M − L (bluish-green) colors. But the extent of learning was higher for +S (lavender) than for −S (yellow-lime); moreover, at plateau performance, the cone contrast at the detection threshold was higher for +S than for −S. These asymmetries may reflect differences in retinal circuitry for S-ON and S-OFF. At plateau performance, the two species also had similar detection thresholds for all colors, although monkeys had shorter reaction times than humans and slightly lower thresholds for colors that modulated L/M cones. We discuss whether these observations, together with previous work showing that monkeys have lower spatial acuity than humans, could be accounted for by selective pressures driving higher chromatic sensitivity at the cost of spatial acuity amongst monkeys, specifically for the more recently evolved L − M mechanism.

Color Vision Variation as Evidenced by Hybrid L/M Opsin Genes in Wild Populations of Trichromatic Alouatta New World Monkeys

Platyrrhine (New World) monkeys possess highly polymorphic color vision owing to allelic variation of the single-locus L/M opsin gene on the X chromosome. Most species consist of female trichromats and female and male dichromats. Howlers (genus Alouatta) are an exception; they are considered to be routinely trichromatic with L and M opsin genes juxtaposed on the X chromosome, as seen in catarrhine primates (Old World monkeys, apes, and humans). Yet it is not known whether trichromacy is invariable in howlers. We examined L/M opsin variation in wild howler populations in Costa Rica and Nicaragua (Alouatta palliata) and Belize (A. pigra), using fecal DNA. We surveyed exon 5 sequences (containing the diagnostic 277th and 285th residues for λ_max) for 8 and 18 X chromosomes from Alouatta palliata and A. pigra, respectively. The wavelengths of maximal absorption (λ_max) of the reconstituted L and M opsin photopigments were 564 nm and 532 nm, respectively, in both species. We found one M–L hybrid sequence with a recombinant 277/285 haplotype in Alouatta palliata and two L–M hybrid sequences in A. pigra. The λ_max values of the reconstituted hybrid photopigments were in the range of 546~554 nm, which should result in trichromat phenotypes comparable to those found in other New World monkey species. Our finding of color vision variation due to high frequencies of L/M hybrid opsin genes in howlers challenges the current view that howlers are routine and uniform trichromats. These results deepen our understanding of the evolutionary significance of color vision polymorphisms and routine trichromacy and emphasize the need for further assessment of opsin gene variation as well as behavioral differences among subtypes of trichromacy.

Gene conversion and purifying selection shape nucleotide variation in gibbon L/M opsin genes

Background

Routine trichromatic color vision is a characteristic feature of catarrhines (humans, apes and Old World monkeys). This is enabled by L and M opsin genes arrayed on the X chromosome and an autosomal S opsin gene. In non-human catarrhines, genetic variation affecting the color vision phenotype is reported to be absent or rare in both L and M opsin genes, despite the suggestion that gene conversion has homogenized the two genes. However, nucleotide variation of both introns and exons among catarrhines has only been examined in detail for the L opsin gene of humans and chimpanzees. In the present study, we examined the nucleotide variation of gibbon (Catarrhini, Hylobatidae) L and M opsin genes. Specifically, we focused on the 3.6~3.9-kb region that encompasses the centrally located exon 3 through exon 5, which encode the amino acid sites functional for the spectral tuning of the genes.

Results

Among 152 individuals representing three genera (Hylobates, Nomascus and Symphalangus), all had both L and M opsin genes and no L/M hybrid genes. Among 94 individuals subjected to the detailed DNA sequencing, the nucleotide divergence between L and M opsin genes in the exons was significantly higher than the divergence in introns in each species. The ratio of the inter-LM divergence to the intra-L/M polymorphism was significantly lower in the introns than that in synonymous sites. When we reconstructed the phylogenetic tree using the exon sequences, the L/M gene duplication was placed in the common ancestor of catarrhines, whereas when intron sequences were used, the gene duplications appeared multiple times in different species. Using the GENECONV program, we also detected that tracts of gene conversions between L and M opsin genes occurred mostly within the intron regions.

Conclusions

These results indicate the historical accumulation of gene conversions between L and M opsin genes in the introns in gibbons. Our study provides further support for the homogenizing role of gene conversion between the L and M opsin genes and for the purifying selection against such homogenization in the central exons to maintain the spectral difference between L and M opsins in non-human catarrhines.

Transmission of colour and acuity signals by parvocellular cells in marmoset monkeys

The red-green axis of colour vision evolved recently in primate evolutionary history. Signals serving red-green colour vision travel together with signals serving spatial vision, in the parvocellular (PC) division of the subcortical visual pathway. However, the question of whether receptive fields of PC pathway cells are specialized to transmit red-green colour signals remains unresolved. We addressed this question in single-cell recordings from the lateral geniculate nucleus of anaesthetized marmosets. Marmosets show a high proportion of dichromatic (red-green colour-blind) individuals, allowing spatial and colour tuning properties of PC cells to be directly compared in dichromatic and trichromatic visual systems. We measured spatial frequency tuning for sine gratings that provided selective stimulation of individual photoreceptor types. We found that in trichromatic marmosets, the foveal visual field representation is dominated by red-green colour-selective PC cells. Colour selectivity of PC cells is reduced at greater eccentricities, but cone inputs to centre and surround are biased to create more selectivity than predicted by a purely 'random wiring' model. Thus, one-to-one connections in the fovea are sufficient, but not necessary, to create colour-selective responses. The distribution of spatial tuning properties for achromatic stimuli shows almost complete overlap between PC cells recorded in dichromatic and trichromatic marmosets. These data indicate that transmission of red-green colour signals has been enabled by centre-surround receptive fields of PC cells, and has not altered the capacity of PC cells to serve high-acuity vision at high stimulus contrast.

Beyond correlation: do color features influence attention in rainforest?

Recent research indicates a direct relationship between low-level color features and visual attention under natural conditions. However, the design of these studies allows only correlational observations and no inference about mechanisms. Here we go a step further to examine the nature of the influence of color features on overt attention in an environment in which trichromatic color vision is advantageous. We recorded eye-movements of color-normal and deuteranope human participants freely viewing original and modified rainforest images. Eliminating red–green color information dramatically alters fixation behavior in color-normal participants. Changes in feature correlations and variability over subjects and conditions provide evidence for a causal effect of red–green color-contrast. The effects of blue–yellow contrast are much smaller. However, globally rotating hue in color space in these images reveals a mechanism analyzing color-contrast invariant of a specific axis in color space. Surprisingly, in deuteranope participants we find significantly elevated red–green contrast at fixation points, comparable to color-normal participants. Temporal analysis indicates that this is due to compensatory mechanisms acting on a slower time scale. Taken together, our results suggest that under natural conditions red–green color information contributes to overt attention at a low-level (bottom-up). Nevertheless, the results of the image modifications and deuteranope participants indicate that evaluation of color information is done in a hue-invariant fashion.

Degeneration of olfactory receptor gene repertories in primates: no direct link to full trichromatic vision

Odor molecules in the environment are detected by olfactory receptors (ORs), being encoded by a large multigene family in mammalian genomes. It is generally thought that primates are vision oriented and dependent weakly on olfaction. Previous studies suggested that Old World monkeys (OWMs) and hominoids lost many functional OR genes after the divergence from New World monkeys (NWMs) due to the acquisition of well-developed trichromatic vision. To examine this hypothesis, here we analyzed OR gene repertoires of five primate species including NWMs, OWMs, and hominoids for which high-coverage genome sequences are available, together with two prosimians and tree shrews with low-coverage genomes. The results showed no significant differences in the number of functional OR genes between NWMs (marmosets) and OWMs/hominoids. Two independent analyses, identification of orthologous genes among the five primates and estimation of the numbers of ancestral genes by the reconciled tree method, did not support a sudden loss of OR genes at the branch of the OWMs/hominoids ancestor but suggested a gradual loss in every lineage. Moreover, we found that humans retain larger numbers of ancestral OR genes that were in the common ancestor of NWMs/OWMs/hominoids than orangutans and macaques and that the OR gene repertoire in humans is more similar to that of marmosets than those of orangutans and macaques. These results suggest that the degeneration of OR genes in primates cannot simply be explained by the acquisition of trichromatic vision, and our sense of smell may not be inferior to other primate species.

Primate color vision: a comparative perspective

Thirty years ago virtually everything known about primate color vision derived from psychophysical studies of normal and color-defective humans and from physiological investigations of the visual system of the macaque monkey, the most popular of human surrogates for this purpose. The years since have witnessed much progress toward the goal of understanding this remarkable feature of primate vision. Among many advances, investigations focused on naturally occurring variations in color vision in a wide range of nonhuman primate species have proven to be particularly valuable. Results from such studies have been central to our expanding understanding of the interrelationships between opsin genes, cone photopigments, neural organization, and color vision. This work is also yielding valuable insights into the evolution of color vision.

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.

Advantage of dichromats over trichromats in discrimination of color-camouflaged stimuli in nonhuman primates

Due to a middle- to long-wavelength-sensitive (M/LWS) cone opsin polymorphism, there is considerable phenotypic variation in the color vision of New World monkeys. Many females have trichromatic vision, whereas some females and all males have dichromatic vision. The selective pressures that maintain this polymorphism are unclear. In the present study we compared the performance of dichromats and trichromats in a discrimination task. We examined tri- and dichromatic individuals of two species: brown capuchin monkeys (Cebus apella) and long-tailed macaques (Macaca fascicularis). We also examined one protanomalous chimpanzee (Pan troglodytes). The subjects' task was to discriminate a circular pattern from other patterns in which textural elements differed in orientation and thickness from the background. After they were trained with stimuli of a single color, the subjects were presented with color-camouflaged stimuli with a green/red mosaic overlaid onto the pattern. The dichromatic monkeys and the protanomalous chimpanzee selected the correct stimulus under camouflaged conditions at rates significantly above chance levels, while the trichromats did not. These findings demonstrate that dichromatic nonhuman primates possess a superior visual ability to discriminate color-camouflaged stimuli, and that such an ability may confer selective advantages with respect to the detection of cryptic foods and/or predators.

Identification of a protanomalous chimpanzee by molecular genetic and electroretinogram analyses

We determined the structures of long (L)-wavelength-sensitive and middle (M)-wavelength-sensitive opsin gene array of 58 male chimpanzees and we investigated relative sensitivity to red and green lights by electroretinogram flicker photometry. One subject had protanomalous color vision, while others had normal color vision. Unlike in humans, a polymorphic difference in the copy number of the genes and a polymorphic base substitution at amino acid position 180 were not frequently observed in chimpanzees.

Forest-climate interactions in fragmented tropical landscapes

In the tropics, habitat fragmentation alters forest-climate interactions in diverse ways. On a local scale (less than 1 km), elevated desiccation and wind disturbance near fragment margins lead to sharply increased tree mortality, thus altering canopy-gap dynamics, plant community composition, biomass dynamics and carbon storage. Fragmented forests are also highly vulnerable to edge-related fires, especially in regions with periodic droughts or strong dry seasons. At landscape to regional scales (10-1000 km), habitat fragmentation may have complex effects on forest-climate interactions, with important consequences for atmospheric circulation, water cycling and precipitation. Positive feedbacks among deforestation, regional climate change and fire could pose a serious threat for some tropical forests, but the details of such interactions are poorly understood.

Evolution and function of routine trichromatic vision in primates

Evolution of the red-green visual subsystem in trichromatic primates has been linked to foraging advantages, namely the detection of either ripe fruits or young leaves amid mature foliage. We tested competing hypotheses globally for eight primate taxa: five with routine trichromatic vision, three without. Routinely trichromatic species ingested leaves that were "red shifted" compared to background foliage more frequently than species lacking this trait. Observed choices were not the reddest possible, suggesting a preference for optimal nutritive gain. There were no similar differences for fruits although red-greenness may sometimes be important in close-range fruit selection. These results suggest that routine trichromacy evolved in a context in which leaf consumption was critical.

Absorption spectra of reconstituted visual pigments of a nocturnal prosimian, Otolemur crassicaudatus

Absorption spectra of visual pigments characterize animal vision. The association between absorption spectra and amino acid (aa) sequences of pigments has been well established for the middle- to-long-wave-sensitive (M/LWS) class of cone visual pigments of vertebrates, known as the "five-sites" rule where amino acid residues at the 180th, 197th, 277th, 285th, and 308th sites mostly determine the spectra. For primate M/LWS pigments, however, applicability of the rule is not clear because of the scarcity of absorbance data collected directly from purified pigments. In particular, no prosimian pigment has been examined in vitro. In this study, we reconstituted visual pigments of a nocturnal prosimian, the greater galago (Otolemur crassicaudatus), which has the M/LWS cone and the rod visual pigments in its retina. The five residues of the galago M/LWS pigment are Ala, His, Tyr, Ala, and Ala, respectively, and its peak absorption spectra (lambda(max)) was measured to be 539 nm, which is virtually identical to the expected value from the rule (538 nm), showing that the five-sites rule holds for this prosimian. The lambda(max) of the rod visual pigment was measured as 502 nm. Accurate estimate of lambda(max) values is essential in establishing the molecular basis of visual pigment evolution.

Behavioral evidence of color vision deficiency in a protanomalia chimpanzee (Pan troglodytes)

Although color vision deficiency is very rare among Old World monkeys and apes, one male chimpanzee (Lucky) was identified as protanomalous by genetic and physiological analyses. This study assessed behavioral phenotypes of Lucky and four chimpanzees with normal color vision by discrimination task using the modified Ishihara pseudo-isochromatic plates. Lucky could not discriminate the stimuli that the other chimpanzees could. This is the first behavioral evidence of color vision deficiency in chimpanzees.

Historical contingency in the evolution of primate color vision

Primates are unique among eutherian mammals for possessing three types of retinal cone. Curiously, catarrhines, platyrrhines, and strepsirhines share this anatomy to different extents, and no hypothesis has hitherto accounted for this variability. Here we propose that the historical biogeography of figs and arborescent palms accounts for the global variation in primate color vision. Specifically, we suggest that primates invaded Paleogene forests characterized by figs and palms, the fruits of which played a keystone function. Primates not only relied on such resources, but also provided high-quality seed dispersal. In turn, figs and palms lost or simply did not evolve conspicuous coloration, as this conferred little advantage for attracting mammals. We suggest that the abundance and coloration of figs and palms offered a selective advantage to foraging groups with mixed capabilities for chromatic distinction. Climatic cooling at the end of the Eocene and into the Neogene resulted in widespread regional extinction or decimation of palms and (probably) figs. In regions where figs and palms became scarce, we suggest primates evolved routine trichromatic vision in order to exploit proteinaceous young leaves as a replacement resource. A survey of the hue and biogeography of extant figs and palms provides some empirical support. Where these resources are infrequent, primates are routinely trichromatic and consume young leaves during seasonal periods of fruit dearth. These results imply a link between the differential evolution of primate color vision and climatic changes during the Eocene-Oligocene transition.

Corridors affect plants, animals, and their interactions in fragmented landscapes

Among the most popular strategies for maintaining populations of both plants and animals in fragmented landscapes is to connect isolated patches with thin strips of habitat, called corridors. Corridors are thought to increase the exchange of individuals between habitat patches, promoting genetic exchange and reducing population fluctuations. Empirical studies addressing the effects of corridors have either been small in scale or have ignored confounding effects of increased habitat area created by the presence of a corridor. These methodological difficulties, coupled with a paucity of studies examining the effects of corridors on plants and plant-animal interactions, have sparked debate over the purported value of corridors in conservation planning. We report results of a large-scale experiment that directly address this debate. In eight large-scale experimental landscapes that control for patch area and test alternative mechanisms of corridor function, we demonstrate that corridors not only increase the exchange of animals between patches, but also facilitate two key plant-animal interactions: pollination and seed dispersal. Our results show that the beneficial effects of corridors extend beyond the area they add, and suggest that increased plant and animal movement through corridors will have positive impacts on plant populations and community interactions in fragmented landscapes.

Visual ecology: coloured fruit is what the eye sees best

Trichromatic vision may have evolved as an aid to frugivory. This hypothesis is supported by the recent demonstration that the spatial characteristics of pictures containing fruit are particularly well matched to the abilities of the human visual system.

Color vision perception in the capuchin monkey (Cebus apella): a re-evaluation of procedures using Munsell papers

In previous experiments based on color vision discrimination of Munsell chips, Pessoa et al. (Behav Brain Res 1997;89:285-8) presented evidence of trichromatic color vision in capuchin monkeys. However, the possibility of subjects using brightness cues was not eliminated. The purpose of the present study was to reassess the color perception in Cebus apella in a similar behavioral paradigm, but using a range of brightness values at each tested hue. We now report that male capuchins show a dichromatic behavior. The results are discussed in terms of the hypothesis of male dichromatism in the New World monkey, the biological role of color vision and cognitive abilities of capuchin monkeys.

Variations in long- and middle-wavelength-sensitive opsin gene loci in crab-eating monkeys

We analyzed variations in long (L)- and middle (M)-wavelength-sensitive opsin gene loci in crab-eating monkeys. Unlike humans, most monkeys have a single L and a single M gene. Two variant genotypes, one with only one opsin gene (dichromatic) and one with tandemly arrayed multiple genes, were also found in the monkeys. However, the frequency of the former was 0.47%, and that of the latter was 5% in the monkeys, while 2% and 66%, respectively, in Caucasian males. The two variants were found only in Java Island, Indonesia, and South Thailand, respectively. The data suggest that the frequency of each genotype is different among Old World primates.

Electroretinogram analysis of relative spectral sensitivity in genetically identified dichromatic macaques

The retinas of macaque monkeys usually contain three types of photopigment, providing them with trichromatic color vision homologous to that of humans. However, we recently used molecular genetic analysis to identify several macaques with a dichromatic genotype. The affected X chromosome of these animals contains a hybrid gene of long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) photopigments instead of separate genes encoding L and M photopigments. The product of the hybrid gene exhibits a spectral sensitivity close to that of M photopigment; consequently, male monkeys carrying the hybrid gene are genetic protanopes, effectively lacking L photopigment. In the present study, we assessed retinal expression of L photopigment in monkeys carrying the hybrid gene. The relative sensitivities to middle-wavelength (green) and long-wavelength (red) light were measured by electroretinogram flicker photometry. We found the sensitivity to red light to be extremely low in protanopic male monkeys compared with monkeys with the normal genotype. In female heterozygotes, sensitivity to red light was intermediate between the genetic protanopes and normal monkeys. Decreased sensitivity to long wavelengths was thus consistent with genetic loss of L photopigment.

Ecological importance of trichromatic vision to primates

Trichromatic colour vision, characterized by three retinal photopigments tuned to peak wavelengths of approximately 430 nm, approximately 535 nm and approximately 562 nm (refs 1, 2), has evolved convergently in catarrhine primates and one genus of New World monkey, the howlers (genus Alouatta). This uniform capacity to discriminate red-green colours, which is not found in other mammals, has been proposed as advantageous for the long-range detection of either ripe fruits or young leaves (which frequently flush red in the tropics) against a background of mature foliage. Here we show that four trichromatic primate species in Kibale Forest, Uganda, eat leaves that are colour discriminated only by red-greenness, a colour axis correlated with high protein levels and low toughness. Despite their divergent digestive systems, these primates have no significant interspecific differences in leaf colour selection. In contrast, eaten fruits were generally discriminated from mature leaves on both red-green and yellow-blue channels and also by their luminance, with a significant difference between chimpanzees and monkeys in fruit colour choice. Our results implicate leaf consumption, a critical food resource when fruit is scarce, as having unique value in maintaining trichromacy in catarrhines.