Sexual selection and trichromatic color vision in primates: statistical support for the preexisting-bias hypothesis

The evolution of trichromatic color vision in primates may improve foraging performance as well as intraspecific communication; however, the context in which color vision initially evolved is unknown. We statistically examined the hypothesis that trichromatic color vision in primates represents a preexisting bias for the evolution of red coloration (pelage and/or skin) through sexual selection. Our analyses show that trichromatic color vision evolved before red pelage and red skin, as well as before gregarious mating systems that would promote sexual selection for visual traits and other forms of intraspecific communication via red traits. We also determined that both red pelage and red skin were more likely to evolve in the presence of color vision and mating systems that promote sexual selection. These results provide statistical support for the hypothesis that trichromatic color vision in primates evolved in a context other than intraspecific communication with red traits, most likely foraging performance, but, once evolved, represented a preexisting bias that promoted the evolution of red traits through sexual selection.     

Fixed green and brown color morphs and a novel color-changing morph of the Pacific tree frog Hyla regilla

Pacific tree frogs Hyla regilla are typically either green or brown in dorsal coloration. The frequency of green and brown individuals is known to fluctuate seasonally. Previous investigators have generally assumed that the green and brown body colors represent a "fixed" polymorphism and that seasonal changes in the proportion of the two body colors are a consequence of differential survival of the two color morphs. Here we report that, in addition to the "fixed" (i.e., non-color-changing) green and brown morphs of H. regilla, there are some individuals that can change hue between green and brown. The distribution of color-change ability in our study population is bimodal, suggesting that "color changers" are a distinct morph rather than one extreme of a continuous distribution of color-change ability. Our findings suggest that background brightness, not hue, triggers color change in the newly discovered morph and that this change requires days to weeks to occur. Such slow color change is not well suited for making short-term changes in color as a frog moves between differently colored substrates. Rather, seasonal changes in habitat characteristics and/or microhabitat use are likely to maintain color-change ability. Color polymorphism and color-change ability appear to represent alternative responses to divergent selection for crypsis in a heterogeneous, seasonally variable environment.     

Long-term occupational exposure to organic solvents affects color vision, contrast sensitivity and visual fields

The purpose of this study was to evaluate the visual outcome of chronic occupational exposure to a mixture of organic solvents by measuring color discrimination, achromatic contrast sensitivity and visual fields in a group of gas station workers. We tested 25 workers (20 males) and 25 controls with no history of chronic exposure to solvents (10 males). All participants had normal ophthalmologic exams. Subjects had worked in gas stations on an average of 9.6±6.2 years. Color vision was evaluated with the Lanthony D15d and Cambridge Colour Test (CCT). Visual field assessment consisted of white-on-white 24–2 automatic perimetry (Humphrey II-750i). Contrast sensitivity was measured for sinusoidal gratings of 0.2, 0.5, 1.0, 2.0, 5.0, 10.0 and 20.0 cycles per degree (cpd). Results from both groups were compared using the Mann–Whitney U test. The number of errors in the D15d was higher for workers relative to controls (p<0.01). Their CCT color discrimination thresholds were elevated compared to the control group along the protan, deutan and tritan confusion axes (p<0.01), and their ellipse area and ellipticity were higher (p<0.01). Genetic analysis of subjects with very elevated color discrimination thresholds excluded congenital causes for the visual losses. Automated perimetry thresholds showed elevation in the 9°, 15° and 21° of eccentricity (p<0.01) and in MD and PSD indexes (p<0.01). Contrast sensitivity losses were found for all spatial frequencies measured (p<0.01) except for 0.5 cpd. Significant correlation was found between previous working years and deutan axis thresholds (rho = 0.59; p<0.05), indexes of the Lanthony D15d (rho = 0.52; p<0.05), perimetry results in the fovea (rho = −0.51; p<0.05) and at 3, 9 and 15 degrees of eccentricity (rho = −0.46; p<0.05). Extensive and diffuse visual changes were found, suggesting that specific occupational limits should be created.     

Molecular basis of abnormal red-green color vision: a family with three types of color vision defects

The molecular nature of three different types of X-linked color-vision defects, protanomaly, deuteranomaly, and protanopia, in a large 3-generation family was determined. In the protanomalous and protanopic males the normal red pigment gene was replaced by a 5' red-3' green fusion gene. The protanomalous male had more red pigment DNA in his fusion gene than did the more severely affected protanopic individual. The deuteranomalous individual had four green pigment genes and one 5' green-3' red fusion gene. These results extend those of Nathans et al., who proposed that most red-green color-vision defects arise as a result of unequal crossing-over between the red and green pigment genes. The various data suggest that differences in severity of color-vision defects associated with fusion genes are caused by differences in crossover sites between the red and green pigment genes. Currently used molecular methodology is not sufficiently sensitive to define these fusion points accurately, and the specific color-vision defect within the deutan or protan class cannot be predicted. The DNA patterns for color-vision genes of female heterozygotes have not previously been described. Patterns of heterozygotes may not be distinguishable from those of normals. However, a definite assignment of the various color pigment gene arrays could be carried out by family study. Two compound heterozygotes for color-vision defects who tested as normal by anomaloscopy were found to carry abnormal fusion genes. In addition, a normal red pigment gene was present on one chromosome and at least one normal green pigment gene was present on the other.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Anomalous color vision in three Mexican populations

Color vision was tested as part of a study of micro-evolution in three historically-related populations in Mexico. The frequencies of color vision anomalies fall within the range observed for contemporary Latin American populations. The present findings do not support the previously proposed hypothesis concerning the relaxation of selective forces in agricultural and urban populations.     

A model of color vision in trichromats and protans

A model which explains the human vision protanopic deficiency and its biologic prototype with the absence of red-absorbing pigment (rabbit) was constructed from neuron-like elements. In behavioral experiments and by means of evoked potential technique it was shown that the rabbit's color space is characterized by a spherical four-dimensional with a reduction of red-coding area. Similar spherical four-dimensional structure of color space is characteristic for a group of protanopic human subjects. The perceptive space of another group of protanopic subjects (protanomals) is characterized by a reduction of both parts of the red-green opponent axis. These disorders are reproduced in the model either by a loss of some color-coding elements (the absence of the red-absorbing pigment as in protanops) or a shift of the spectral characteristics of the red pigment towards those of the green one (protanomals).     

Variations in primate color vision: Mechanisms and utility

Among mammals, only the primates have acquired the biological machinery needed for highly acute color vision. That distinction led Gordon Walls, perhaps the foremost authority on comparative vision of this century, to suggest long ago that “the color vision of the higher primates is assuredly a law unto itself, genetically and historically speaking.”¹ Primate color vision is indeed unique. One manifestation of this uniqueness is that color vision abilities vary significantly, not only between some groupings of primate species, but, remarkably, among individuals of a considerable number of species. Although the functional significance of these variations remains, in large measure, to be sorted out, the past decade has brought much progress in revealing the mechanisms that underlie variation.     

Visual perception: mind and brain see eye to eye

Recent functional imaging studies have identified neural activity that is closely associated with the perception of illusory motion. The mapping of the mind onto the bin appears to be one-to-one: activity in visual 'motion area' MT is highly correlated with perceptual experience.     

Visual fields, eye movements, and scanning behavior of a sit-and-wait predator, the black phoebe (Sayornis nigricans)

Foraging mode influences the dominant sensory modality used by a forager and likely the strategies of information gathering used in foraging and anti-predator contexts. We assessed three components of visual information gathering in a sit-and-wait avian predator, the black phoebe (Sayornis nigricans): configuration of the visual field, degree of eye movement, and scanning behavior through head-movement rates. We found that black phoebes have larger lateral visual fields than similarly sized ground-foraging passerines, as well as relatively narrower binocular and blind areas. Black phoebes moved their eyes, but eye movement amplitude was relatively smaller than in other passerines. Black phoebes may compensate for eye movement constraints with head movements. The rate of head movements increased before attacking prey in comparison to non-foraging contexts and before movements between perches. These findings suggest that black phoebes use their lateral visual fields, likely subtended by areas of high acuity in the retina, to track prey items in a three-dimensional space through active head movements. These head movements may increase depth perception, motion detection and tracking. Studying information gathering through head movement changes, rather than body posture changes (head-up, head-down) as generally presented in the literature, may allow us to better understand the mechanisms of information gathering from a comparative perspective.     

Visual computation and saccadic eye movements: a theoretical perspective

A simple instance of parallel computation in neural networks occurs when the eye orients to a novel visual target. Consideration of target-elicited saccadic eye movements opens the question of how spatial position is represented in the visual pathways involved in this response. It is argued that a point-for-point retinotopic coding of spatial position (the 'local sign' approach) is inadequate to account for the characteristics of the response. An alternative approach based on distributed coding is developed.     

Preliminary investigation into a neural net theory of color vision

Summary This paper reports results of an investigation of the problem of vertebrate color vision by means of a theoretical model, which, although it uses one kind of receptor, can be adapted to a multireceptor concept. It is assumed (1) that the time constant of the change of the receptor potential conveys the color information of the stimulus, whereas the magnitude of the potential is correlated with stimulus intensity and (2) that a group of cells, tentatively identified as ganglion cells, are associated with each receptor field. These cells fire only if the time constant falls within a certain range. Thus, the visual spectrum is divided into regions and the information is transmitted to the central nervous system. Wave length discrimination in the theoretical model is accomplished by one kind of retinal neural nets that are biased differentially. An analog computer was used in this initial phase of the investigation. Care has been taken to ensure that the model satisfies current anatomical and physiological knowledge. It has produced results similar to Granit's (1955) spectral sensitivity and Kelly's (1961) amplitude sensitivity curves. The model, which will predict subjective color phenomena at appropriate frequencies, has raised questions amenable to psychophysiological techniques.

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Zusammenfassung Dieser Bericht enthält die Ergebnisse einer Untersuchung des Farbensehens der Wirbeltiere, dargestellt in einem theoretischen Modell, das, obgleich es nur einen Receptor besitzt, auch auf mehrere Receptoren erweitert werden kann. Es wird angenommen 1., daß die Zeitkonstante der Veränderung des Receptor-Potentials die Farbeninformation des Lichtreizes überträgt, während die Potentialgröße mit der Intensität des Lichtreizes zusammenhängt, und 2., daß eine Gruppe von Zellen, die vorläufig als Ganglionzellen angenommen werden, mit jedem Receptorfeld assoziiert sind. Diese Zellen werden nur dann aktiviert, wenn die Zeitkonstante in einen bestimmten Bereich fällt. Demgemäß kann das visuelle Spektrum in Bereiche eingeteilt werden, die die Information aus diesem Bereich an das Zentralnervensystem weiterleiten. Die Unterscheidung der Wellenlängen in dem theoretischen Modell erreicht man durch ein Teilmodell der Retina, das differentiell beeinflußt wird. Ein Analogrechner wurde bei dieser Voruntersuchung verwandt. Es wurde besonders darauf geachtet, daß das Modell mit dem heutigen Stand des Wissens der Anatomie und Physiologie übereinstimmt. Die Ergebnisse ähneln den Verläufen von Granits Spektralempfindlichkeit und Kellys Amplitudenempfindlichkeit. Das Modell, das Subjektive Farben-Erscheinungen bei passenden Frequenzen voraussagt, wirft Fragen auf, die psychophysiologischen Methoden zugänglich sind.