Understanding how neural responses contribute to the diversity of avian colour vision

Long-wavelength-sensitive (lws) opsin gene expression, foraging and visual communication in coral reef fishes

Coral reef fishes are diverse in ecology and behaviour and show remarkable colour variability. Investigating the visual pigment gene (opsin) expression in these fishes makes it possible to associate their visual genotype and phenotype (spectral sensitivities) to visual tasks, such as feeding strategy or conspecific detection. By studying all major damselfish clades (Pomacentridae) and representatives from five other coral reef fish families, we show that the long-wavelength-sensitive (lws) opsin is highly expressed in algivorous and less or not expressed in zooplanktivorous species. Lws is also upregulated in species with orange/red colours (reflectance >520 nm) and expression is highest in orange/red-coloured algivores. Visual models from the perspective of a typical damselfish indicate that sensitivity to longer wavelengths does enhance the ability to detect the red to far-red component of algae and orange/red-coloured conspecifics, possibly enabling social signalling. Character state reconstructions indicate that in the early evolutionary history of damselfishes, there was no lws expression and no orange/red coloration. Omnivory was most often the dominant state. Although herbivory was sometimes dominant, zooplanktivory was never dominant. Sensitivity to long wavelength (increased lws expression) only emerged in association with algivory but never with zooplanktivory. Higher lws expression is also exploited by social signalling in orange/red, which emerged after the transition to algivory. Although the relative timing of traits may deviate by different reconstructions and alternative explanations are possible, our results are consistent with sensory bias whereby social signals evolve as a correlated response to natural selection on sensory system properties in other contexts.

The Perceptual and Cognitive Processes That Govern Egg Rejection in Hosts of Avian Brood Parasites

Hosts of avian brood parasites are under intense selective pressure to prevent or reduce the cost of parasitism. Many have evolved refined egg discrimination abilities, which can select for eggshell mimicry in their parasite. A classic assumption underlying these coevolutionary dynamics is that host egg recognition depends on the perceivable difference between their own eggs and those of their parasite. Over the past two decades, the receptor noise-limited (RNL) model has contributed to our understanding of these coevolutionary interactions by providing researchers a method to predict a host’s ability to discriminate a parasite’s egg from its own. Recent research has shown that some hosts are more likely to reject brown eggs than blue eggs, regardless of the perceived differences to their own. Such responses suggest that host egg recognition may be due to perceptual or cognitive processes not currently predictable by the RNL model. In this perspective, we discuss the potential value of using the RNL model as a null model to explore alternative perceptual processes and higher-order cognitive processes that could explain how and why some hosts make seemingly counter-intuitive decisions. Further, we outline experiments that should be fruitful for determining the perceptual and cognitive processing used by hosts for egg recognition tasks.

Major Flower Pigments Originate Different Colour Signals to Pollinators

Flower colour is mainly due to the presence and type of pigments. Pollinator preferences impose selection on flower colour that ultimately acts on flower pigments. Knowing how pollinators perceive flowers with different pigments becomes crucial for a comprehensive understanding of plant-pollinator communication and flower colour evolution. Based on colour space models, we studied whether main groups of pollinators, specifically hymenopterans, dipterans, lepidopterans and birds, differentially perceive flower colours generated by major pigment groups. We obtain reflectance data and conspicuousness to pollinators of flowers containing one of the pigment groups more frequent in flowers: chlorophylls, carotenoids and flavonoids. Flavonoids were subsequently classified in UV-absorbing flavonoids, aurones-chalcones and the anthocyanins cyanidin, pelargonidin, delphinidin, and malvidin derivatives. We found that flower colour loci of chlorophylls, carotenoids, UV-absorbing flavonoids, aurones-chalcones, and anthocyanins occupied different regions of the colour space models of these pollinators. The four groups of anthocyanins produced a unique cluster of colour loci. Interestingly, differences in colour conspicuousness among the pigment groups were almost similar in the bee, fly, butterfly, and bird visual space models. Aurones-chalcones showed the highest chromatic contrast values, carotenoids displayed intermediate values, and chlorophylls, UV-absorbing flavonoids and anthocyanins presented the lowest values. In the visual model of bees, flowers with UV-absorbing flavonoids (i.e., white flowers) generated the highest achromatic contrasts. Ours findings suggest that in spite of the almost omnipresence of floral anthocyanins in angiosperms, carotenoids and aurones-chalcones generates higher colour conspicuousness for main functional groups of pollinators.

Color discrimination properties arising from optimal decoding in the early stages of visual systems

In order to interpret animal behavior we need to understand how they see the world. As directly testing color discrimination in animals is difficult and time consuming, it is important to develop theoretical models based in the properties of visual systems. One of the most successful for the prediction of color discrimination behavior is the receptor noise-limited (RNL) model, which depends only on the level of noise in photoreceptors and opponent mechanisms. Here a complementary approach to model construction is used, and optimal color discrimination properties are obtained using information theoretical tools, for the early stages of visual systems. It is shown here that, for most biologically relevant conditions the optimal discrimination function of an ideal observer coincides with the one obtained with the RNL model. Furthermore, within this framework the influence of opponency can be studied by considering models with and without that mechanism but with exactly the same parameters at the level of photoreceptors. As an example, it is shown here that opponency is necessary to explain the discrimination of monochromatic stimuli in honeybees, but not in budgerigars. Since this is a consequence of the narrowing of absorption spectra of photoreceptors, produced by the presence of oil droplets, this could also be true for most other species of birds. This suggests that in order to study opponency in birds, stimuli should have a relatively wide spectrum.

Comparison of Categorical Color Perception in Two Estrildid Finches

AbstractSensory systems are predicted to be adapted to the perception of important stimuli, such as signals used in communication. Prior work has shown that female zebra finches perceive the carotenoid-based orange-red coloration of male beaks-a mate choice signal-categorically. Specifically, females exhibited an increased ability to discriminate between colors from opposite sides of a perceptual category boundary than equally different colors from the same side of the boundary. The Bengalese finch, an estrildid finch related to the zebra finch, is black, brown, and white, lacking carotenoid coloration. To explore the relationship between categorical color perception and signal use, we tested Bengalese finches using the same orange-red continuum as in zebra finches, and we also tested how both species discriminated among colors differing systematically in hue and brightness. Unlike in zebra finches, we found no evidence of categorical perception of an orange-red continuum in Bengalese finches. Instead, we found that the combination of chromatic distance (hue difference) and Michelson contrast (difference in brightness) strongly correlated with color discrimination ability on all tested color pairs in Bengalese finches. The pattern was different in zebra finches: this strong correlation held when discriminating between colors from different categories but not when discriminating between colors from within the same category. These experiments suggest that categorical perception is not a universal feature of avian-or even estrildid finch-vision. Our findings also provide further insights into the mechanism underlying categorical perception and are consistent with the hypothesis that categorical perception is adapted for signal perception.

Wild hummingbirds discriminate nonspectral colors

Many animals have the potential to discriminate nonspectral colors. For humans, purple is the clearest example of a nonspectral color. It is perceived when two color cone types in the retina (blue and red) with nonadjacent spectral sensitivity curves are predominantly stimulated. Purple is considered nonspectral because no monochromatic light (such as from a rainbow) can evoke this simultaneous stimulation. Except in primates and bees, few behavioral experiments have directly examined nonspectral color discrimination, and little is known about nonspectral color perception in animals with more than three types of color photoreceptors. Birds have four color cone types (compared to three in humans) and might perceive additional nonspectral colors such as UV+red and UV+green. Can birds discriminate nonspectral colors, and are these colors behaviorally and ecologically relevant? Here, using comprehensive behavioral experiments, we show that wild hummingbirds can discriminate a variety of nonspectral colors. We also show that hummingbirds, relative to humans, likely perceive a greater proportion of natural colors as nonspectral. Our analysis of plumage and plant spectra reveals many colors that would be perceived as nonspectral by birds but not by humans: Birds' extra cone type allows them not just to see UV light but also to discriminate additional nonspectral colors. Our results support the idea that birds can distinguish colors throughout tetrachromatic color space and indicate that nonspectral color perception is vital for signaling and foraging. Since tetrachromacy appears to have evolved early in vertebrates, this capacity for rich nonspectral color perception is likely widespread.