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Abstract Carotenoid pigments are the basis for much red, orange, and yellow coloration in nature and central to visual signaling. However, as pigment concentration increases, carotenoid signals not only darken and become more saturated but they also redshift; for example, orange pigments can look red at higher concentration. This occurs because light experiences exponential attenuation, and carotenoid‐based signals have spectrally asymmetric reflectance in the visible range. Adding pigment disproportionately affects the high‐absorbance regions of the reflectance spectra, which redshifts the perceived hue. This carotenoid redshift is substantial and perceivable by animal observers. In addition, beyond pigment concentration, anything that increases the path length of light through pigment causes this redshift (including optical nano‐ and microstructures). For example, maleRamphocelustanagers appear redder than females, despite the same population and concentration of carotenoids, due to microstructures that enhance light–pigment interaction. This mechanism of carotenoid redshift has sensory and evolutionary consequences for honest signaling in that structures that redshift carotenoid ornaments may decrease signal honesty. More generally, nearly all colorful signals vary in hue, saturation, and brightness as light–pigment interactions change, due to spectrally asymmetrical reflectance within the visible range of the relevant species. Therefore, the three attributes of color need to be considered together in studies of honest visual signaling. 

Glass frogs actively maintain transparency by removing red blood cells from the circulation and concealing them in their livers. 

Abstract Predators can strongly influence disease transmission and evolution, particularly when they prey selectively on infected hosts. Although selective predation has been observed in numerous systems, why predators select infected prey remains poorly understood. Here, we use a mathematical model of predator vision to test a long‐standing hypothesis about the mechanistic basis of selective predation in aDaphnia–microparasite system, which serves as a model for the ecology and evolution of infectious diseases. Bluegill sunfish feed selectively onDaphniainfected by a variety of parasites, particularly in water uncolored by dissolved organic carbon. The leading hypothesis for selective predation in this system is that infection‐induced changes in the transparency ofDaphniarender them more visible to bluegill. Rigorously evaluating this hypothesis requires that we quantify the effect of infection on the visibility of prey from the predator's perspective, rather than our own. Using a model of the bluegill visual system, we show that three common parasites,Metschnikowia bicuspidata,Pasteuria ramosa, andSpirobacillus cienkowskii, decrease the transparency ofDaphnia, rendering infectedDaphniadarker against a background of bright downwelling light. As a result of this increased brightness contrast, bluegill can see infectedDaphniaat greater distances than uninfectedDaphnia—between 19% and 33% further, depending on the parasite.PasteuriaandSpirobacillusalso increase the chromatic contrast ofDaphnia. These findings lend support to the hypothesis that selective predation by fish on infectedDaphniacould result from the effects of infection onDaphnia's visibility. However, contrary to expectations, the visibility ofDaphniawas not strongly impacted by water color in our model. Our work demonstrates that models of animal visual systems can be useful in understanding ecological interactions that impact disease transmission.