Evolutionary trade‐offs may interact with physiological constraints to maintain color variation

Abstract

Animal coloration is a multifaceted trait with many ecological roles and related to a variety of developmental and physiological processes. Consequently, coloration is often subject to a variety of selective pressures, leading to the evolutionary maintenance of variation. In this study, we investigated hypotheses related to the maintenance of dorsal color variation in wood frogs (Rana sylvatica). First, we tested for multimodality, and whether color correlates with body size or condition or varies by sex or age-class. We combined behavioral trials with visual modeling to test for sex recognition. We also considered visual models for predators and tested for an interaction between discriminability indexes (JND) of color channel (chromatic vs. achromatic) and predator type (birds vs. snakes), as well as for a within individual trade-off between the JND of chromatic and achromatic coloration. Finally, we tested for disruptive viability selection on color using predation trials, and for antagonistic directional selection between viability selection and reproductive investment of females. We found that wood frogs present continuous color variation that does not correlate with body size or condition, but that changes with age. Wood frogs present subtle sexual dichromatism, but we found no evidence for a role of color in sex recognition. Instead, we discuss the possibility that sex differences might, at least in part, have a demographic explanation. Predator visual models indicated that wood frogs cannot solely rely on dorsal coloration for camouflage. Moreover, different predators might present selective pressures in different color channels, while individuals' achromatic and chromatic coloration trade-off in JND. Therefore, different selective pressures caused by different predators might interact with ontogenetic changes and developmental/physiological trade-offs to maintain color variation. We found no relationship between color and survival or reproductive investment, suggesting further work is required to fully understand selection on color. Our results highlight the importance of understanding evolutionary trade-offs and developmental/physiological constraints in combination with one another, and suggest the potential for an interaction between these proximate and ultimate mechanisms in the evolutionary maintenance of variation. These results likely extend beyond color expression in amphibians, and exemplify a more general process for such evolutionary outcomes.

Color morphs of the fire salamander are discriminated at night by conspecifics and predators

“…We modelled the vision of M. putorius setting cone maximum absorbances at 430 (SWS) and 558 nm (LWS) in a proportion of 1:14 (Calderone & Jacobs, 2003). To model the vision of Natrix sp., we considered a closely related Natricine species ( Thamnophis sirtalis ), which has maximum absorbances at 360, 482 and 554 nm (Sillman et al., 1997) and cone proportions of 1:1.6:7.3 (UVWS:MWS:LWS) (Goedert et al., 2021). To account for nocturnal light conditions in the visual models, we used starlight and full moonlight irradiance data for all models (Johnsen et al., 2006).…”

A wolf in sheep's clothing: Predatory fish have convergent consumptive effects but divergent predation‐risk effects

“…Larvae were weighed, photographed in standardized positions on a white balance card with scales for morphometric and color calibrations, and then returned to their respective mesocosm. Following methods of Touchon and Warkentin (2008), we measured larval body size (body length), tail size (tail length, tail depth, and tail muscle depth), and tail color, for which we utilized the hue (color), saturation (intensity), and brightness (HSV) color model (Goedert et al, 2020). ImageJ (Schneider et al, 2012) was used for morphometrics and color measurements.…”

Larval newts from turbid and transparent waterbodies exhibit different morphological and behavioural traits

“…Also, given that the colour of prey may mediate their predation risk, be it by means of crypsis (Vignieri et al, 2010) or various types of mimicry (Finkbeiner et al, 2018; Kikuchi et al, 2020), colouration might influence predator–prey interactions. Indeed, the pressure exerted by visual predators in a given context might be key in determining the variation in prey colouration (Goedert et al, 2021). In fact, optimum colouration against predators is known to be environment‐dependent (Wuthrich et al, 2022).…”