Beyond morphs: Inter-individual colour variation despite strong genetic determinism of colour morphs in a wild bird

Categorizing individuals into discrete forms in colour polymorphic species can overlook more subtle patterns in coloration that can be of functional significance. Thus, quantifying inter-individual variation in these species at both within- and between-morph levels is critical to understand the evolution of colour polymorphisms. Here we present analyses of inter-individual colour variation in the Reunion grey white-eye (Zosterops borbonicus), a colour polymorphic wild bird endemic to the island of Reunion in which all highland populations contain two sympatric colour morphs, with birds showing predominantly grey or brown plumage, respectively. We first quantified colour variation across multiple body areas by using a continuous plumage colour score to assess variation in brown-grey coloration as well as smaller scale variation in light patches. To examine the possible causes of among-individual variation, we tested if colour variation in plumage component elements could be explained by genotypes at two markers near a major-effect locus previously related to back coloration in this species, and by other factors such as age, sex and body condition. Overall, grey-brown coloration was largely determined by genetic factors and was best described by three distinct clusters that were associated to genotypic classes (homozygotes and heterozygote), with no effect of age or sex, whereas variation in smaller light patches was primarily related to age and sex. Our results highlight the importance of characterizing subtle plumage variation beyond morph categories that are readily observable since multiple patterns of colour variation may be driven by different mechanisms, have different functions and will likely respond in different ways to selection.

Electrocommunication signals and aggressive behavior vary among male morphs in an apteronotid fish, Compsaraia samueli

Within-species variation in male morphology is common among vertebrates and is often characterized by dramatic differences in behavior and hormonal profiles. Males with divergent morphs also often use communication signals in a status-dependent way. Weakly electric knifefish are an excellent system for studying variation in male morphology and communication and its hormonal control. Knifefish transiently modulate the frequency of their electric organ discharge (EOD) during social encounters to produce chirps and rises. In the knifefish Compsaraia samueli, males vary extensively in jaw length. EODs and their modulations (chirps and rises) have never been investigated in this species, so it is unclear whether jaw length is related to the function of these signals. We used three behavioral assays to analyze EOD modulations in male C. samueli: (1) artificial playbacks, (2) relatively brief, live agonistic dyadic encounters, and (3) long-term overnight recordings. We also measured circulating levels of two androgens, 11-ketotestosterone and testosterone. Chirp structure varied within and across individuals in response to artificial playback, but was unrelated to jaw length. Males with longer jaws were more often dominant in dyadic interactions. Chirps and rises were correlated with and preceded attacks regardless of status, suggesting these signals function in aggression. In longer-term interactions, chirp rate declined after one week of pairing, but was unrelated to male morphology. Levels of circulating androgens were low and not predictive of jaw length or EOD signal parameters. These results suggest that communication signals and variation in male morphology are linked to outcomes of non-breeding agonistic contests.

Epistatic interactions between pterin and carotenoid genes modulate intra-morph color variation in a lizard

Color polymorphisms have become a major topic in evolutionary biology and substantial efforts have been devoted to the understanding of the mechanisms responsible for originating such colorful systems. Within-morph continuous variation, on the other hand, has been neglected in most of the studies. Here, we combine spectrophotometric/visual modeling and genetic data to study the mechanisms promoting continuous variation within categorical color morphs of Podarcis muralis. Our results suggest that intra-morph variability in the pterin-based orange morph is greater compared to white and yellow morphs. We also show that continuous variation within the orange morph is partially discriminable by conspecifics. Genotyping results indicate that allelic variants at the BCO2 locus (responsible for deposition of yellow carotenoids) contribute to generate continuous variation in orange individuals. However, other intrinsic and/or extrinsic mechanisms, such as body size, might be involved, opening a new avenue for future research on the drivers of continuous variation within-morphs.

Female preferences for discrete and continuous male colour expression may help reinforce colour polymorphism in a desert lizard

Despite recognition that colour can vary continuously, colour expression in colour polymorphic species is usually treated as discrete. We conducted three experiments to evaluate the extent that discrete and continuous male coloration influenced female mating preferences in long-tailed brush lizards (Urosaurus graciosus). Each experiment provided females with a different social context: a dimorphic choice between a yellow and an orange male (coloration treated as discrete), and a choice between either two orange males or two yellow males (coloration treated as continuous variation). Females preferred orange males over yellow males in the first experiment, and the findings of our second experiment suggested that males with moderate orange coloration were most preferred. In contrast, females behaved randomly with respect to two yellow males. Our findings show that females in colour polymorphic species can evaluate both discrete and continuous aspects of morph coloration during mate assessment, which may help maintain their polymorphism.

Do ectoparasites affect survival of three species of lizards of the genus Sceloporus?

The short-term effects that ectoparasites cause to their hosts, such as local wounds and secondary infections that occur within a few hours or days after infection, are well documented in a wide variety of taxa, whereas long-term negative effects on the fitness of hosts, which result from chronic infections and are evident after several months, are less understood. Lizards are hosts of distinct species of mites and ticks that cause short-term negative effects such as ulcers, sores and local inflammation. However, the negative effects that these ectoparasites may have on the long-term survival of lizards have not been evaluated. In this study, we collected two years of capture-mark-recapture data and implemented a multi-model inference framework to examine if high ectoparasite loads have negative effects on the long-term survival probability of three lizard species of the genus Sceloporus (S. grammicus, S. megalepidurus, and S. torquatus). In addition, we considered that the potential negative effect of ectoparasites on survival may vary depending on sex, body condition, reproductive season, or climatic season. Contrary to our expectations, our results did not support the hypothesis that high ectoparasite loads reduce the survival probability of these lizards. In S. grammicus and S. megalepidurus we found no evidence of an effect of ectoparasite load on host survival. In S. torquatus ectoparasites influenced survival probability, but the effect was opposite to what we predicted: survival increased substantially as ectoparasite load increased. This unexpected result might be explained by mites discriminating between hosts and attaching more frequently to lizards in better health status, or by high-quality lizards having greater chances of contracting ectoparasites, because these individuals move around large areas and frequently engage in social interactions.

Discrete or indiscrete? Redefining the colour polymorphism of the land snail Cepaea nemoralis

Biologists have long tried to describe and name the different phenotypes that make up the shell polymorphism of the land snail Cepaea nemoralis. Traditionally, the view is that the ground colour of the shell is one of a few major colour classes, either yellow, pink or brown, but in practise it is frequently difficult to distinguish the colours, and define different shades of the same colour. To understand whether colour variation is in reality continuous, and to investigate how the variation may be perceived by an avian predator, we applied psychophysical models of colour vision to shell reflectance measures. We found that both achromatic and chromatic variation are indiscrete in Cepaea nemoralis, being continuously distributed over many perceptual units. Nonetheless, clustering analysis based on the density of the distribution did reveal three groups, roughly corresponding to human-perceived yellow, pink and brown shells. We also found large-scale geographic variation in the frequency of these groups across Europe, and some covariance between shell colour and banding patterns. Although further studies are necessary, the observation of continuous variation in colour is intriguing because the traditional theory is that the underlying supergene that determines colour has evolved to prevent phenotypes from “dissolving” into continuous trait distributions. The findings thus have significance for understanding the Cepaea polymorphism, and the nature of the selection that acts upon it, as well as more generally highlighting the need to measure colour objectively in other systems.

Through the eye of a lizard: hue discrimination in a lizard with ventral polymorphic coloration

Colour polymorphisms are thought to be maintained by complex evolutionary processes some of which require that the colours of the alternative morphs function as chromatic signals to conspecifics. Unfortunately, a key aspect of this hypothesis has rarely been studied: whether the study species perceives its own colour variation as discrete rather than continuous. The European common wall lizard (Podarcis muralis) presents a striking colour polymorphism: the ventral surface of adults of both sexes may be coloured orange, white, yellow, or with a mosaic of scales combining two colours (orange-white, orange-yellow). Here we use a discrimination learning paradigm to test if P. muralis is capable of discriminating colour stimuli designed to match the ventral colours of conspecifics. We trained 20 lizards to eat from colour-coded wells bored in wooden blocks. Blocks had four colour-coded wells (orange, white, yellow, and an achromatic control), but only one contained food (mealworm larvae). After six trials, the lizards performed significantly better than expected by chance, showing a decrease in both the number of wells explored and the latency to finding the food. Using visual modelling techniques we found that, based on their spectral properties and the lizards’ cone sensitivities, the ventral colours of P. muralis correspond to discrete rather than continuous colour categories, and that colour discriminability (i.e. distance in perceptual space) varies depending on the morphs compared, which may have implications for signal detection and discrimination. These results suggest that P. muralis can discriminate hue differences matching their own ventral colour variation.