The weaker sex: Male lingcod (Ophiodon elongatus) with blue color polymorphism are more burdened by parasites than are other sex-color combinations

The unusual blue color polymorphism of lingcod (Ophiodon elongatus) is the subject of much speculation but little empirical research; ~20% of lingcod individuals exhibit this striking blue color morph, which is discrete from and found within the same populations as the more common brown morph. In other species, color polymorphisms are intimately linked with host–parasite interactions, which led us to ask whether blue coloration in lingcod might be associated with parasitism, either as cause or effect. To test how color and parasitism are related in this host species, we performed parasitological dissection of 89 lingcod individuals collected across more than 26 degrees of latitude from Alaska, Washington, and California, USA. We found that male lingcod carried 1.89 times more parasites if they were blue than if they were brown, whereas there was no difference in parasite burden between blue and brown female lingcod. Blue individuals of both sexes had lower hepatosomatic index (i.e., relative liver weight) values than did brown individuals, indicating that blueness is associated with poor body condition. The immune systems of male vertebrates are typically less effective than those of females, due to the immunocompromising properties of male sex hormones; this might explain why blueness is associated with elevated parasite burdens in males but not in females. What remains to be determined is whether parasites induce physiological damage that produces blueness or if both blue coloration and parasite burden are driven by some unmeasured variable, such as starvation. Although our study cannot discriminate between these possibilities, our data suggest that the immune system could be involved in the blue color polymorphism–an exciting jumping-off point for future research to definitively identify the cause of lingcod blueness and a hint that immunocompetence and parasitism may play a role in lingcod population dynamics.

Antibiotic treatment increases yellowness of carotenoid feather coloration in male greenfinches (Chloris chloris)

Carotenoid plumage coloration is an important sexually selected trait in many bird species. However, the mechanisms ensuring the honesty of signals based on carotenoid pigments remain unclear. It has recently been suggested that intestinal integrity, which is affected by gut parasites and microbiota and influences nutrient absorption and acquisition, mediates the relationship between carotenoid ornamentation and individual quality. Here, we test whether carotenoid plumage coloration in greenfinches (Chloris chloris) is affected by the treatment of an antibiotic or an antiparasitic drug. We captured wild greenfinches (N = 71) and administered anticoccidial medication toltrazuril (TOLTRA) to one group, antibiotic metronidazole (METRO) to the second group to target trichomonosis, and the third group received no medication. In the METRO group, feathers grown during the experiment had significantly higher chroma of yellow parts, but there was no effect of TOLTRA on feather chroma. The results suggest that METRO increased the efficiency of carotenoid modification or deposition to the feathers rather than nutrient acquisition and/or freed energy resources that could be invested in coloration. Alternatively, though not measured, METRO might have affected microbial community and host physiology as microbial metabolites can modulate mitochondrial and immune function.

Behavioral, energetic, and color trait integration in male guppies: testing the melanocortin hypothesis

Individuals of the same population differ consistently from each other in the average expression of behavioral and physiological traits. Often, such traits are integrated and thus correlated with each other. However, the underlying proximate mechanisms generating and maintaining this among-individual covariation are still poorly understood. The melanocortin hypothesis suggests that the melanocortin pathways can have pleiotropic effects linking the expression of melanin-based coloration with physiological and behavioral traits. In the present study, we test this hypothesis in adult male guppies (Poecilia reticulata), by estimating among individual correlations between behaviors (activity, feeding, boldness, display, and chase during courtship), stress response (peak metabolic rate), and coloration (black spot, fuzzy black, and orange). The lack of correlation of any behavior or metabolism with black coloration indicates that the melanocortin hypothesis is not supported in this species. However, we observed covariation among coloration traits, as well as among behavioral traits. Our findings suggest that, although there appear to be constraints within sets of related traits, coloration, physiology, and behaviors can potentially evolve as independent modules in response to selection in this species.

Melanin-based coloration and host-parasite interactions under global change

The role of parasites in shaping melanin-based colour polymorphism, and the consequences of colour polymorphism for disease resistance, remain debated. Here we review recent evidence of the links between melanin-based coloration and the behavioural and immunological defences of vertebrates against their parasites. First we propose that (1) differences between colour morphs can result in variable exposure to parasites, either directly (certain colours might be more or less attractive to parasites) or indirectly (variations in behaviour and encounter probability). Once infected, we propose that (2) immune variation between differently coloured individuals might result in different abilities to cope with parasite infection. We then discuss (3) how these different abilities could translate into variable sexual and natural selection in environments varying in parasite pressure. Finally, we address (4) the potential role of parasites in the maintenance of melanin-based colour polymorphism, especially in the context of global change and multiple stressors in human-altered environments. Because global change will probably affect both coloration and the spread of parasitic diseases in the decades to come, future studies should take into account melanin-based coloration to better predict the evolutionary responses of animals to changing disease risk in human-altered environments.

Prevalence and genotyping of Trichomonas infections in wild birds in central Germany

Avian trichomonosis is a widespread disease in columbids and other birds, caused by ingestion of the unicellular flagellate Trichomonas gallinae which proliferate primarily in the upper respiratory tracts. Studies using genetic analyses have determined some highly pathogenic lineages in birds, but the prevalence and distribution of potentially pathogenic and non-pathogenic T. gallinae lineages in wild birds is still not well known. We examined 440 oral swab samples of 35 bird species collected between 2015 and 2017 in Hesse, central Germany, for Trichomonas spp. infection and for determining the genetic lineages. Of these birds, 152 individuals were caught in the wild and 288 individuals were admitted from the wild to a veterinary clinic. The overall Trichomonas spp. prevalence was 35.6%. We observed significant differences between bird orders, with the highest prevalence in owls (58%) and columbids (50%), while other orders had slightly lower prevalences, with 36% in Accipitriformes, 28% in Falconiformes and 28% in Passeriformes. Among 71 successfully sequenced samples, we found 13 different haplotypes, including two previously described common lineages A/B (20 samples) and C/V/N (36 samples). The lineage A/B has been described as pathogenic, causing lesions and mortality in columbids, raptors and finches. This lineage was found in 11 of the 35 species, including columbids (feral pigeon, woodpigeon, stock dove), passerines (greenfinch, chaffinch, blackbird) and raptors (common kestrel, sparrowhawk, red kite, peregrine falcon and common buzzard). One new lineage (R) was found in a sample of a chaffinch. In conclusion, we found that the prevalence of Trichomonas spp. infection in wild birds was high overall, and the potentially pathogenic lineage A/B was widespread. Our findings are worrying, as epidemic outbreaks of trichomonosis have already been observed in Germany in several years and can have severe negative effects on bird populations. This disease may add to the multiple pressures that birds face in areas under high land-use intensity.

Toward Understanding the Repeated Occurrence of Associations between Melanin-Based Coloration and Multiple Phenotypes

Melanin is the most widespread pigment in organisms. Melanin-based coloration has been repeatedly observed to be associated with the same traits and in the same direction in different vertebrate and insect species. However, whether any factors that are common to different taxa account for the repeated evolution of melanin-phenotype associations remains unclear. We propose to approach this question from the perspective of convergent and parallel evolution to clarify to what extent different species have evolved the same associations owing to a shared genetic basis and being subjected to similar selective pressures. Our current understanding of the genetic basis of melanin-phenotype associations allows for both convergent and parallel evolution, but this understanding is still limited. Further research is needed to clarify the generality and interdependencies of the different proposed mechanisms (supergenes, pleiotropy based on hormones, or neural crest cells). The general ecological scenarios whereby melanin-based coloration is under selection-protection from ultraviolet radiation, thermoregulation in cold environments, or as a signal of social status-offer a good opportunity to study how melanin-phenotype associations evolve. Reviewing these scenarios shows that some traits associated with melanin-based coloration might be selected together with coloration by also favoring adaptation but that other associated traits might impede adaptation, which may be indicative of genetic constraints. We therefore encourage further research on the relative roles that selection and genetic constraints play in shaping multiple melanin-phenotype associations. Placed into a phylogenetic context, this will help clarify to what extent these associations result from convergent or parallel evolutionary processes and why melanin-phenotype associations are so common across the tree of life.

Playing by the rules? Phenotypic adaptation to temperate environments in an American marsupial

Phenotypic variation along environmental gradients can provide evidence suggesting local adaptation has shaped observed morphological disparities. These differences, in traits such as body and extremity size, as well as skin and coat pigmentation, may affect the overall fitness of individuals in their environments. The Virginia opossum (Didelphis virginiana) is a marsupial that shows phenotypic variation across its range, one that has recently expanded into temperate environments. It is unknown, however, whether the variation observed in the species fits adaptive ecogeographic patterns, or if phenotypic change is associated with any environmental factors. Using phenotypic measurements of over 300 museum specimens of Virginia opossum, collected throughout its distribution range, we applied regression analysis to determine if phenotypes change along a latitudinal gradient. Then, using predictors from remote-sensing databases and a random forest algorithm, we tested environmental models to find the most important variables driving the phenotypic variation. We found that despite the recent expansion into temperate environments, the phenotypic variation in the Virginia opossum follows a latitudinal gradient fitting three adaptive ecogeographic patterns codified under Bergmann's, Allen's and Gloger's rules. Temperature seasonality was an important predictor of body size variation, with larger opossums occurring at high latitudes with more seasonal environments. Annual mean temperature predicted important variation in extremity size, with smaller extremities found in northern populations. Finally, we found that precipitation and temperature seasonality as well as low temperatures were strong environmental predictors of skin and coat pigmentation variation; darker opossums are distributed at low latitudes in warmer environments with higher precipitation seasonality. These results indicate that the adaptive mechanisms underlying the variation in body size, extremity size and pigmentation are related to the resource seasonality, heat conservation, and pathogen-resistance hypotheses, respectively. Our findings suggest that marsupials may be highly susceptible to environmental changes, and in the case of the Virginia opossum, the drastic phenotypic evolution in northern populations may have arisen rapidly, facilitating the colonization of seasonal and colder habitats of temperate North America.

Simple and noninvasive method for assessment of digestive efficiency: Validation of fecal steatocrit in greenfinch coccidiosis model

Animals’ capability to absorb energy and nutrients from food poses a major internal constraint that affects the amount of resources available for allocation to maintenance, growth, signaling, and reproduction. Intestinal surface is the largest area of contact between immune system and microbial antigens; gut thus appears the main arena where trade‐offs between immune function and other components of fitness arise. Assessment of the integrity of digestive machinery should therefore be of high priority in ecophysiological research. Traditional methods of digestive physiology, however, appear unsuitable for most ecological applications due to lethality or complexity of the procedure.

Here, we test the reliability of a simple, cheap, and noninvasive procedure, an acid steatocrit that assesses fat content in feces. It is based on centrifugation of a fecal sample, diluted in acid medium, in hematocrit capillary tube and quantifying the percentage of fat in fecal matter. The method has been previously validated in humans and mice; here, we apply it for the first time in birds.

When applied to captive wild‐caught greenfinches, the method showed reasonable internal consistency (r_s = 0.71 for steatocrit values, sampled from the same fecal aliquot in duplicate but processed separately). Individual steatocrit values were significantly repeatable in time in different intervals from eight to at least 20 days (r_s = 0.32–0.49). The relationship between intestinal health and steatocrit values was tested by experimental manipulations. Medication against coccidiosis (a naturally pervasive intestinal infection) reduced, and experimental infection with heterologous coccidian strains increased steatocrit. Individual changes in steatocrit correlated negatively with changes of two markers of nutritional state—plasma triglyceride levels and body mass.

Findings of this study suggest that steatocrit has a wide application potential as a marker of intestinal health in ecophysiological research. In particular, we see the perspective of this method for increasingly popular immunoecological research, conservation medicine, and studies of animal coloration.

A novel role for pigment genes in the stress response in rainbow trout (Oncorhynchus mykiss)

In many vertebrate species visible melanin-based pigmentation patterns correlate with high stress- and disease-resistance, but proximate mechanisms for this trait association remain enigmatic. Here we show that a missense mutation in a classical pigmentation gene, melanocyte stimulating hormone receptor (MC1R), is strongly associated with distinct differences in steroidogenic melanocortin 2 receptor (MC2R) mRNA expression between high- (HR) and low-responsive (LR) rainbow trout (Oncorhynchus mykiss). We also show experimentally that cortisol implants increase the expression of agouti signaling protein (ASIP) mRNA in skin, likely explaining the association between HR-traits and reduced skin melanin patterning. Molecular dynamics simulations predict that melanocortin 2 receptor accessory protein (MRAP), needed for MC2R function, binds differently to the two MC1R variants. Considering that mRNA for MC2R and the MC1R variants are present in head kidney cells, we hypothesized that MC2R activity is modulated in part by different binding affinities of the MC1R variants for MRAP. Experiments in mammalian cells confirmed that trout MRAP interacts with the two trout MC1R variants and MC2R, but failed to detect regulation of MC2R signaling, possibly due to high constitutive MC1R activity.

A dark cuticle allows higher investment in immunity, longevity and fecundity in a beetle upon a simulated parasite attack

Cuticle melanism in insects is linked to a number of life history traits: a positive relationship is hypothesized between melanism, immune function, fecundity and lifespan. However, it is not clear how activation of the immune system affects trade-offs between life history traits in female mealworm beetles (Tenebrio molitor) differing in cuticle melanization. The females with tan, brown and black cuticles examined in the present study did not differ in the intensity of encapsulation response, fecundity and longevity when their immune system was not activated. However, we found that immune activation and cuticle melanization have a significant effect on life history traits. Offspring number and lifespan decreased in females with tan and brown cuticles, while the fecundity and lifespan of black females were not affected. Importantly, we inserted the implants again and found a significant decrease in the strength of encapsulation response in females with tan and brown cuticles. In contrast, black females increased melanotic reactions against the nylon implant, suggesting immunological priming. The results show that cuticle melanization plays an important adaptive role under the risk of being infected, while the lack of these benefits before the insertion of nylon monofilaments suggests that there are costs associated with an activated immunity system.

Viability selection affects black but not yellow plumage colour in greenfinches

Much of the debate surrounding the selective forces responsible for the expression of conspicuous plumage colouration is centred on the question of precisely which individual qualities are signalled by carotenoid- and melanin-based pigments. To examine this and other related issues, we performed viability selection analyses in wild-caught captive male greenfinches (Carduelis chloris) in Estonia during winters between 2003 and 2014. Based on our measurements, birds with a darker black eumelanin-based colouration of tail feathers survived better than those whose tail feathers had a paler black colouration. The carotenoid-based yellow colouration of the same feathers was not associated with mortality in captivity and showed much less between-year variation in the field than the black colouration. Between year-variation in the black (but not yellow) colouration of feathers was parallel in wild-grown feathers (on birds in the wild) and laboratory-grown ones (on birds held temporarily in captivity). Taken together, these findings imply that eumelanotic colouration in greenfinches is currently under selection and suggest the presence of sufficient genetic variation for a rapid response to selection. In particular, tail feathers have become darker black since the emergence of avian trichomonosis, which is known to selectively kill paler individuals.