Critical review of the phytohemagglutinin assay for assessing amphibian immunity

Infectious diseases are a major driver of the global amphibian decline. In addition, many factors, including genetics, stress, pollution, and climate change can influence the response to pathogens. Therefore, it is important to be able to evaluate amphibian immunity in the laboratory and in the field. The phytohemagglutinin (PHA) assay is an inexpensive and relatively non-invasive tool that has been used extensively to assess immunocompetence, especially in birds, and more recently in amphibians. However, there is substantial variation in experimental methodology among amphibian PHA studies in terms of species and life stages, PHA doses and injection sites, and use of experimental controls. Here, we compile and compare all known PHA studies in amphibians to identify knowledge gaps and develop best practices for future work. We found that research has only been conducted on a limited number of species, which may not reflect the diversity of amphibians. There is also a lack of validation studies in most species, so that doses and timing of PHA injection and subsequent swelling measurements may not effectively evaluate immunocompetence. Based on these and other findings, we put forward a set of recommendations to make future PHA studies more consistent and improve the ability to utilize this assay in wild populations, where immune surveillance is greatly needed.

Relationships between male secondary sexual traits, physiological state and offspring viability in the three-spined stickleback

BACKGROUND

Sexual signals produced by males play a central role in sexual selection, but the relationship between these traits and the quality of the bearer are often ambiguous. Secondary sexual traits may represent genetic quality of the bearer, resulting in positive relationships with physiological state, or may be costly to produce, showing trade-off with physiological state. A number of studies have explored the relationships between secondary sexual traits and other functional traits, but few have studied their fitness consequences. We studied the link between diverse physiological traits and both morphological and behavioural sexual traits and examined how their interplay influences offspring viability in the three-spined stickleback.

RESULTS

Male sticklebacks showing nest building and courtship behaviour were smaller than those not investing in reproductive activities. There was no evidence that the expression of red nuptial colouration and the quality of courtship behaviour of males are positively related to their metabolic rates, swim ability, oxidative damage and mtDNA copy number. However, individuals showing larger red nuptial colour areas had higher levels of oxidative DNA damage in their sperm. Male courtship behaviour and aggressiveness, but not red colour area, were good predictors of offspring hatching and survival.

CONCLUSIONS

Our results suggest that, in our study population at the southern edge of the species' distribution, sexual colouration of male sticklebacks was not a good indicator of their body state, but both courtship quality and aggressiveness during the courtship are reliable cues of their gamete quality, influencing the viability of their offspring. Thus, females that choose mates based on their courtship behaviour will have high fitness. In the study population, which represents a fast pace-of-life with high reproductive rate and short lifespan, sexual ornaments of males may not honestly signal their physiological and physical state because they invest at maximum in a single reproductive season despite high costs.

Androgen Elevation Accelerates Reproductive Senescence in Three-Spined Stickleback

Costs of reproduction shape the life-history evolution of investment in current and future reproduction and thereby aging. Androgens have been proposed to regulate the physiology governing these investments. Furthermore, androgens are hypothesized to play a central role in carotenoid-dependent sexual signaling, regulating how much carotenoids are diverted to ornamentation and away from somatic maintenance, increasing oxidative stress, and accelerating aging. We investigated these relationships in male three-spined stickleback in which we elevated 11-ketotestosterone and supplied vitamin E, an antioxidant, in a 2 × 2 design. Androgen elevation shortened the time stickleback maintained reproductive activities. We suspect that this effect is caused by 11-ketotestosterone stimulating investment in current reproduction, but we detected no evidence for this in our measurements of reproductive effort: nest building, body composition, and breeding coloration. Carotenoid-dependent coloration was even slightly decreased by 11-ketotestosterone elevation and was left unaffected by vitamin E. Red coloration correlated with life expectancy and reproductive capacity in a quadratic manner, suggesting overinvestment of the individuals exhibiting the reddest bellies. In contrast, blue iris color showed a negative relationship with survival, suggesting physiological costs of producing this aspect of nuptial coloration. In conclusion, our results support the hypothesis that androgens regulate investment in current versus future reproduction, yet the precise mechanisms remain elusive. The quadratic relationships between sexual signal expression and aspects of quality have wider consequences for how we view sexual selection on ornamentation and its relationship with aging.

The 150th anniversary of The Descent of Man: Darwin and the impact of sex-role reversal on sexual selection research

The year 2021 marks the 150th anniversary of the publication of Charles Darwin’s extraordinary book The Descent of Man and Selection in Relation to Sex. Here, we review the history and impact of a single profound insight from The Descent of Man: that, in some few species, females rather than males compete for access to mates. In other words, these species are ‘sex-role reversed’ with respect to mating competition and sexual selection compared to the majority of species in which sexual selection acts most strongly on males. Over the subsequent 150 years, sex-role-reversed species have motivated multiple key conceptual breakthroughs in sexual selection. The surprising mating dynamics of such species challenged scientists’ preconceptions, forcing them to examine implicit assumptions and stereotypes. This wider worldview has led to a richer and more nuanced understanding of animal mating systems and, in particular, to a proper appreciation for the fundamental role that females play in shaping these systems. Sex-role-reversed species have considerable untapped potential and will continue to contribute to sexual selection research in the decades to come.

The dark-ventral-patch of male red deer, a sexual signal that conveys the degree of involvement in rutting behavior

Background

In polygynous mammals, signalling may play a decisive role in mating behavior, mediating the intensity of male fights and female mate choice. During the rutting season, male red deer may show a visible dark patch in their ventral fur. Recently, this patch has been suggested to act as a flexible sexual signal, due to its relationships with other variables such as age, body size, antler development, volatile compounds, or the competitive environment. The analysis of fur pigmentation at the ventral patch suggests that this might also visually indicate the male intrinsic predisposition to take part in mating competition.

Results

To assess the possible role of this trait as a communicative signal related to mate competition, we used red deer behavioral observations during the rut in Doñana National Park (Spain) to examine the link between the degree of expression of the dark ventral patch and the rutting activity (assessed from both intra-and-inter-sexual behaviors). Consistent with our predictions, we found in a field study that males with large dark patches showed a higher frequency of rutting behaviors (mainly roaring and flehmen), more interactions with females, and attained larger harem sizes.

Conclusions

The dark ventral patch was a better predictor of male behavior than antler tines or territory holding, thus standing as a short-term indicator of male willingness to invest in mating competition.

Physiological costs and age constraints of a sexual ornament: an experimental study in a wild bird

Sexual ornaments are often considered honest signals of quality because potential costs or constraints prevent their display by low-quality individuals. Testing for potential physiological costs of ornaments is difficult, as this requires experimentally forcing individuals to produce and display elaborate ornaments. We use this approach to test whether a sexually selected trait is physiologically costly to male superb fairy-wrens (Malurus cyaneus). Male fairy-wrens molt from brown to blue breeding plumage at different times of the year, and females strongly prefer the few males that are blue early, during winter. We used short-acting testosterone implants to stimulate males to produce “early-blue” plumage and assessed costs during and after molt using a panel of physiological indices. Testosterone-implanted, T-males molted in winter and produced blue plumage 6 weeks before control-implanted, C-males. T-males molted while in lower body condition, tended to have lower fat reserves, and were more likely to be parasitized by lice. However, we detected no negative effects on immune function, blood parasites, exposure to stressors, or survival. Juvenile males never naturally display early-blue plumage, but we found no evidence for increased costs paid by juvenile T-males. Instead, juvenile T-males molted later than adult T-males, suggesting that age presents an absolute constraint on ornament exaggeration that cannot be fully overcome by testosterone treatment. Together, these small costs and large, age-related constraints may enforce signal honesty, and explain female preference for early-blue males.

Testosterone regulates CYP2J19-linked carotenoid signal expression in male red-backed fairywrens (Malurus melanocephalus)

Carotenoid pigments produce most red, orange and yellow colours in vertebrates. This coloration can serve as an honest signal of quality that mediates social and mating interactions, but our understanding of the underlying mechanisms that control carotenoid signal production, including how different physiological pathways interact to shape and maintain these signals, remains incomplete. We investigated the role of testosterone in mediating gene expression associated with a red plumage sexual signal in red-backed fairywrens (Malurus melanocephalus). In this species, males within a single population can flexibly produce either red/black nuptial plumage or female-like brown plumage. Combining correlational analyses with a field-based testosterone implant experiment and quantitative polymerase chain reaction, we show that testosterone mediates expression of carotenoid-based plumage in part by regulating expression of CYP2J19, a ketolase gene associated with ketocarotenoid metabolism and pigmentation in birds. This is, to our knowledge, the first time that hormonal regulation of a specific genetic locus has been linked to carotenoid production in a natural context, revealing how endocrine mechanisms produce sexual signals that shape reproductive success.

Interactions between Environmental Contaminants and Gastrointestinal Parasites: Novel Insights from an Integrative Approach in a Marine Predator

Environmental contaminants and parasites are ubiquitous stressors that can affect animal physiology and derive from similar dietary sources (co-exposure). To unravel their interactions in wildlife, it is thus essential to quantify their concurring drivers. Here, the relationship between blood contaminant residues (11 trace elements and 17 perfluoroalkyl substances) and nonlethally quantified gastrointestinal parasite loads was tested while accounting for intrinsic (sex, age, and mass) and extrinsic factors (trophic ecology inferred from stable isotope analyses and biologging) in European shags Phalacrocorax aristotelis. Shags had high mercury (range 0.65-3.21 μg g^-1 wet weight, ww) and extremely high perfluorooctanoic acid (PFOA) and perfluorononanoic acid (PFNA) residues (3.46-53 and 4.48-44 ng g^-1 ww, respectively). Males had higher concentrations of arsenic, mercury, PFOA, and PFNA than females, while the opposite was true for selenium, perfluorododecanoic acid (PFDoA), and perfluooctane sulfonic acid (PFOS). Individual parasite loads (Contracaecum rudolphii) were higher in males than in females. Females targeted pelagic-feeding prey, while males relied on both pelagic- and benthic-feeding organisms. Parasite loads were not related to trophic ecology in either sex, suggesting no substantial dietary co-exposure with contaminants. In females, parasite loads increased strongly with decreasing selenium:mercury molar ratios. Females may be more susceptible to the interactive effects of contaminants and parasites on physiology, with potential fitness consequences.

Whither the challenge hypothesis?

Almost fifty years ago the advent of assay methods to measure circulating levels of hormones revolutionized endocrinology in relation to investigations of free-living and captive animals. This new field "environmental endocrinology" revealed that endocrine profiles in animals in their natural habitat were not only different from captive animals, but often deviated from predictions. It quickly became apparent that the organization and analysis of data from the field should be sorted by life history stages such as for reproductive processes, migration, molt etc. and spaced in time according to natural duration of those processes. Presentation of data by calendar date alone gives much simpler, even misleading, patterns. Stage-organized analyses revealed species-specific patterns of hormone secretion and dramatic inter-individual differences. The "Challenge Hypothesis" sparked exploration of these results, which diverged from expectations of hormone-behavior interactions. The hypothesis led to specific predictions about how the hypothalamo-pituitary-gonad axis, and particularly circulating patterns of testosterone, might respond to social challenges such as simulated territorial intrusions. Initially, a group of studies on free-living and captive birds played a key role in the formulation of the hypothesis. Over the decades since, the effects of social challenge and environmental context on hormonal responses have been tested in all vertebrate taxa, including humans, as well as in insects. Although it is now clear that the Challenge Hypothesis in its original form is simplistic, field and laboratory tests of the hypothesis have led to other concepts that have become seminal to the development of environmental endocrinology as a field. In this special issue these developments are addressed and examples from many different taxa enrich the emerging concepts, paving the way for investigations using recent technologies for genetic and transcriptome analyses.

Immune challenge of mating effort: steroid hormone profile, dark ventral patch and parasite burden in relation to intrasexual competition in male Iberian red deer

Testosterone secretion may regulate the reproductive effort and the development of sexual traits, but it may also involve costs at the immunological and metabolic levels. However, the evidence for this trade-off in wild populations is scarce. Cortisol also plays an important role in mediating the reproductive and immune functions. In this study, we analyzed whether the endoparasite burden relates to hormonal levels (fecal testosterone and cortisol metabolites) and/or morphological sexual traits (size of the dark ventral patch, a trait that indicates reproductive effort in males) in male Iberian red deer. For this purpose, we sampled male red deer harvested during hunting actions in 2 types of populations in south western Spain that differed in structure, affecting the level of male-male competition for mates. We used coprological analyses to estimate the parasite burden mainly of gastrointestinal and bronchopulmonary nematodes and of protozoa, and assessed testosterone and cortisol metabolite levels from fecal pellets. We found a positive relationship of host parasitation with both testosterone levels and the size of the dark ventral patch, but these relationships depended on the intensity of male-male competition in the population, being only found under the high-competition scenario. These results are discussed under the hypothesis of the testosterone immunocompetence handicap, suggesting a cost at the immunological level, and, therefore, higher susceptibility to parasite infection in males that make a greater reproductive effort. However, this effect seems to be modulated by the social environment (male-male competition) that might lead to different optima in testosterone production and sexual trait development.

Reproductive and Somatic Senescence in the European Badger (Meles meles): Evidence from Lifetime Sex-Steroid Profiles

Among the Carnivora, there is sparse evidence for any substantive fitness benefits of post reproductive lifespan (PRLS, survival after reproductive cessation, RC). Using the European badger (Meles meles) as a model species, we analyzed sex-specific cross-sectional endocrinological and morphological data to investigate: 1) age-dependent reproductive decline in sex-steroid levels versus prime reproductive age; 2) age-dependent declines in somatic condition and reproductive advertisement (from subcaudal scent gland secretion); 3) changes in reproductive success with age due to somatic and endocrinological decline; 4) occurrence of RC, PRLS, and post reproductive representation (PrR) in the population with reference to pre-pubescent hormone levels and evidenced by fewer cub assignments from pedigree. We provide strong evidence for a gradual, not abrupt, decline in sex-steroid levels with age, with both sexes following a concave (down) quadratic trend. For both sexes, the onset of decline in somatic condition commenced at the age of 3 years. In contrast, decline in reproductive hormones started at age ca. 5.5 years in females and 6 years in males, with similar rates of decline thereafter. Subcaudal gland secretion volume also decreased in both sexes, especially after age 5, suggesting less investment in reproductive advertisement. After age 3, fewer (surviving) females were assigned cubs. This coincided with the onset of somatic decline but came earlier than hormonal decline (5.5 years onwards). The decrease in offspring assignments commenced later in males at age 5-6 years; concomitant with onset of testosterone decline at 6 years. This suggests that, contrary to females, in males declining body condition does not preclude reproductive success (no 'restraint') in advance of hormonal senescence ('constraint'). There was evidence of female PRLS, with very old adults living up to 2.59 ± 1.29 years after RC; although in males this evidence was weaker. We discuss the implications of these findings for RC and PRLS in the context of adaptive and non-adaptive hypotheses. There was evidence of over 2 years of Post Reproductive Life Span in both sexes.

Endless forms of sexual selection

In recent years, the field of sexual selection has exploded, with advances in theoretical and empirical research complementing each other in exciting ways. This perspective piece is the product of a "stock-taking" workshop on sexual selection and sexual conflict. Our aim is to identify and deliberate on outstanding questions and to stimulate discussion rather than provide a comprehensive overview of the entire field. These questions are organized into four thematic sections we deem essential to the field. First we focus on the evolution of mate choice and mating systems. Variation in mate quality can generate both competition and choice in the opposite sex, with implications for the evolution of mating systems. Limitations on mate choice may dictate the importance of direct vs. indirect benefits in mating decisions and consequently, mating systems, especially with regard to polyandry. Second, we focus on how sender and receiver mechanisms shape signal design. Mediation of honest signal content likely depends on integration of temporally variable social and physiological costs that are challenging to measure. We view the neuroethology of sensory and cognitive receiver biases as the main key to signal form and the 'aesthetic sense' proposed by Darwin. Since a receiver bias is sufficient to both initiate and drive ornament or armament exaggeration, without a genetically correlated or even coevolving receiver, this may be the appropriate 'null model' of sexual selection. Thirdly, we focus on the genetic architecture of sexually selected traits. Despite advances in modern molecular techniques, the number and identity of genes underlying performance, display and secondary sexual traits remains largely unknown. In-depth investigations into the genetic basis of sexual dimorphism in the context of long-term field studies will reveal constraints and trajectories of sexually selected trait evolution. Finally, we focus on sexual selection and conflict as drivers of speciation. Population divergence and speciation are often influenced by an interplay between sexual and natural selection. The extent to which sexual selection promotes or counteracts population divergence may vary depending on the genetic architecture of traits as well as the covariance between mating competition and local adaptation. Additionally, post-copulatory processes, such as selection against heterospecific sperm, may influence the importance of sexual selection in speciation. We propose that efforts to resolve these four themes can catalyze conceptual progress in the field of sexual selection, and we offer potential avenues of research to advance this progress.

Age- and quality-dependent DNA methylation correlate with melanin-based coloration in a wild bird

Secondary sexual trait expression can be influenced by fixed individual factors (such as genetic quality) as well as by dynamic factors (such as age and environmentally induced gene expression) that may be associated with variation in condition or quality. In particular, melanin-based traits are known to relate to condition and there is a well-characterized genetic pathway underpinning their expression. However, the mechanisms linking variable trait expression to genetic quality remain unclear. One plausible mechanism is that genetic quality could influence trait expression via differential methylation and differential gene expression. We therefore conducted a pilot study examining DNA methylation at a candidate gene (agouti-related neuropeptide: AgRP) in the black grouse Lyrurus tetrix. We specifically tested whether CpG methylation covaries with age and multilocus heterozygosity (a proxy of genetic quality) and from there whether the expression of a melanin-based ornament (ultraviolet-blue chroma) correlates with DNA methylation. Consistent with expectations, we found clear evidence for age- and heterozygosity-specific patterns of DNA methylation, with two CpG sites showing the greatest DNA methylation in highly heterozygous males at their peak age of reproduction. Furthermore, DNA methylation at three CpG sites was significantly positively correlated with ultraviolet-blue chroma. Ours is the first study to our knowledge to document age- and quality-dependent variation in DNA methylation and to show that dynamic sexual trait expression across the lifespan of an organism is associated with patterns of DNA methylation. Although we cannot demonstrate causality, our work provides empirical support for a mechanism that could potentially link key individual factors to variation in sexual trait expression in a wild vertebrate.

Morph-Specific Patterns of Reproductive Senescence: Connections to Discrete Reproductive Strategies

How reproductive strategies contribute to patterns of senescence in natural populations remains contentious. We studied reproductive senescence in the dimorphic white-throated sparrow, an excellent species for exploring this issue. Within both sexes the morphs use distinct reproductive strategies, and disassortative pairing by morph results in pair types with distinct parental systems. White morph birds are more colorful and aggressive than tan counterparts, and white males compete for extrapair matings, whereas tan males are more parental. Tan males and white females share parental care equally, whereas white males provide little parental support to tan females. We found morph-specific patterns of reproductive senescence in both sexes. White males exhibited greater reproductive senescence than tan males. This result likely reflects the difficulty of sustaining a highly competitive reproductive strategy as aging progresses rather than high physiological costs of competitiveness, since white males were also long-lived. Moreover, morph was not consistently related to reproductive senescence across the sexes, arguing against especially high costs of the traits associated with white morph identity. Rather, tan females exhibited earlier reproductive senescence than white females and were short-lived, perhaps reflecting the challenges of unsupported motherhood. Results underscore the importance of social dynamics in determining patterns of reproductive senescence.

Stabilising selection on immune response in male black grouse Lyrurus tetrix

Illnesses caused by a variety of micro- and macro- organisms can negatively affect individuals’ fitness, leading to the expectation that immunity is under positive selection. However, immune responses are costly and individuals must trade-off their immune response with other fitness components (e.g. survival or reproductive success) meaning that individuals with intermediate response may have the greatest overall fitness. Such a process might be particularly acute in species with strong sexual selection because the condition-dependence of male secondary sexual-traits might lead to striking phenotypic differences amongst males of different immune response levels. We tested whether there is selection on immune response by survival and reproduction in yearling and adult male black grouse (Lyrurus tetrix) following an immune challenge with a novel antigen and tested the hypothesis that sexual signals and body mass are honest signals of the immune response. We show that yearling males with highest immune response to these challenges had higher survival, but the reverse was true for adults. Adults with higher responses had highest mass loss and adult males with intermediate immune response had highest mating success. Tail length was related to baseline response in adults and more weakly in yearlings. Our findings reveal the complex fitness consequences of mounting an immune response across age classes. Such major differences in the direction and magnitude of selection in multiple fitness components is an alternative route underpinning the stabilising selection of immune responses with an intermediate immune response being optimal.

Can Neglected Tropical Diseases Compromise Human Wellbeing in Sex-, Age-, and Trait-Specific Ways?

Traits that facilitate competition for reproductive resources or that influence mate choice have evolved to signal resilience to infectious disease and other stressors. As a result, the dynamics of competition and choice can, in theory, be used to generate predictions about sex-, age-, and trait-specific vulnerabilities for any sexually reproducing species, including humans. These dynamics and associated vulnerabilities are reviewed for nonhuman species, focusing on traits that are compromised by exposure to parasites. Using the same approach, sex-, age-, and trait-specific vulnerabilities to parasitic disease are illustrated for children's and adolescent's physical growth and fitness. Suggestions are then provided for widening the assessment of human vulnerabilities to include age-appropriate measures of behavioral (e.g., children's play) and cognitive (e.g., language fluency) traits. These are traits that are likely to be compromised by infection in age- and sex-specific ways. Inclusion of these types of measures in studies of neglected tropic diseases has the potential to provide a more nuanced understanding of how these diseases undermine human wellbeing and may provide a useful means to study the efficacy of associated treatments.

The role of parasite-driven selection in shaping landscape genomic structure in red grouse (Lagopus lagopus scotica)

Landscape genomics promises to provide novel insights into how neutral and adaptive processes shape genome-wide variation within and among populations. However, there has been little emphasis on examining whether individual-based phenotype-genotype relationships derived from approaches such as genome-wide association (GWAS) manifest themselves as a population-level signature of selection in a landscape context. The two may prove irreconcilable as individual-level patterns become diluted by high levels of gene flow and complex phenotypic or environmental heterogeneity. We illustrate this issue with a case study that examines the role of the highly prevalent gastrointestinal nematode Trichostrongylus tenuis in shaping genomic signatures of selection in red grouse (Lagopus lagopus scotica). Individual-level GWAS involving 384 SNPs has previously identified five SNPs that explain variation in T. tenuis burden. Here, we examine whether these same SNPs display population-level relationships between T. tenuis burden and genetic structure across a small-scale landscape of 21 sites with heterogeneous parasite pressure. Moreover, we identify adaptive SNPs showing signatures of directional selection using F(ST) outlier analysis and relate population- and individual-level patterns of multilocus neutral and adaptive genetic structure to T. tenuis burden. The five candidate SNPs for parasite-driven selection were neither associated with T. tenuis burden on a population level, nor under directional selection. Similarly, there was no evidence of parasite-driven selection in SNPs identified as candidates for directional selection. We discuss these results in the context of red grouse ecology and highlight the broader consequences for the utility of landscape genomics approaches for identifying signatures of selection.

Genome-wide association and genome partitioning reveal novel genomic regions underlying variation in gastrointestinal nematode burden in a wild bird

Identifying the genetic architecture underlying complex phenotypes is a notoriously difficult problem that often impedes progress in understanding adaptive eco-evolutionary processes in natural populations. Host-parasite interactions are fundamentally important drivers of evolutionary processes, but a lack of understanding of the genes involved in the host's response to chronic parasite insult makes it particularly difficult to understand the mechanisms of host life history trade-offs and the adaptive dynamics involved. Here, we examine the genetic basis of gastrointestinal nematode (Trichostrongylus tenuis) burden in 695 red grouse (Lagopus lagopus scotica) individuals genotyped at 384 genome-wide SNPs. We first use genome-wide association to identify individual SNPs associated with nematode burden. We then partition genome-wide heritability to identify chromosomes with greater heritability than expected from gene content, due to harbouring a multitude of additive SNPs with individually undetectable effects. We identified five SNPs on five chromosomes that accounted for differences of up to 556 worms per bird, but together explained at best 4.9% of the phenotypic variance. These SNPs were closely linked to genes representing a range of physiological processes including the immune system, protein degradation and energy metabolism. Genome partitioning indicated genome-wide heritability of up to 29% and three chromosomes with excess heritability of up to 4.3% (total 8.9%). These results implicate SNPs and novel genomic regions underlying nematode burden in this system and suggest that this phenotype is somewhere between being based on few large-effect genes (oligogenic) and based on a large number of genes with small individual but large combined effects (polygenic).

Patterns of ectoparasitism in North American red squirrels (Tamiasciurus hudsonicus): Sex-biases, seasonality, age, and effects on male body condition

Within many species, males are often more heavily parasitised than females. Several hypotheses have been proposed to explain this phenomenon, including immunocompetence handicaps, sexual size dimorphism and behavioural differences. Here we set out to test the latter two hypotheses and make inferences about the former by assessing patterns of ectoparasitism across various life-history stages in a population of North American red squirrels (Tamiasciurus hudsonicus). We also conducted an ectoparasite removal experiment to investigate the effects of ectoparasites on male body condition. We found that males were more intensely parasitized than females, but only during the mating period. There was no difference in ectoparasite intensity between male and female juveniles at birth or at emergence, suggesting that ectoparasites do not exploit male red squirrels for longer-range natal dispersal. Male red squirrels in our population were slightly heavier than females, however we did not find any evidence that this dimorphism drives male-biased ectoparasitism. Finally, we could not detect an effect of ectoparasite removal on male body mass. Our results lend support to the hypothesis that ectoparasites exploit their male hosts for transmission and that male red squirrels are important for the transmission dynamics of ectoparasites in this population; however, the mechanisms (i.e., immunocompetence, testosterone) are not known.

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Host life-history stage and sex modulate observed patterns of ectoparasitism.

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Red squirrels experienced highest flea intensities as young juveniles in the nest.

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Sex-biases in ectoparasite intensity were only observable in adult hosts.

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Ectoparasitism did not appear to affect the mass of adult male red squirrels.

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Patterns of parasitism should be considered within the context of host life-history.

Host and pathogen ecology drive the seasonal dynamics of a fungal disease, white-nose syndrome

Seasonal patterns in pathogen transmission can influence the impact of disease on populations and the speed of spatial spread. Increases in host contact rates or births drive seasonal epidemics in some systems, but other factors may occasionally override these influences. White-nose syndrome, caused by the emerging fungal pathogen Pseudogymnoascus destructans, is spreading across North America and threatens several bat species with extinction. We examined patterns and drivers of seasonal transmission of P. destructans by measuring infection prevalence and pathogen loads in six bat species at 30 sites across the eastern United States. Bats became transiently infected in autumn, and transmission spiked in early winter when bats began hibernating. Nearly all bats in six species became infected by late winter when infection intensity peaked. In summer, despite high contact rates and a birth pulse, most bats cleared infections and prevalence dropped to zero. These data suggest the dominant driver of seasonal transmission dynamics was a change in host physiology, specifically hibernation. Our study is the first, to the best of our knowledge, to describe the seasonality of transmission in this emerging wildlife disease. The timing of infection and fungal growth resulted in maximal population impacts, but only moderate rates of spatial spread.

Host sex and parasitism in Red-winged Blackbirds (Agelaius phoeniceus): examining potential causes of infection biases in a sexually dimorphic species

Sex biases in parasitism rates are widely reported in the literature. Among vertebrates in particular, males are more frequently parasitized than females. These sex-linked differences are often attributed to different investment strategies in current versus future reproduction at the ultimate level and to different levels of circulating androgens at the proximate level. But there are other factors that can influence parasitism rates that are often neglected. In this study, we examined multiple measures of parasitism in male and female Red-winged Blackbirds (Agelaius phoeniceus (L., 1766)) to determine whether sex biases occur and sought to identify physiological and ecological factors that may be shaping such differences. We assessed three groups of parasites (ectoparasites, intestinal parasites, blood parasites), one measure of immune function (bacteria-killing ability), and diet. We found that male-biased infections were common only for intestinal parasites, but could not be attributed to dietary or immunological differences. We also found that immune function was associated with an individual’s infection status, appearing elevated in males infected with helminths. Our results suggest that, for this species, sex biases in parasitism do occur but are not the norm. Furthermore, their existence may depend on the nature of the host–parasite relationship.

Physiological control of elaborate male courtship: female choice for neuromuscular systems

Males of many animal species perform specialized courtship behaviours to gain copulations with females. Identifying physiological and anatomical specializations underlying performance of these behaviours helps clarify mechanisms through which sexual selection promotes the evolution of elaborate courtship. Our knowledge about neuromuscular specializations that support elaborate displays is limited to a few model species. In this review, we focus on the physiological control of the courtship of a tropical bird, the golden-collared manakin, which has been the focus of our research for nearly 20 years. Male manakins perform physically elaborate courtship displays that are quick, accurate and powerful. Females seem to choose males based on their motor skills suggesting that neuromuscular specializations possessed by these males are driven by female choice. Male courtship is activated by androgens and androgen receptors are expressed in qualitatively and quantitatively unconventional ways in manakin brain, spinal cord and skeletal muscles. We propose that in some species, females select males based on their neuromuscular capabilities and acquired skills and that elaborate steroid-dependent courtship displays evolve to signal these traits.

Fine-scale population epigenetic structure in relation to gastrointestinal parasite load in red grouse (Lagopus lagopus scotica)

Epigenetic modification of cytosine methylation states can be elicited by environmental stresses and may be a key process affecting phenotypic plasticity and adaptation. Parasites are potent stressors with profound physiological and ecological effects on their host, but there is little understanding in how parasites may influence host methylation states. Here, we estimate epigenetic diversity and differentiation among 21 populations of red grouse (Lagopus lagopus scotica) in north-east Scotland and test for association of gastrointestinal parasite load (caecal nematode Trichostrongylus tenuis) with hepatic genome-wide and locus-specific methylation states. Following methylation-sensitive AFLP (MSAP), 129 bands, representing 73 methylation-susceptible and 56 nonmethylated epiloci, were scored across 234 individuals. The populations differed significantly in genome-wide methylation levels and were also significantly epigenetically (F_SC = 0.0227; P < 0.001) and genetically (F_SC = 0.0058; P < 0.001) differentiated. Parasite load was not associated with either genome-wide methylation levels or epigenetic differentiation. Instead, we found eight disproportionately differentiated epilocus-specific methylation states (F_ST outliers) using bayescan software and significant positive and negative association of 35 methylation states with parasite load from bespoke generalized estimating equations (GEE), simple logistic regression (sam) and Bayesian environmental analysis (bayenv2). Following Sanger sequencing, genome mapping and geneontology (go) annotation, some of these epiloci were linked to genes involved in regulation of cell cycle, signalling, metabolism, immune system and notably rRNA methylation, histone acetylation and small RNAs. These findings demonstrate an epigenetic signature of parasite load in populations of a wild bird and suggest intriguing physiological effects of parasite-associated cytosine methylation.

The evolution of honest communication: integrating social and physiological costs of ornamentation

Much research on animal communication has addressed how costs such as social costs or physiological costs favor the accuracy of signals. Previous work has largely considered these costs separately, but we may be missing essential connections by studying costs in isolation. After all, social interactions produce rapid changes in hormone titers which can then affect individual behavior and physiology. As a result, social costs are likely to have widespread physiological consequences. Here, I present a new perspective on the factors that maintain honest signals by describing how the interplay between social costs and physiological costs may maintain an accurate link between an animal's abilities and ornament elaboration. I outline three specific mechanisms by which the interaction between social behavior and hormones could favor honest signals and present specific predictions for each of the three models. Then, I review how ornaments alter agonistic behavior, agonistic behavior influences hormones, and how these hormonal effects influence fitness. I also describe the few previous studies that have directly tested how ornaments influence hormones. Finally, opportunities for future work are discussed. Considering the interaction between social behavior and physiology may address some challenges associated with both social and physiological models of costs. Understanding the dynamic feedbacks between physiology and social costs has potential to transform our understanding of the stability of animals' communication systems.

Is sexual ornamentation an honest signal of male quality in the Chinese grouse (Tetrastes sewerzowi)?

We examined the variation in sexual ornamentation of male Chinese grouse (Tetrastes sewerzowi) in the Gansu Province, China, seeking to identify factors involved in whether ornament size and brightness are honest signals of male quality. Compared to unmated males, mated males had significantly larger and redder combs and, although they did not have significantly larger territories, they defended them more vigorously. Mated males had significantly higher blood carotenoid and testosterone levels, significantly better body condition, and significantly lower parasite loads than unmated males. Our findings are thus consistent with the hypothesis that comb size and color are honest signals of better male quality in the grouse, mediated through lower parasite loads and/or higher testosterone levels.

Two sexes, one genome: the evolutionary dynamics of intralocus sexual conflict

As the evolutionary interests of males and females are frequently divergent, a trait value that is optimal for the fitness of one sex is often not optimal for the other. A shared genome also means that the same genes may underlie the same trait in both sexes. This can give rise to a form of sexual antagonism, known as intralocus sexual conflict (IASC). Here, a tug-of-war over allelic expression can occur, preventing the sexes from reaching optimal trait values, thereby causing sex-specific reductions in fitness. For some traits, it appears that IASC can be resolved via sex-specific regulation of genes that subsequently permits sexual dimorphism; however, it seems that whole-genome resolution may be impossible, due to the genetic architecture of certain traits, and possibly due to the changing dynamics of selection. In this review, we explore the evolutionary mechanisms of, and barriers to, IASC resolution. We also address the broader consequences of this evolutionary feud, the possible interactions between intra- and interlocus sexual conflict (IRSC: a form of sexual antagonism involving different loci in each sex), and draw attention to issues that arise from using proxies as measurements of conflict. In particular, it is clear that the sex-specific fitness consequences of sexual dimorphism require characterization before making assumptions concerning how this relates to IASC. Although empirical data have shown consistent evidence of the fitness effects of IASC, it is essential that we identify the alleles mediating these effects in order to show IASC in its true sense, which is a “conflict over shared genes.”

Social context decouples the relationship between a sexual ornament and testosterone levels in a male wild bird

In order to maximise fitness individuals should adjust their level of signalling according to their surrounding social environment. However, field experiments showing such adjustment of current signalling associated to changes in social context are lacking. Here, we manipulated levels of male aggressive- and dominance-related displays in a wild bird in our treated area by increasing testosterone levels using implants in a subset of males. We then compared the expression of sexual signals (i.e. comb size) between non-treated red grouse Lagopus lagopus scoticus males from control and treatment areas. We further explored the potential endocrinological mechanism linking social environment and signal expression by analysing testosterone levels in all males. Our treatment successfully increased overall aggressive- and dominance-related behaviours in the treatment area. Furthermore, testosterone-implanted birds increased their comb size as repeatedly shown in previous studies in male red grouse. Interestingly, untreated males living in the treatment area decreased their comb size, whilst increasing testosterone levels. Since comb size is a signal of dominance, untreated males from the treatment area may have perceived themselves as subordinate individuals and decreased their signalling levels to avoid confrontations with testosterone-treated, dominant individuals. In conclusion, our findings show that social context has the potential to regulate sexual signalling and testosterone levels. Our results highlight the role of social context when exploring the link between testosterone and behaviour, as it may reverse the relationship between both traits. Our results suggest that social context affects signalling and testosterone independently.