Avian colouration in a polluted world: a meta-analysis

Brilliant, diverse colour ornaments of birds were one of the crucial cues that led Darwin to the idea of sexual selection. Although avian colouration plays many functions, including concealment, thermoregulation, or advertisement as a distasteful prey, a quality-signalling role in sexual selection has attracted most research attention. Sexually selected ornaments are thought to be more susceptible to external stressors than naturally selected traits, and as such, they might be used as a test for environmental quality. For this reason, the last two decades have seen numerous studies on the impact of anthropogenic pollution on the expression of various avian colour traits. Herein, we provide the first meta-analytical summary of these results and examine whether there is an interaction between the mechanism of colour production (carotenoid-based, melanin-based and structural) and the type of anthropogenic factor (categorised as heavy metals, persistent organic pollutants, urbanisation, or other). Following the assumption of heightened condition dependence of ornaments under sexual selection, we also expected the magnitude of effect sizes to be higher in males. The overall effect size was close to significance and negative, supporting a general detrimental impact of anthropogenic pollutants on avian colouration. In contrast to expectations, there was no interaction between pollution types and colour-producing mechanisms. Yet there were significant differences in sensitivity between colour-producing mechanisms, with carotenoid-based colouration being the most affected by anthropogenic environmental disturbances. Moreover, we observed no significant tendency towards heightened sensitivity in males. We identified a publication gap on structural colouration, which, compared to pigment-based colouration, remains markedly understudied and should thus be prioritised in future research. Finally, we call for the unification of methods used in colour quantification in ecological research to ensure comparability of results among studies.

Synthesis of sexual selection: a systematic map of meta-analyses with bibliometric analysis

Sexual selection has been a popular subject within evolutionary biology because of its central role in explaining odd and counterintuitive traits observed in nature. Consequently, the literature associated with this field of study became vast. Meta-analytical studies attempting to draw inferences from this literature have now accumulated, varying in scope and quality, thus calling for a synthesis of these syntheses. We conducted a systematic literature search to create a systematic map with a report appraisal of meta-analyses on topics associated with sexual selection, aiming to identify the conceptual and methodological gaps in this secondary literature. We also conducted bibliometric analyses to explore whether these gaps are associated with the gender and origin of the authors of these meta-analyses. We included 152 meta-analytical studies in our systematic map. We found that most meta-analyses focused on males and on certain animal groups (e.g. birds), indicating severe sex and taxonomic biases. The topics in these studies varied greatly, from proximate (e.g. relationship of ornaments with other traits) to ultimate questions (e.g. formal estimates of sexual selection strength), although the former were more common. We also observed several common methodological issues in these studies, such as lack of detailed information regarding searches, screening, and analyses, which ultimately impairs the reliability of many of these meta-analyses. In addition, most of the meta-analyses' authors were men affiliated to institutions from developed countries, pointing to both gender and geographical authorship biases. Most importantly, we found that certain authorship aspects were associated with conceptual and methodological issues in meta-analytical studies. Many of our findings might simply reflect patterns in the current state of the primary literature and academia, suggesting that our study can serve as an indicator of issues within the field of sexual selection at large. Based on our findings, we provide both conceptual and analytical recommendations to improve future studies in the field of sexual selection.

Evolution of fertilization ability in obligatorily outcrossing populations of Caenorhabditis elegans

In species reproducing by selfing, the traits connected with outcrossing typically undergo degeneration, a phenomenon called selfing syndrome. In Caenorhabditis elegans nematodes, selfing syndrome affects many traits involved in mating, rendering cross-fertilization highly inefficient. In this study, we investigated the evolution of cross-fertilization efficiency in populations genetically modified to reproduce by obligatory outcrossing. Following the genetic modification, replicate obligatorily outcrossing were maintained for over 100 generations, at either optimal (20 °C) or elevated (24 °C) temperatures, as a part of a broader experimental evolution program. Subsequently, fertilization rates were assayed in the evolving populations, as well as their ancestors who had the obligatory outcrossing introduced but did not go through experimental evolution. Fertilization effectivity was measured by tracking the fractions of fertilized females in age-synchronized populations, through 8 h since reaching adulthood. In order to check the robustness of our measurements, each evolving population was assayed in two or three independent replicate blocks. Indeed, we found high levels of among-block variability in the fertilization trajectories, and in the estimates of divergence between evolving populations and their ancestors. We also identified five populations which appear to have evolved increased fertilization efficiency, relative to their ancestors. However, due to the abovementioned high variability, this set of populations should be treated as candidate, with further replications needed to either confirm or disprove their divergence from ancestors. Furthermore, we also discuss additional observations we have made concerning fertilization trajectories.

Offspring plumage coloration as a condition-dependent signal in the blue tit

In many species, offspring display conspicuous coloration already early in life, even though they might be very vulnerable to predation at this stage. However, most attention has been drawn to the conspicuous plumage displayed by adult individuals in a sexual context, while other signaling functions have been explored much less. Here, we investigated whether the yellow breast plumage of blue tit (Cyanistes caeruleus) nestlings shows patterns of condition dependence and hence signals individual quality, as has been described for adult birds. During three consecutive breeding seasons, we, therefore, explored the association between nestling body mass and three color components of the yellow breast plumage (i.e., UV chroma, carotenoid chroma, and total brightness), considering both within and among nest effects. Variation in carotenoid chroma was not related to body mass. However, UV chroma and total brightness varied with body mass on an among-nest level, suggesting that they might signal aspects of genetic quality or parental rearing capacity. Interestingly, we also found a within-nest effect of body mass on total brightness, suggesting that this is a good candidate for a condition-dependent signal within the family. Thus, other family members could rely on brightness to adjust their behavioral strategies, such as feeding behavior in parents. Our study thus reveals that certain color components of the yellow breast plumage might signal different aspects of offspring quality, and they might have a correlated signaling value across life-history stages.

No detectable changes in reproductive behaviour of Caenorhabditis elegans males after 97 generations under obligatory outcrossing

In Caenorhabditis elegans, a species reproducing mostly via self-fertilization, numerous signatures of selfing syndrome are observed, including differences in reproductive behaviour compared to related obligatory outcrossing species. In this study we investigated the effect of nearly 100 generations of obligatory outcrossing on several characteristics of male reproductive behaviour. A genetically uniform ancestral population carrying a mutation changing the reproductive system to obligatory outcrossing was split into four independent populations. We predicted that the transition from the natural reproductive system, where males were extremely rare, to obligatory outcrossing, where males comprise 50% of the population and are necessary for reproduction, will increase the selection pressure on higher effectiveness of mating behaviour. Several characteristics of male mating behaviour during a 15 min interaction as well as copulation success were compared between the ancestral and evolved populations. No significant differences in male mating behaviour or fertilization success were detected between generations 1 and 97 of obligatory outcrossing populations. We found, however, that longer contact with females increased chances of successful copulation, although this effect did not differ between populations. We conclude that either selection acting on male mating behaviour has not been strong enough, or mutational input of new adaptive variants has not been sufficient to cause noticeable behavioural differences after 97 generations of evolution starting from genetically uniform population.

Quantifying research waste in ecology

Research inefficiencies can generate huge waste: evidence from biomedical research has shown that most research is avoidably wasted and steps have been taken to tackle this costly problem. Although other scientific fields could also benefit from identifying and quantifying waste and acting to reduce it, no other estimates of research waste are available. Given that ecological issues interweave most of the United Nations Sustainable Development Goals, we argue that tackling research waste in ecology should be prioritized. Our study leads the way. We estimate components of waste in ecological research based on a literature review and a meta-analysis. Shockingly, our results suggest only 11-18% of conducted ecological research reaches its full informative value. All actors within the research system-including academic institutions, policymakers, funders and publishers-have a duty towards science, the environment, study organisms and the public, to urgently act and reduce this considerable yet preventable loss. We discuss potential ways forward and call for two major actions: (1) further research into waste in ecology (and beyond); (2) focused development and implementation of solutions to reduce unused potential of ecological research.

Sexual dichromatism, size dimorphism, and microscale anatomy of white wing stripe in blue tits

Achromatic patches are a common element of plumage patterns in many bird species and there is growing body of evidence that in many avian taxa they can play a signaling role in mate choice. Although the blue tit Cyanistes caeruleus is a well-established model species in the studies on coloration, its white wing patch has never been examined in the context of sex-specific trait expression. In this exploratory study, we examined sexual size dimorphism and dichromatism of greater covert’s dots creating white wing patch and analyzed its correlations with current body condition and crown coloration—a trait with established role in sexual selection. Further, we qualitatively analyzed microstructural barb morphology underlying covert’s coloration. We found significant sexual dimorphism in the dot size independent of covert size and sexual dichromatism in both white dot and blue outer covert’s vane spectral characteristics. Internal structure of covert barbs within the white dot was similar to the one found in barbs from the blue part that is, with a medullary area consisting of dead keratinocytes containing channel-type ß-keratin spongy nanostructure and centrally located air cavities. However, it lacked melanosomes which was the main observed difference. Importantly, UV chroma of covert’s blue vane was positively correlated with crown UV chroma and current condition (the latter only in males), which should be a premise for further research on the signal function of the wing stripe.

Are plumage colour and song redundant ornaments in great tits (Parus major)? It depends on the colour

According to current theory, the development of different conspicuous ornaments may provide multiple, redundant or unreliable information. However, multiple visual and acoustic traits have not often been tested at the same time. In this study, we analyse whether black tie size, yellow breast colour indexes (chroma and hue) and song parameters (repertoire size, strophe length, maximum and minimum frequency) are inter-correlated in male great tits (Parus major). We found that males with larger black ties sang greater repertoires and had higher song frequencies, providing redundant information. However, song parameters did not correlate with hue and chroma of the yellow breast coloration, supporting the idea that they convey different units of information. Results therefore stress that since different plumaged patches can transmit very different units of information, bird song will correlate to plumage coloration traits depending on the patch and the colour considered, an aspect rarely taken into account in previous studies relating song to plumage colour traits.

Mutual mate preferences and assortative mating in relation to a carotenoid-based color trait in blue tits

Choosing an appropriate sexual partner is a critical decision for many animal species. However, many mechanisms involved in mate choice are still poorly understood. Do both males and females choose their sexual partners, do both sexes use the same criteria for choosing, and do their own phenotype influence the choices they make, are questions that need further investigation. Over two successive experiments conducted in captivity with hand-reared blue tits (Cyanistes caeruleus), we manipulated the color of the chest plumage, a secondary sexual trait that reflects an individual’s condition, to create two different color morphs (one pale and one colored). We then tested whether both sexes express a preference, whether they are attracted to the same morphs, and if the subjects’ own chest color influences the preference they show. Our data reveal that both sexes are choosy, with females tending to be slightly choosier than males. We also show that both sexes preferentially select individuals with a pale chest plumage over colorful individuals, and this was again more pronounced in females. Finally, paler individuals tend to be selected by birds that are themselves pale, even if this phenotype matching was not very robust. Such a preference for paler individuals is intriguing because mates are predicted to associate with individuals displaying higher, not lower, value of quality signals. It could result from adaptive mechanisms related to avoidance of aggressiveness in a confined environment, avoidance of conflicting sexual signals within individuals, or from cultural mechanisms leading to a preference for individuals that match its own phenotype.

Male size and reproductive performance in three species of livebearing fishes (Gambusia spp.): A systematic review and meta-analysis

The genus Gambusia represents approximately 45 species of polyandrous livebearing fishes with reversed sexual size dimorphism (i.e. males smaller than females) and with copulation predominantly via male coercion. Male body size has been suggested as an important sexually selected trait, but despite abundant research, evidence for sexual selection on male body size in this genus is mixed. Studies have found that large males have an advantage in both male-male competition and female choice, but that small males perform sneaky copulations better and at higher frequency and thus may sire more offspring in this coercive mating system. Here, we synthesized this inconsistent body of evidence using pre-registered methods and hypotheses. We performed a systematic review and meta-analysis of summary and primary (raw) data combining both published (n = 19 studies, k = 106 effect sizes) and unpublished effect sizes (n = 17, k = 242) to test whether there is overall selection on male body size across studies in Gambusia. We also tested several specific hypotheses to understand the sources of heterogeneity across effects. Meta-analysis revealed an overall positive correlation between male size and reproductive performance (r = 0.23, 95% confidence interval: 0.10-0.35, n = 36, k = 348, 4,514 males, three Gambusia species). Despite high heterogeneity, the large-male advantage appeared robust across all measures studied (i.e. female choice, mating success, paternity, sperm quantity and quality), and was considerably larger for female choice (r = 0.43, 95% confidence interval: 0.28-0.59, n = 14, k = 43). Meta-regressions found several important factors explaining heterogeneity across effects, including type of sperm characteristic, male-to-female ratio, female reproductive status and environmental conditions. We found evidence of publication bias; however, its influence on our estimates was attenuated by including a substantial amount of unpublished effects, highlighting the importance of open primary data for more accurate meta-analytic estimates. In addition to positive selection on male size, our study suggests that we need to rethink the role and form of sexual selection in Gambusia and, more broadly, to consider the ecological factors that affect reproductive behaviour in livebearing fishes.

Adaptive coloration in pied flycatchers (Ficedula hypoleuca)-The devil is in the detail

Understanding the origin and persistence of phenotypic variation within and among populations is a major goal in evolutionary biology. However, the eagerness to find unadulterated explanatory models in combination with difficulties in publishing replicated studies may lead to severe underestimations of the complexity of selection patterns acting in nature. One striking example is variation in plumage coloration in birds, where the default adaptive explanation often is that brightly colored individuals signal superior quality across environmental conditions and therefore always should be favored by directional mate choice. Here, we review studies on the proximate determination and adaptive function of coloration traits in male pied flycatchers (Ficedula hypoleuca). From numerous studies, we can conclude that the dark male color phenotype is adapted to a typical northern climate and functions as a dominance signal in male-male competition over nesting sites, and that the browner phenotypes are favored by relaxed intraspecific competition with more dominant male collared flycatchers (Ficedula albicollis) in areas where the two species co-occur. However, the role of avoidance of hybridization in driving character displacement in plumage between these two species may not be as important as initially thought. The direction of female choice on male coloration in pied flycatchers is not simply as opposite in direction in sympatry and allopatry as traditionally expected, but varies also in relation to additional contexts such as climate variation. While some of the heterogeneity in the observed relationships between coloration and fitness probably indicate type 1 errors, we strongly argue that environmental heterogeneity and context-dependent selection play important roles in explaining plumage color variation in this species, which probably also is the case in many other species studied in less detail.

Meta-analysis challenges a textbook example of status signalling and demonstrates publication bias

The status signalling hypothesis aims to explain within-species variation in ornamentation by suggesting that some ornaments signal dominance status. Here, we use multilevel meta-analytic models to challenge the textbook example of this hypothesis, the black bib of male house sparrows (Passer domesticus). We conducted a systematic review, and obtained primary data from published and unpublished studies to test whether dominance rank is positively associated with bib size across studies. Contrary to previous studies, the overall effect size (i.e. meta-analytic mean) was small and uncertain. Furthermore, we found several biases in the literature that further question the support available for the status signalling hypothesis. We discuss several explanations including pleiotropic, population- and context-dependent effects. Our findings call for reconsidering this established textbook example in evolutionary and behavioural ecology, and should stimulate renewed interest in understanding within-species variation in ornamental traits.

Disentangling sex allocation in a viviparous reptile with temperature-dependent sex determination: a multifactorial approach

Females are predicted to alter sex allocation when ecological, physiological and behavioural variables have different consequences on the fitness of male and female offspring. Traditionally, tests of sex allocation have examined single causative factors, often ignoring possible interactions between multiple factors. Here, we used a multifactorial approach to examine sex allocation in the viviparous skink, Niveoscincus ocellatus. We integrated a 16-year observational field study with a manipulative laboratory experiment to explore whether the effects of the maternal thermal environment interact with the resources available to females for reproduction to affect sex allocation decisions. We found strong effects of temperature on sex allocation in the field, with females born in warm conditions and males in cold conditions; however, this was not replicated in the laboratory. In contrast, we found no effect of female resource availability on sex allocation, either independently, or in interaction with temperature. These results corresponded with an overall lack of an effect of resource availability on any of the life history traits that we predicted would mediate the benefits of differential sex allocation in this system, suggesting that selection for sex allocation in response to resource availability may be relatively weak. Combined, these results suggest that temperature may be the predominant factor driving sex allocation in this system.

No mutual mate choice for quality in zebra finches: Time to question a widely held assumption

Studies of mate choice typically assume that individuals prefer high quality mates and select them based on condition-dependent indicator traits. In species with biparental care, mutual mate choice is expected to result in assortative mating for quality. When assortment is not perfect, the lower quality pair members are expected to compensate by increased parental investment to secure their partner (positive differential allocation). This framework has been assumed to hold for monogamous species like the zebra finch (Taeniopygia guttata), but progress has been hampered by the difficulty to define individual quality. By combining multiple measures of causes (inbreeding, early nutrition) and consequences (ornaments, displays, fitness components) of variation in quality into a single principal component, we here show that quality variation can be quantified successfully. We further show that variation in quality indeed predicts individual pairing success, presumably because it reflects an individual's vigor or ability to invest in reproduction. However, despite high statistical power, we found no evidence for either assortative mating or for positive differential allocation. We suggest that zebra finch ornaments and displays are not sufficiently reliable for the benefits of choosiness to exceed the costs of competition for the putative best partner. To assess the generality of these findings unbiased quantification of signal honesty and preference strength is required, rather than selective reporting of significant results.

Striving for transparent and credible research: practical guidelines for behavioral ecologists

Science is meant to be the systematic and objective study of the world but evidence suggests that scientific practices are sometimes falling short of this expectation. In this invited idea, we argue that any failure to conduct research according to a documented plan (lack of reliability) and/or any failure to ensure that reconducting the same project would provide the same finding (lack of reproducibility), will result in a low probability of independent studies reaching the same outcome (lack of replicability). After outlining the challenges facing behavioral ecology and science more broadly and incorporating advice from international organizations such as the Center for Open Science (COS), we present clear guidelines and tutorials on what we think open practices represent for behavioral ecologists. In addition, we indicate some of the currently most appropriate and freely available tools for adopting these practices. Finally, we suggest that all journals in our field, such as Behavioral Ecology, give additional weight to transparent studies and therefore provide greater incentives to align our scientific practices to our scientific values. Overall, we argue that producing demonstrably credible science is now fully achievable for the benefit of each researcher individually and for our community as a whole.

Assortative mating by colored ornaments in blue tits: space and time matter

Assortative mating is a potential outcome of sexual selection, and estimating its level is important to better understand local adaptation and underlying trait evolution. However, assortative mating studies frequently base their conclusions on small numbers of individuals sampled over short periods of time and limited spatial scales even though spatiotemporal variation is common. Here, we characterized assortative mating patterns over 10 years in four populations of the blue tit (Cyanistes caeruleus), a passerine bird. We focused on two plumage ornaments-the blue crown and the yellow breast patch. Based on data for 1,657 pairs of birds, we found large interannual variation: assortative mating varied from positive to negative. To determine whether there was nonetheless a general trend in the data, we ran a within-study meta-analysis. It revealed that assortative mating was moderately positive for both ornaments. It also showed that mating patterns differed among populations and especially between two neighboring populations that displayed phenotypic divergence. Our results therefore underscore that long-term studies are needed to draw broad conclusions about mating patterns in natural populations. They also call for studying the potential role of assortative mating in local adaptation and evolution of ornaments in both sexes.

Colour ornamentation in the blue tit: quantitative genetic (co)variances across sexes

Although secondary sexual traits are commonly more developed in males than females, in many animal species females also display elaborate ornaments or weaponry. Indirect selection on correlated traits in males and/or direct sexual or social selection in females are hypothesized to drive the evolution and maintenance of female ornaments. Yet, the relative roles of these evolutionary processes remain unidentified, because little is known about the genetic correlation that might exist between the ornaments of both sexes, and few estimates of sex-specific autosomal or sex-linked genetic variances are available. In this study, we used two wild blue tit populations with 9 years of measurements on two colour ornaments: one structurally based (blue crown) and one carotenoid based (yellow chest). We found significant autosomal heritability for the chromatic part of the structurally based colouration in both sexes, whereas carotenoid chroma was heritable only in males, and the achromatic part of both colour patches was mostly non heritable. Power limitations, which are probably common among most data sets collected so far in wild populations, prevented estimation of sex-linked genetic variance. Bivariate analyses revealed very strong cross-sex genetic correlations in all heritable traits, although the strength of these correlations was not related to the level of sexual dimorphism. In total, our results suggest that males and females share a majority of their genetic variation underlying colour ornamentation, and hence the evolution of these sex-specific traits may depend greatly on correlated responses to selection in the opposite sex.

Transparency in Ecology and Evolution: Real Problems, Real Solutions

To make progress scientists need to know what other researchers have found and how they found it. However, transparency is often insufficient across much of ecology and evolution. Researchers often fail to report results and methods in detail sufficient to permit interpretation and meta-analysis, and many results go entirely unreported. Further, these unreported results are often a biased subset. Thus the conclusions we can draw from the published literature are themselves often biased and sometimes might be entirely incorrect. Fortunately there is a movement across empirical disciplines, and now within ecology and evolution, to shape editorial policies to better promote transparency. This can be done by either requiring more disclosure by scientists or by developing incentives to encourage disclosure.

Structural (UV) and carotenoid-based plumage coloration - signals for parental investment?

Parental care increases parental fitness through improved offspring condition and survival but comes at a cost for the caretaker(s). To increase life-time fitness, caring parents are, therefore, expected to adjust their reproductive investment to current environmental conditions and parental capacities. The latter is thought to be signaled via ornamental traits of the bearer. We here investigated whether pre- and/or posthatching investment of blue tit (Cyanistes caeruleus) parents was related to ornamental plumage traits (UV crown coloration and carotenoid-based plumage coloration) expressed by either the individual itself (i.e. "good parent hypothesis") or its partner (i.e. "differential allocation hypothesis"). Our results show that neither prehatching (that is clutch size and offspring begging intensity) nor posthatching parental investment (provisioning rate, offspring body condition at fledging) was related to an individual's UV crown coloration or to that of its partner. Similar observations were made for carotenoid-based plumage coloration, except for a consistent positive relationship between offspring begging intensity and maternal carotenoid-based plumage coloration. This sex-specific pattern likely reflects a maternal effect mediated via maternally derived egg substances, given that the relationship persisted when offspring were cross-fostered. This suggests that females adjust their offspring's phenotype toward own phenotype, which may facilitate in particular mother-offspring co-adaptation. Overall, our results contribute to the current state of evidence that structural or pigment-based plumage coloration of blue tits are inconsistently correlated with central life-history traits.

Replicating research in ecology and evolution: feasibility, incentives, and the cost-benefit conundrum

We believe that replicating studies in ecology and evolution is extremely valuable, but replication within species and systems is troublingly rare, and even ‘quasi-replications’ in different systems are often insufficient. We make a case for supporting multiple types of replications and point out that the current incentive structure needs to change if ecologists and evolutionary biologist are to value scientific replication sufficiently.

Coadaptation of offspring begging and parental provisioning--an evolutionary ecological perspective on avian family life

Offspring begging and parental provisioning are the two central social behaviours expressed during the period of parental care. Both behaviours influence each other and it is, therefore, hypothesized that they should ultimately become (genetically) correlated, stabilized by fitness costs to parents and/or offspring. By reciprocally exchanging entire clutches in canaries (Serinus canaria), we tested (1) whether there is covariation between these behaviours and (2) whether a mismatch - as introduced by cross-fostering - entails costs. Begging was scored in a standardized begging test and parental provisioning was measured via (a) the actual feeding rate and (b) using the growth rate of the foster nestlings as a proxy. Costs were established in terms of future reproductive investment in subsequent clutches and offspring growth. We found a positive and significant phenotypic covariation between offspring begging and parental feeding when using the growth rate as a proxy and, to a lesser extent, in case of the parental feeding rate. Female parents suffered no future reproductive costs when feeding foster nestlings that were more demanding than their own nestlings. Neither growth measured amongst all offspring nor the reproductive investment measured amongst the female offspring as adults was influenced by their begging behaviour. However, the reproductive investment of female offspring tended to depend on the parental qualities of their foster parents. Thus, offspring may only be able to extract resources within the limit of generosity of their foster parents. This suggests parental control of feeding, which is also supported by the positive covariation between offspring begging and parental feeding.