Demographic causes of adult sex ratio variation and their consequences for parental cooperation

Pre-Copulatory Sexual Selection Predicts Sexual Size Dimorphism: A Meta-Analysis of Comparative Studies

Size differences between males and females are common across the tree of life (termed sexual size dimorphism; SSD), and have fundamental implications for ecology, life history and behaviour of both sexes. Conventionally, SSD is thought to evolve in response to sex-specific sexual selection but more recent work suggests that ecological processes can also promote sex-differences in size. Here, we provide a global test for the role of sexual selection in the evolution of sexual size dimorphism using data from 77 comparative studies spanning the major classes of the animal kingdom. We show that intense sexual selection typically correlates with male-biased SSD across species. Importantly, pre-copulatory but not post-copulatory sexual selection predicts SSD, suggesting a pervasive role of premating male-male competition and female choice to drive sex differences in body size. Collectively, our findings suggest that pre-copulatory sexual selection plays a major role in the evolution of male-biased SSD.

Gender inequality in workloads explained by operational sex ratio

Ecological differences between human populations can affect the relative strength of sexual selection, and hence drive gender inequality. Here, we exploit the cultural diversity of southwestern China, where some village sex ratios are female-biased, in part due to a proportion of males entering monastic celibacy, to evaluate the role of sex ratio on the sexual division of labor. We used a detachable activity tracker to measure workload by step counts in both sexes among 561 individuals in 55 villages in six different areas. We show that a lower sex ratio and a higher prevalence of monasticism are associated with higher women's workloads and reduced men's workloads in the non-celibate population. As the operational sex ratio increases, gender inequality diminishes. This study offers valuable insights into the origins of gender disparities by examining the role of sex ratio on the sexual division of labor.

The evolution of sex roles: The importance of ecology and social environment

Males and females often have different roles in reproduction, although the origin of these differences has remained controversial. Explaining the enigmatic reversed sex roles where males sacrifice their mating potential and provide full parental care is a particularly long-standing challenge in evolutionary biology. While most studies focused on ecological factors as the drivers of sex roles, recent research highlights the significance of social factors such as the adult sex ratio. To disentangle these propositions, here, we investigate the additive and interactive effects of several ecological and social factors on sex role variation using shorebirds (sandpipers, plovers, and allies) as model organisms that provide the full spectrum of sex role variation including some of the best-known examples of sex-role reversal. Our results consistently show that social factors play a prominent role in driving sex roles. Importantly, we show that reversed sex roles are associated with both male-skewed adult sex ratios and high breeding densities. Furthermore, phylogenetic path analyses provide general support for sex ratios driving sex role variations rather than being a consequence of sex roles. Together, these important results open future research directions by showing that different mating opportunities of males and females play a major role in generating the evolutionary diversity of sex roles, mating system, and parental care.

Sex ratios and gender norms: why both are needed to understand sexual conflict in humans

Sexual conflict theory has been successfully applied to predict how in non-human animal populations, sex ratios can lead to conflicting reproductive interests of females and males and affect their bargaining positions in resolving such conflicts of interests. Recently this theory has been extended to understand the resolution of sexual conflict in humans, but with mixed success. We argue that an underappreciation of the complex relationship between gender norms and sex ratios has hampered a successful understanding of sexual conflict in humans. In this paper, we review and expand upon existing theory to increase its applicability to humans, where gender norms regulate sex ratio effects on sexual conflict. Gender norms constrain who is on the marriage market and how they are valued, and may affect reproductive decision-making power. Gender norms can also directly affect sex ratios, and we hypothesize that they structure how individuals respond to market value gained or lost through biased sex ratios. Importantly, gender norms are in part a product of women's and men's sometimes conflicting reproductive interests, but these norms are also subject to other evolutionary processes. An integration of sexual conflict theory and cultural evolutionary theory is required to allow for a full understanding of sexual conflict in humans.

Long-term female bias in sex ratios across life stages of Harpy Eagle, a large raptor exhibiting reverse sexual size dimorphism

The primary (PSR), secondary (SSR) and adult (ASR) sex ratios of sexually reproducing organisms influence their life histories. Species exhibiting reversed sexual size dimorphism (RSD) may imply a higher cost of female production or lower female survival, thus generating biases in PSR, SSR and/or ASR towards males. The Harpy Eagle is the world's largest eagle exhibiting RSD. This species is found in the Neotropical region and is currently threatened with extinction. We used molecular markers to determine the sex of 309 Harpy Eagles spanning different life stages-eaglets, subadults and adults-from 1904 to 2021 within the Amazon Rainforest and Atlantic Forest. Sex ratios for all life stages revealed a female-biased deviation across all periods and regions. Our results suggest that the population bias towards females is an evolutionary ecological pattern of this species, and SSR and ASR likely emerged from the PSR. This natural bias towards females may be compensated by an earlier sexual maturation age of males, implying a longer reproductive lifespan and a higher proportion of sexually active males. A better understanding of the Harpy Eagle's life history can contribute to understanding sex-role evolution and enable more appropriate conservation strategies for the species.

Effects of season length and uniparental care efficacy on the evolution of parental care

Parental care patterns differ enormously among and even within species. This is exemplified by Chinese penduline tits Remiz consobrinus, where biparental care, female-only care, male-only care and biparental desertion all occur in the same population; moreover, the distribution of these care patterns differs systematically between populations. The eco-evolutionary determinants of this diversity are largely unknown. We developed an individual-based model that allows us to investigate the effects of season length and offspring needs (expressed by the efficacy with which a clutch can be raised by a single parent) on the evolution of parental care patterns. The model is largely conceptual, aiming at general conclusions. However, to keep the model realistic, its set-up and the choice of parameters are motivated by field studies on Chinese penduline tits. Exploring a wide range of parameters, we investigate how parental care patterns are affected by season length and offspring needs and whether and under what conditions diverse parental care patterns can stably coexist. We report five main findings. First, under a broad range of conditions, different care patterns (e.g. male care and biparental care) coexist at equilibrium. Second, for the same parameters, alternative evolutionary equilibria are possible; this can explain differences in care patterns across populations. Third, rapid evolutionary transitions can occur between alternative equilibria; this can explain the often-reported evolutionary lability of parental care patterns. Fourth, season length has a strong but nonmonotonic effect on the evolved care patterns. Fifth, when uniparental care efficacy is low, biparental care tends to evolve; however, in many scenarios uniparental care is still common at equilibrium. In addition, our study sheds new light on Trivers' hypothesis that the sex with the highest prezygotic investment is predestined to invest more postzygotically as well. Our study highlights that diversity in parental care can readily evolve and it shows that even in the absence of environmental change parental care patterns can be evolutionary labile. In the presence of directional environmental change, systematic shifts in care patterns are to be expected.

Sex roles and sex ratios in animals

In species with separate sexes, females and males often differ in their morphology, physiology and behaviour. Such sex-specific traits are functionally linked to variation in reproductive competition, mate choice and parental care, which have all been linked to sex roles. At the 150th anniversary of Darwin's theory on sexual selection, the question of why patterns of sex roles vary within and across species remains a key topic in behavioural and evolutionary ecology. New theoretical, experimental and comparative evidence suggests that variation in the adult sex ratio (ASR) is a key driver of variation in sex roles. Here, we first define and discuss the historical emergence of the sex role concept, including recent criticisms and rebuttals. Second, we review the various sex ratios with a focus on ASR, and explore its theoretical links to sex roles. Third, we explore the causes, and especially the consequences, of biased ASRs, focusing on the results of correlational and experimental studies of the effect of ASR variation on mate choice, sexual conflict, parental care and mating systems, social behaviour, hormone physiology and fitness. We present evidence that animals in diverse societies are sensitive to variation in local ASR, even on short timescales, and propose explanations for conflicting results. We conclude with an overview of open questions in this field integrating demography, life history and behaviour.

Adult sex ratios: causes of variation and implications for animal and human societies

Converging lines of inquiry from across the social and biological sciences target the adult sex ratio (ASR; the proportion of males in the adult population) as a fundamental population-level determinant of behavior. The ASR, which indicates the relative number of potential mates to competitors in a population, frames the selective arena for competition, mate choice, and social interactions. Here we review a growing literature, focusing on methodological developments that sharpen knowledge of the demographic variables underlying ASR variation, experiments that enhance understanding of the consequences of ASR imbalance across societies, and phylogenetic analyses that provide novel insights into social evolution. We additionally highlight areas where research advances are expected to make accelerating contributions across the social sciences, evolutionary biology, and biodiversity conservation.

Long-term trends and drought: Spatiotemporal variation in juvenile sex ratios of North American ducks

Sex ratios affect population dynamics and individual fitness, and changing sex ratios can be indicative of shifts in sex-specific survival at different life stages. While climate and landscape changes alter sex ratios of wild bird populations, long-term, landscape scale assessments of sex ratios are rare. Further, little work has been done to understand changes in sex ratios in avian communities. In this manuscript, we analyze long-term (1961-2015) data on five species of ducks across five broad climatic regions of the United States to estimate the effects of drought and long-term trends on the proportion of juvenile females captured at banding. As waterfowl have a 1:1 sex ratio at hatch, we interpret changes in sex ratios of captured juveniles as changes in sex-specific survival rates during early life. Seven of 12 species-region pairs exhibited evidence for long-term trends in the proportion of juvenile females at banding. The proportion of juvenile females at banding increased for duck populations in the western United States and typically declined for duck populations in the eastern United States. We only observed evidence for an effect of drought in two of the 12 species-region pairs, where the proportion of females declined during drought. As changes to North American landscapes and climate continue and intensify, we expect continued changes in sex-specific juvenile survival rates. More broadly, we encourage further research examining the mechanisms underlying long-term trends in juvenile sex ratios in avian communities.

Sex-Specific Life History Affected by Stocking in Juvenile Brown Trout

Salmonids are a socioeconomically and ecologically important group of fish that are often managed by stocking. Little is known about potential sex-specific effects of stocking, but recent studies found that the sexes differ in their stress tolerances already at late embryonic stage, i.e., before hatchery-born larvae are released into the wild and long before morphological gonad formation. It has also been speculated that sex-specific life histories can affect juvenile growth and mortality, and that a resulting sex-biassed demography can reduce population growth. Here we test whether juvenile brown trout (Salmo trutta) show sex-specific life histories and whether such sex effects differ in hatchery- and wild-born fish. We modified a genetic sexing protocol to reduce false assignment rates and used it to study the timing of sex differentiation in a laboratory setting, and in a large-scale field experiment to study growth and mortality of hatchery- and wild-born fish in different environments. We found no sex-specific mortality in any of the environments we studied. However, females started sex differentiation earlier than males, and while growth rates were similar in the laboratory, they differed significantly in the field depending on location and origin of fish. Overall, hatchery-born males grew larger than hatchery-born females while wild-born fish showed the reverse pattern. Whether males or females grew larger was location-specific. We conclude that juvenile brown trout show sex-specific growth that is affected by stocking and by other environmental factors that remain to be identified.

Cascading effects of pre-adult survival on sexual selection

Sexual selection influences broad-scale patterns of biodiversity. While a large body of research has investigated the effect of mate competition on sexual selection, less work has examined how pre-adult life history influences sexual selection. We used a mathematical framework to explore the influence of pre-adult survival on sexual selection. Our model suggests that pre-adult male mortality will affect the strength of sexual selection when a fixed number of adult males have an advantageous mate-acquisition trait. When a fixed number of males have an advantageous mate-acquisition trait, sexual selection is expected to increase when pre-adult mortality is relatively low. By contrast, if a fixed proportion (rather than number) of adult males have a mate-acquisition trait, pre-adult male mortality is not expected to affect the strength of sexual selection. Further, if the advantageous mating trait affects pre-adult survival, natural and sexual selection can interact to influence the overall selection on the mating trait. Given that pre-adult mortality is often shaped by natural selection, our results highlight conditions under which natural selection can have cascading effects on sexual selection.

Sex differences in immune gene expression in the brain of a small shorebird

Males and females often exhibit differences in behaviour, life histories, and ecology, many of which are typically reflected in their brains. Neuronal protection and maintenance include complex processes led by the microglia, which also interacts with metabolites such as hormones or immune components. Despite increasing interest in sex-specific brain function in laboratory animals, the significance of sex-specific immune activation in the brain of wild animals along with the variables that could affect it is widely lacking. Here, we use the Kentish plover (Charadrius alexandrinus) to study sex differences in expression of immune genes in the brain of adult males and females, in two wild populations breeding in contrasting habitats: a coastal sea-level population and a high-altitude inland population in China. Our analysis yielded 379 genes associated with immune function. We show a significant male-biased immune gene upregulation. Immune gene expression in the brain did not differ in upregulation between the coastal and inland populations. We discuss the role of dosage compensation in our findings and their evolutionary significance mediated by sex-specific survival and neuronal deterioration. Similar expression profiles in the coastal and inland populations suggest comparable genetic control by the microglia and possible similarities in pathogen pressures between habitats. We call for further studies on gene expression of males and females in wild population to understand the implications of immune function for life-histories and demography in natural systems.

Spatiotemporal variation in the adult sex ratio, male aggregation, and movement of two tropical cloud forest dung beetles

While theory suggests that at conception the sex ratio should be balanced (1:1), this can be variable across space and time in wild populations. Currently, studies of the environmental factors that regulate adult sex ratio (ASR) in species with different life history traits are scarce. Using capture-recapture over a year, we analyzed the influence of habitat type (forest and nonforest) and season (rainy and dry) on variation in ASR, male aggregation and the trajectory movement of 2 dung beetle species with different life history traits: Deltochilum mexicanum (a hornless roller species) and Dichotomius satanas (a tunneler species with horns on its head and thorax). We found opposite tendencies. The D. mexicanum population tends to be female-biased, but the population of D. satanas tends to be predominantly male, and observed values were not related to habitat type or season. However, the 95% confidence intervals estimated were highly variable between seasons depending on habitat. On examining the monthly variation in ASR for both habitats, we found that it depends on the species. In addition, male aggregation differed between species depending on habitat type and season, and species movement patterns were closely related to their habitat preferences. Based on our results, we argue that comparative population studies of species with different life history traits are necessary to understand the variation in demographic parameters as well as its ecological and evolutionary implications in the face of spatial and climatic environmental variation.

Reproductive value and the evolution of altruism

Altruism is favored by natural selection provided that it delivers sufficient benefits to relatives. An altruist's valuation of her relatives depends upon the extent to which they carry copies of her genes - relatedness - and also on the extent to which they are able to transmit their own genes to future generations - reproductive value. However, although relatedness has received a great deal of attention with regard to altruism, reproductive value has been surprisingly neglected. We review how reproductive value modulates patterns of altruism in relation to individual differences in age, sex, and general condition, and discuss how social partners may manipulate each other's reproductive value to incentivize altruism. This topic presents opportunities for tight interplay between theoretical and empirical research.

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.

Evolution of large males is associated with female-skewed adult sex ratios in amniotes

Body size often differs between the sexes (leading to sexual size dimorphism, SSD), as a consequence of differential responses by males and females to selection pressures. Adult sex ratio (ASR, the proportion of males in the adult population) should influence SSD because ASR relates to both the number of competitors and available mates, which shape the intensity of mating competition and thereby promotes SSD evolution. However, whether ASR correlates with SSD variation among species has not been yet tested across a broad range of taxa. Using phylogenetic comparative analyses of 462 amniotes (i.e., reptiles, birds, and mammals), we fill this knowledge gap by showing that male bias in SSD increases with increasingly female-skewed ASRs in both mammals and birds. This relationship is not explained by the higher mortality of the larger sex because SSD is not associated with sex differences in either juvenile or adult mortality. Phylogenetic path analysis indicates that higher mortality in one sex leads to skewed ASR, which in turn may generate selection for SSD biased toward the rare sex. Taken together, our findings provide evidence that skewed ASRs in amniote populations can result in the rarer sex evolving large size to capitalize on enhanced mating opportunities.

Consequences of combining sex-specific traits

Males and females follow distinct life-history strategies that have co-evolved with several sex-specific traits. Higher investment into parental investment (PI) demands an increased lifespan. Thus, resource allocation toward an efficient immune system is mandatory. In contrast, resources allocated toward secondary sexual signals (ornamentation) may negatively correlate with investment into immunity and ultimately result in a shorter lifespan. Previous studies have addressed how resource allocation toward single sex-specific traits impacts lifetime reproductive success (LRS). However, the trade-offs between diverse sex-specific characteristics and their impact on LRS remain largely unassessed impeding our understanding of life-history evolution. We have designed a theoretical framework (informed by experimental data and evolutionary genetics) that explores the effects of multiple sex-specific traits and assessed how they influence LRS. From the individual sex-specific traits, we inferred the consequences at the population level by evaluating adult sex ratios (ASR). Our theory implies that sex-specific resource allocation toward the assessed traits resulted in a biased ASR. Our model focuses on the impact of PI, ornamentation, and immunity as causal to biased ASR. The framework developed herein can be employed to understand the combined impact of diverse sex-specific traits on the LRS and the eventual population dynamics of particular model systems.

Sex Ratios in a Warming World: Thermal Effects on Sex-Biased Survival, Sex Determination, and Sex Reversal

Rising global temperatures threaten to disrupt population sex ratios, which can in turn cause mate shortages, reduce population growth and adaptive potential, and increase extinction risk, particularly when ratios are male biased. Sex ratio distortion can then have cascading effects across other species and even ecosystems. Our understanding of the problem is limited by how often studies measure temperature effects in both sexes. To address this, the current review surveyed 194 published studies of heat tolerance, finding that the majority did not even mention the sex of the individuals used, with <10% reporting results for males and females separately. Although the data are incomplete, this review assessed phylogenetic patterns of thermally induced sex ratio bias for 3 different mechanisms: sex-biased heat tolerance, temperature-dependent sex determination (TSD), and temperature-induced sex reversal. For sex-biased heat tolerance, documented examples span a large taxonomic range including arthropods, chordates, protists, and plants. Here, superior heat tolerance is more common in females than males, but the direction of tolerance appears to be phylogenetically fluid, perhaps due to the large number of contributing factors. For TSD, well-documented examples are limited to reptiles, where high temperature usually favors females, and fishes, where high temperature consistently favors males. For temperature-induced sex reversal, unambiguous cases are again limited to vertebrates, and high temperature usually favors males in fishes and amphibians, with mixed effects in reptiles. There is urgent need for further work on the full taxonomic extent of temperature-induced sex ratio distortion, including joint effects of the multiple contributing mechanisms.

Neoptile feathers contribute to outline concealment of precocial chicks

Camouflage is a widespread strategy to increase survival. The cryptic plumage colouration of precocial chicks improves camouflage often through disruptive colouration. Here, we examine whether and how fringed neoptile feathers conceal the outline of chicks. We first conducted a digital experiment to test two potential mechanisms for outline concealment through appendages: (1) reduction of edge intensity and (2) luminance transition. Local Edge Intensity Analysis showed that appendages decreased edge intensity whereas a mean luminance comparison revealed that the appendages created an intermediate transition zone to conceal the object's outline. For edge intensity, the outline diffusion was strongest for a vision system with low spatial acuity, which is characteristic of many mammalian chick predators. We then analysed photographs of young snowy plover (Charadrius nivosus) chicks to examine whether feathers increase outline concealment in a natural setting. Consistent with better camouflage, the outline of digitally cropped chicks with protruding feathers showed lower edge intensities than the outline of chicks without those feathers. However, the observed mean luminance changes did not indicate better concealment. Taken together, our results suggest that thin skin appendages such as neoptile feathers improve camouflage. As skin appendages are widespread, this mechanism may apply to many organisms.

Offspring desertion with care? Chick mortality and plastic female desertion in Snowy Plovers

Offspring desertion is often a plastic behavioral strategy that requires precise timing as the termination of parental care may have profound consequences for the fitness of parents and offspring. However, the decision process involved with termination of care is still poorly understood. Snowy Plovers Charadrius nivosus show highly flexible brood care with some females deserting the brood early and re-mate, whereas others provide extended care until the young are independent. Using a dynamic modeling framework, we investigated the effect of multiple factors on the decision-making process of female brood care in Ceuta, Mexico over a 7-year period. Females were more likely to stay with larger broods, while their probability of care was lower at the beginning of the season, when re-mating opportunities are higher than later in the season. Offspring condition at hatching did not influence the length of female care. Chick death and offspring desertion frequently coincided, suggesting that deteriorating offspring condition may trigger female desertion. Females deserted broods with high survival prospects when their absence did not impact negatively chick survival. Conversely, females deserted broods with low survival prospects when chick mortality despite female care reduced the value of the brood and re-mating was still possible. This suggests that female Snowy Plovers are sensitive to the needs and the value of their broods and adjust their parental care strategy accordingly. Taken together, we conclude that offspring desertion is a highly plastic behavior that allows females to maximize their reproductive success in a stochastic environment.

Mortality cost of sex-specific parasitism in wild bird populations

Sex-specific mortality is frequent in animals although the causes of different male versus female mortalities remain poorly understood. Parasitism is ubiquitous in nature with widespread detrimental effects to hosts, making parasitism a likely cause of sex-specific mortalities. Using sex-specific blood and gastrointestinal parasite prevalence from 96 and 54 avian host species, respectively, we test the implications of parasites for annual mortality in wild bird populations using phylogenetic comparative methods. First, we show that parasite prevalence is not different between adult males and females, although Nematodes showed a statistically significant but small male-biased parasite prevalence. Second, we found no correlation between sex-biased host mortalities and sex-biased parasite prevalence. These results were consistent in both blood and gastrointestinal parasites. Taken together, our results show little evidence for sex-dependent parasite prevalence in adults in wild bird populations, and suggest that parasite prevalence is an unlikely predictor of sex difference in adult mortalities, not withstanding sampling limitations. We propose that to understand causes of sex-biased mortalities, more complex analyses are needed that incorporate various ecological and life history components of animals life that may include sex differences in exposure to predators, immune capacity and cost of reproduction.

Causes and consequences of an unusually male-biased adult sex ratio in an unmanaged feral horse population

The adult sex ratio (ASR) is important within ecology due to its predicted effects on behaviour, demography and evolution, but research examining the causes and consequences of ASR bias have lagged behind the studies of sex ratios at earlier life stages. Although ungulate ASR is relatively well-studied, exceptions to the usual female-biased ASR challenge our understanding of the underlying drivers of biased ASR and provide an opportunity to better understand its consequences. Some feral ungulate populations, including multiple horse populations, exhibit unusually male-biased ASR. For example, research suggests that the feral horse Equus ferus caballus population on Sable Island, Nova Scotia, Canada may exhibit a male-biased ASR. Such exceptions to the rule provide a valuable opportunity to reveal the contributions of environmental context and trait differences to ASR bias. We aimed to test for bias in Sable Island horse ASR, identify the demographic drivers of bias, and explore its demographic and social consequences. To do this, we used life history, movement and group membership information for hundreds of horses followed through a long-term individual-based study between 2007 and 2018. Sable Island horse ASR is male biased and this skew has increased over time, reaching 62% male in 2018. Our life table response experiment suggested that ASR skew was driven predominantly by male-biased adult survival. Further analyses pointed to sex-biased survival being driven by reduced female survival post-reproduction. Male-biased ASR was associated with reduced harem sizes, an increase in the number of social groups on the island, and reduced reproduction in young females. Our results support the idea that male-biased ASR in feral ungulate populations may be caused by a combination of high population density and high reproductive output. We suggest that female-biased mortality may be caused by females continuing to reproduce at high density, and thus being more susceptible to resource shortages. Thus, our results highlight the strong context dependence of ASR. Furthermore, our work indicates the potential for ASR to substantially alter a population's social organisation. Such changes in social structure could have knock-on consequences for demography by altering the formation/stability of social relationships, or competition for matings.

Successful breeding predicts divorce in plovers

When individuals breed more than once, parents are faced with the choice of whether to re-mate with their old partner or divorce and select a new mate. Evolutionary theory predicts that, following successful reproduction with a given partner, that partner should be retained for future reproduction. However, recent work in a polygamous bird, has instead indicated that successful parents divorced more often than failed breeders (Halimubieke et al. in Ecol Evol 9:10734-10745, 2019), because one parent can benefit by mating with a new partner and reproducing shortly after divorce. Here we investigate whether successful breeding predicts divorce using data from 14 well-monitored populations of plovers (Charadrius spp.). We show that successful nesting leads to divorce, whereas nest failure leads to retention of the mate for follow-up breeding. Plovers that divorced their partners and simultaneously deserted their broods produced more offspring within a season than parents that retained their mate. Our work provides a counterpoint to theoretical expectations that divorce is triggered by low reproductive success, and supports adaptive explanations of divorce as a strategy to improve individual reproductive success. In addition, we show that temperature may modulate these costs and benefits, and contribute to dynamic variation in patterns of divorce across plover breeding systems.

An effect size statistical framework for investigating sexual dimorphism in non-avian dinosaurs and other extinct taxa

Despite reports of sexual dimorphism in extinct taxa, such claims in non-avian dinosaurs have been rare over the last decade and have often been criticized. Since dimorphism is widespread in sexually reproducing organisms today, under-reporting in the literature might suggest either methodological shortcomings or that this diverse group exhibited highly unusual reproductive biology. Univariate significance testing, especially for bimodality, is ineffective and prone to false negatives. Species recognition and mutual sexual selection hypotheses, therefore, may not be required to explain supposed absence of sexual dimorphism across the grade (a type II error). Instead, multiple lines of evidence support sexual selection and variation of structures consistent with secondary sexual characteristics, strongly suggesting sexual dimorphism in non-avian dinosaurs. We propose a framework for studying sexual dimorphism in fossils, focusing on likely secondary sexual traits and testing against all alternate hypotheses for variation in them using multiple lines of evidence. We use effect size statistics appropriate for low sample sizes, rather than significance testing, to analyse potential divergence of growth curves in traits and constrain estimates for dimorphism magnitude. In many cases, estimates of sexual variation can be reasonably accurate, and further developments in methods to improve sex assignments and account for intrasexual variation (e.g. mixture modelling) will improve accuracy. It is better to compare estimates for the magnitude of and support for dimorphism between datasets than to dichotomously reject or fail to reject monomorphism in a single species, enabling the study of sexual selection across phylogenies and time. We defend our approach with simulated and empirical data, including dinosaur data, showing that even simple approaches can yield fairly accurate estimates of sexual variation in many cases, allowing for comparison of species with high and low support for sexual variation.

Deer Indirectly Alter the Reproductive Strategy and Operational Sex Ratio of an Unpalatable Forest Perennial

Environmental conditions impose restrictions and costs on reproduction. Multiple reproductive options exist when increased reproductive costs drive plant populations toward alternative reproductive strategies. Using 4 years of demographic data across a deer impact gradient, where deer alter the abiotic environment, we parameterize a size-dependent integral projection model for a sexually labile and unpalatable forest perennial to investigate the demographic processes driving differentiation in the operational sex ratio (OSR) of local populations. In addition to a relative increase in asexual reproduction, our results illustrate that nontrophic indirect effects by overabundant deer on this perennial result in delayed female sex expression to unsustainably large plant sizes and lead to more pronounced plant shrinkage following female sex expression, effectively increasing the cost of reproduction. Among plants of reproductive age, increased deer impact decreases the size-dependent probability of flowering and reduces reproductive consistency over time. This pattern in sex expression skews populations toward female-biased OSRs at low deer impact sites and male-biased OSRs at intermediate and high deer impact sites. While this shift toward a male-biased OSR may ameliorate pollen limitation, it also decreases the effective population size when coupled with increased asexual reproduction. The divergence of reproductive strategies and reduced lifetime fitness in response to indirect deer impacts illustrate the persistent long-term effects of overabundant herbivores on unpalatable understory perennials.

Mate fidelity in a polygamous shorebird, the snowy plover (Charadrius nivosus)

Social monogamy has evolved multiple times and is particularly common in birds. However, it is not well understood why some species live in long-lasting monogamous partnerships while others change mates between breeding attempts. Here, we investigate mate fidelity in a sequential polygamous shorebird, the snowy plover (Charadrius nivosus), a species in which both males and females may have several breeding attempts within a breeding season with the same or different mates. Using 6 years of data from a well-monitored population in Bahía de Ceuta, Mexico, we investigated predictors and fitness implications of mate fidelity both within and between years. We show that in order to maximize reproductive success within a season, individuals divorce after successful nesting and re-mate with the same partner after nest failure. Therefore, divorced plovers, counterintuitively, achieve higher reproductive success than individuals that retain their mate. We also show that different mating decisions between sexes predict different breeding dispersal patterns. Taken together, our findings imply that divorce is an adaptive strategy to improve reproductive success in a stochastic environment. Understanding mate fidelity is important for the evolution of monogamy and polygamy, and these mating behaviors have implications for reproductive success and population productivity.

Sex roles and the evolution of parental care specialization

Males and females are defined by the relative size of their gametes (anisogamy), but secondary sexual dimorphism in fertilization, parental investment and mating competition is widespread and often remarkably stable over evolutionary timescales. Recent theory has clarified the causal connections between anisogamy and the most prevalent differences between the sexes, but deviations from these patterns remain poorly understood. Here, we study how sex differences in parental investment and mating competition coevolve with parental care specialization. Parental investment often consists of two or more distinct activities (e.g. provisioning and defence) and parents may care more efficiently by specializing in a subset of these activities. Our model predicts that efficient care specialization broadens the conditions under which biparental investment can evolve in lineages that historically had uniparental care. Major transitions in sex roles (e.g. from female-biased care with strong male mating competition to male-biased care with strong female competition) can arise following ecologically induced changes in the costs or benefits of different care types, or in the sex ratio at maturation. Our model provides a clear evolutionary mechanism for sex-role transitions, but also predicts that such transitions should be rare. It consequently contributes towards explaining widespread phylogenetic inertia in parenting and mating systems.

Adult sex ratio influences mate choice in Darwin's finches

The adult sex ratio (ASR) is an important property of populations. Comparative phylogenetic analyses have shown that unequal sex ratios are associated with the frequency of changing mates, extrapair mating (EPM), mating system and parental care, sex-specific survival, and population dynamics. Comparative demographic analyses are needed to validate the inferences, and to identify the causes and consequences of sex ratio inequalities in changing environments. We tested expected consequences of biased sex ratios in two species of Darwin's finches in the Galápagos, where annual variation in rainfall, food supply, and survival is pronounced. Environmental perturbations cause sex ratios to become strongly male-biased, and when this happens, females have increased opportunities to choose high-quality males. The choice of a mate is influenced by early experience of parental morphology (sexual imprinting), and since morphological traits are highly heritable, mate choice is expressed as a positive correlation between mates. The expected assortative mating was demonstrated when the Geospiza scandens population was strongly male-biased, and not present in the contemporary Geospiza fortis population with an equal sex ratio. Initial effects of parental imprinting were subsequently overridden by other factors when females changed mates, some repeatedly. Females of both species were more frequently polyandrous in male-biased populations, and fledged more offspring by changing mates. The ASR ratio indirectly affected the frequency of EPM (and hybridization), but this did not lead to social mate choice. The study provides a strong demonstration of how mating patterns change when environmental fluctuations lead to altered sex ratios through differential mortality.

Equitable Chick Survival in Three Species of the Non-Migratory Shorebird Despite Species-Specific Sexual Dimorphism of the Young

Simple Summary

We aimed to determine whether seasonal brood sex-ratio, sex-biased chick survival, and sex specific dimorphism at hatching or during growth occurs among three species of resident Australian shorebird. Our results describe no sex-bias in chick production, survival or growth rates between sexes for any of the three species studied.

Abstract

Sex-biases in populations can have important implications for species’ social biology, population demography and mating systems. It has recently been suggested that in some shorebirds, sex-specific bias in survival of precocial young may occur. This may be driven by variation in the brood sex-ratio and/or the sexual size dimorphism of young birds, which may influence predator escape capacity. Understanding the survival of young birds remains a significant knowledge gap for many taxa, especially when young birds are mobile and cryptic. Our aims were to estimate the sex-ratio variation in three species of Australian resident shorebird, specifically to determine: (1) whether seasonal brood sex-ratio variation at hatching is occurring, (2) the extent of any sex-biased chick survival, (3) if sex specific dimorphism at hatching or during growth occurs; and, (4) whether escape capacity differs between the sexes. We radio-tracked 50 Masked Lapwing Vanellus miles, 42 Red-capped Plover Charadrius ruficapillus and 27 Hooded Plover Thinornis cucullatus chicks from individual broods, examined the likelihood of hatchlings being male or female based on the hatching date within the breeding season, and compared size at hatching, growth and mortality of chicks of different sexes. There was no sex-bias with the hatching date across the breeding season, nor were there differences in survival or growth rates between sexes for any of the three species studied. In one species, male hatchlings had longer tarsi than females, but this did not result in differential escape propensity or improved survival. In conclusion, the hatching date, survival and growth of chicks from three species of resident shorebird was not influenced by their sex.

Natal habitat and sex-specific survival rates result in a male-biased adult sex ratio

The adult sex ratio (ASR) is a crucial component of the ecological and evolutionary forces shaping the dynamics of a population. Although in many declining populations ASRs have been reported to be skewed, empirical studies exploring the demographic factors shaping ASRs are still rare. In this study of the socially monogamous and sexually dimorphic Black-tailed Godwit (Limosa limosa limosa), we aim to evaluate the sex ratio of chicks at hatch and the subsequent sex-specific survival differences occurring over 3 subsequent life stages. We found that, at hatch, the sex ratio did not deviate from parity. However, the survival of pre-fledged females was 15-30% lower than that of males and the sex bias in survival was higher in low-quality habitat. Additionally, survival of adult females was almost 5% lower than that of adult males. Because survival rates of males and females did not differ during other life-history stages, the ASR in the population was biased toward males. Because females are larger than males, food limitations during development or sex-specific differences in the duration of development may explain the lower survival of female chicks. Differences among adults are less obvious and suggest previously unknown sex-related selection pressures. Irrespective of the underlying causes, by reducing the available number of females in this socially monogamous species, a male-biased ASR is likely to contribute to the ongoing decline of the Dutch godwit population.

Sex-biased survival contributes to population decline in a long-lived seabird, the Magellanic Penguin

We developed a Hidden Markov mark-recapture model (R package marked) to examine sex-specific demography in Magellanic Penguins (Spheniscus magellanicus). Our model was based on 33 yr of resightings at Punta Tombo, Argentina, where we banded ~44,000 chicks from 1983 to 2010. Because we sexed only 57% of individuals over their lifetime, we treated sex as an uncertain state in our model. Our goals were to provide insight into the population dynamics of this declining colony, to inform conservation of this species, and to highlight the importance of considering sex-specific vital rates in demographic seabird studies. Like many other seabirds, Magellanic Penguins are long-lived, serially monogamous, and exhibit obligate biparental care. We found that the non-breeding-season survival of females was lower than that of males and that the magnitude of this bias was highest for juveniles. Biases in survival accumulated as cohorts aged, leading to increasingly skewed sex ratios. The survival bias was greatest in years when overall survival was low, that is, females fared disproportionality worse when conditions were unfavorable. Our model-estimated survival patterns are consistent with independent data on carcasses from the species' non-breeding grounds, showing that mortality is higher for juveniles than for adults and higher for females than for males. Juveniles may be less efficient foragers than adults are and, because of their smaller size, females may show less resilience to food scarcity than males. We used perturbation analysis of a population matrix model to determine the impact of sex-biased survival on adult sex ratio and population growth rate at Punta Tombo. We found that adult sex ratio and population growth rate have the greatest proportional response, that is, elasticity, to female pre-breeder and adult survival. Sex bias in juvenile survival (i.e., lower survival of females) made the greatest contribution to population declines from 1990 to 2009. Because starvation is a leading cause of morality in juveniles and adults, precautionary fisheries and spatial management in the region could help to slow population decline. Our data add to growing evidence that knowledge of sex-specific demography and sex ratios are necessary for accurate assessment of seabird population trends.