Sexual Selection Research Articles (Page 1)

Evidence for the evolution of male and female cardinals' plumage colouration being affected by both natural and sexual selection.

Iris color is a conspicuous and diverse trait across animals, but its evolutionary drivers are poorly understood. In over 1000 species of Tyranni passerines, we tested hypotheses relating iris color to signaling and camouflage, its main putative functions. We expected that, if playing a role in signaling, brighter irises would be positively associated with the strength of social and sexual selection as well as signal conspicuousness, i.e., larger relative eye sizes and contrasting, darker plumage colors around the eyes (contrasting local backgrounds). If aiding in camouflage, darker irises would be associated with higher vulnerability to predation, darker habitats, larger relative eye sizes and matching, darker plumage colors around the eyes (matching local backgrounds). In support of the signaling hypothesis, we found that brighter irises are associated with darker plumage around the eyes, larger body sizes, territoriality, and sexual size dimorphism. In support of the camouflage hypothesis, we found that darker irises are associated with higher vulnerability to predation (exposed nests, migratory behavior, and terrestrial lifestyles), and larger relative eye sizes, which are associated with darker habitats. Our findings suggest that bright irises are social/sexual signals mostly associated with species under weaker selection favoring camouflage, which has implications for sensory ecology and macroevolutionary patterns of visual signaling.

Sexual selection due to mate preference for certain traits can maintain phenotypic diversity within populations and species. In taxa with discrete phenotypes, such as color polymorphic species, assortative mating may lead to disruptive selection and sympatric divergence, yet how such interactions vary over species' ranges remains poorly understood. To address this shortcoming, we examined spatial patterns of sexual size dimorphism and presumptive male-female mating pairs based on body size and coloration in the color polymorphic Eastern Red-backed Salamander, Plethodon cinereus. This species exhibits two common color morphs, striped and unstriped, that also differ in other elements of their biology, including ecology, behavior, and morphology. Across six populations that vary in color morph frequency, we predicted each site would show female-biased sexual size dimorphism and a positive body size relationship between presumptive mating pairs. In polymorphic sites, we also predicted that morphs would demonstrate color-based assortative mating, with larger body size as an associated trait. We found female-biased sexual size dimorphism in three of six study sites, while a positive body size relationship between male and female pairs was documented in four populations, and no evidence of male-female associations by color and body size was found in polymorphic populations. The spatial variation across all of our study axes demonstrates the importance of geographic context in shaping sexual selection dynamics and patterns of local adaptation.

Although sexual selection is a well-established part of evolutionary biology, controversies remain about the roles of males and females. For instance, despite clear evidence of male mate choice across a very broad range of species, traditional views of male and female sex roles-the former competitive, the latter choosy-are still common. In addition, studies looking at mate choice in natural populations, especially in terms of male mate choice, remain limited. Here, we consider body size, an important phenotype in mate choice in many species, and its association with patterns of non-random mating in wild populations of two species of seed bugs, Spilostethus pandurus and Lygaeus creticus. We found strong directional pre-copulatory sexual selection for larger females in both species. On the other hand, patterns of selection on male size differed between the two species. There was directional sexual selection for larger individuals in L. creticus, and stabilizing selection for intermediate-sized males in S. pandurus. Our results suggest that while males and females in both species mate non-randomly with respect to the body size of their partner, male pre-copulatory mate choice may be an important component of selection on females in the wild.

ABSTRACT Domestic animals living in the wild pose a serious threat to wildlife, with anthropogenic hybridization and disease transmission being two of the primary concerns. However, the behavioral and ecological mechanisms behind these risks remain poorly understood, especially for rare and elusive taxa. The European wildcat has been the target of a number of studies focused on both hybridization with domestic cats and opportunistic disease surveys, but little attention has been paid to determining the ultimate mechanisms that govern the relationships between these two cat species. Through a seven‐year (2017–2024) field study based on telemetry, camera trapping, and direct observation, we investigated the spatial relationships between wildcats and domestic cats and the prevalence of shared pathogens within a Mediterranean area of sympatry in southeastern Spain. We found evidence of behavioral barriers for both hybridization and disease transmission between European wildcats and domestic cats. This includes hierarchical interspecies exclusion enforced by wildcats, as well as sexual selection exerted by wildcat females. Our results have important implications for the conservation strategies of the European wildcat, and our methodological protocol could guide further etho‐ecological investigations on this and other rare and/or elusive taxa.

The ecology of mating interactions determines a species' mating system, yet whether environmental change can alter the mating system of a species remains unclear. Elevated temperatures can cause male sterility, prompting females to remate for fertility assurance. In monandrous systems, heat-induced male infertility poses a significant extinction risk, as females may mate exclusively with infertile males. A key question is whether male sterility could drive polyandry in a typically monandrous system. Here we address this by examining genetic variance underlying both male fertility resilience to heat stress and facultative polyandry, and assessing the fitness consequences of each mating system. We used isofemales lines of Drosophila subobscura, a monandrous species, exposing males to developmental heat stress. Male heat stress generated sterility and females mated to these males typically remated. While significant genetic variation in male fertility sensitivity and female remating emerged at moderate to high temperatures, we found little genetic variation in plasticity for polyandry. These results indicate evolutionary potential in both traits, but that a shift in mating system would arise through selection on genes associated with polyandry, rather than plasticity. Polyandry improved offspring production after initially mating to a sterile male, but did not fully restore reproductive output relative to fertile monandrous pairs, and mating with heat-stressed males increased female mortality. Heat stress also altered mating behaviour which could impact female mate choice. Together, these findings show that increasing temperatures may shape species' mating systems and the interplay between thermal ecology and sexual selection under climate change.

Contaminants influence the outcomes of the male-male competition component of sexual selection under field conditions.

In plants, sexual selection is argued to operate through reproduction to shape plant traits. Pollen size and style length are two traits that potentially reflect male-male competition and female choice. Adaptations in one trait may elicit an evolutionary response in the other, leading to correlated evolution at higher taxonomic levels. Many studies have examined their correlation across diverse taxonomic groups, with inconsistent results that limit our understanding of the selective forces and processes affecting these traits. To address this, we conducted a comprehensive meta-analysis on 51 studies from 43 published articles. The results uncovered a significant positive correlation between pollen size and style length across studies. To validate this finding further, we compiled a data set comprising 1041 species from 421 genera in 89 families, with measurements of both pollen size and style length. Phylogenetically controlled analyses confirmed a consistent positive correlation between these traits. Furthermore, our analysis revealed a significant effect of pollination vector on pollen size variation. Specifically, mammal-pollinated species produced substantially larger pollen grains, while wind-pollinated species showed no significant difference in size relative to those pollinated by Diptera, Hymenoptera, or Aves. Pollen size also exhibited a negative correlation with pollen number at macroevolutionary scales. Overall, this study underscores a correlated evolutionary pattern between pollen size and style length across angiosperms, calling for more in-depth investigations on the mechanisms by which sexual selection shapes traits that convey male competitive ability and female preference.

In many migratory birds, arrival at breeding grounds is sexually asynchronous. This pattern is often explained by the idea that early arrival benefits the sex under stronger sexual selection by improving access to mates through territoriality. However, in species where females arrive first and are non-territorial, this explanation falls short. One alternative is that early-arriving females are simply in better condition. Here we report, for the first time, evidence of migratory protogyny in the Icelandic population of Red-necked Phalaropes (Phalaropus lobatus), a shorebird with reversed sexual dimorphism. We also present the first direct test of the condition-dependent hypothesis in a protogynous species, using arrival data, morphometric measurements, and leukocyte profiles. Although females arrived significantly earlier than males, we found no evidence that early arrival was linked to superior body condition. Leukocyte profiles did not suggest marked sex differences either, although we had insufficient statistical power to test this. Our results point instead to the role of sex-specific migratory strategies and intrasexual competition in shaping arrival timing. With its unique life history, P. lobatus offers a powerful model for studying the evolution of sex-biased migration. We encourage further research integrating fine-scale tracking, reproductive monitoring, and physiological data to better understand ...

Abstract Understanding the drivers of biodiversity is a central goal in evolutionary biology. In particular, sexual selection has long been proposed as a potential catalyst of speciation, but empirical evidence remains inconclusive. Here, we present a comprehensive meta-analysis synthesizing 145 effect sizes from 50 comparative studies testing the relationship between proxies of sexual selection and species diversity across the animal kingdom. Our results reveal a modest but consistent positive association (global effect size: r = 0.201; 95% confidence interval: 0.035–0.366), supporting the hypothesis that sexual selection contributes to speciation. However, the global effect size corresponds to an R2 of only 0.04, suggesting that sexual selection is not a dominant driver of speciation. We also uncover substantial heterogeneity among effect sizes, largely attributable to between-study variation and taxonomic affinities of effect sizes. Studies that fail to account for phylogenetic non-independence tend to report stronger effects. In contrast, other tested methodological and biological moderators, such as the proxies used to estimate the strength of sexual selection or proxies of speciation, do not explain the observed heterogeneity in effect sizes. Sensitivity analyses confirm the robustness of our results, and we find no signatures of publication bias. We highlight the need for broader taxonomic coverage and a greater focus on understudied mechanisms, such as post-copulatory sexual selection, to refine our understanding of the role of sexual selection in shaping species diversity.

Abstract Background Evolution education is a central tenet of biology education in school, yet the topic is conceptually complex and students’ understanding is fraught with misconceptions. Learning Progression Analytics (LPA) aims to trace students’ conceptual development along established learning progressions. For this purpose, data from students’ interactions with tasks in digital learning environments are analysed. This is done with the intention to make conceptual change and knowledge integration processes accessible to teachers for formative assessment and feedback. One assessment strategy that can mirror such processes in conceptually complex topics is concept mapping. This study presents an initial attempt to analyse concept maps that students created over the course of a digital teaching unit on factors of evolution (i.e., mutation, natural and sexual selection, genetic drift, gene flow). Our aim was to determine which metrics could be suitable for the use in LPA. Method We collected data from 250 high school students who participated in a hybrid teaching unit on five factors of evolution in the school year 2022/23. Students completed a pre- and posttest and created a total of five concept maps over the course of the unit, repeatedly revising and reworking their previous maps. We split the students into three groups based on their gain from pre- to posttest and analysed their maps for differences (1) between the different measurement points and (2) between the groups at each measurement point regarding (a) their similarity to expert concept maps, (b) concept scores, and (c) different network metrics. Results We found significant differences between most of the consecutive measurement points for all calculated metrics (e.g., number of nodes and links, concept scores) across the sample. We found significant differences between the three groups for the average degree and number of edges at two measurement points. Conclusions From our results, we conclude that the most promising metrics from our study for the use of concept maps in LPA are those focusing on the connections (i.e., average degree and number of edges). Further research is needed to refine these assessments in controlled environments and determine their value for automated assessment and feedback more definitely.

Abstract Interactions between natural and sexual selection have been integral to the development of sexual selection theory, yet the role of natural selection in sexual trait evolution has received far less empirical attention than the role of sexual selection, and the extent to which natural and sexual selection are aligned remains an important empirical question. Insect cuticular hydrocarbons (CHCs) provide a good model for exploring interactions between natural and sexual selection because of their dual roles in sexual signalling and desiccation resistance. We used the geometric framework for nutrition to explore the potential for, and magnitude of trade‐offs between male attractiveness, desiccation resistance and the abundance and blend of CHCs in the cricket Teleogryllus oceanicus . Attractiveness and desiccation resistance were both maximized on a diet that was rich in carbohydrate relative to protein. Although maximum expressions occurred at significantly different locations in nutritional space, the difference in angles between maximum attractiveness and desiccation resistance were small. Attractiveness and desiccation resistance were both associated with CHCs of moderate, rather than maximum, abundance and with a blend that was rich in two shorter chained alkenes relative to a single longer chained alkane. Partial correlations between fitness traits suggested that CHCs contribute indirectly to male attractiveness via their role in desiccation resistance. Our findings are consistent with the idea that natural and sexual selection on CHCs can be broadly aligned, facilitating rapid adaptation to ecological conditions and promoting speciation. Read the free Plain Language Summary for this article on the Journal blog.

Sexual selection theory predicts that males with larger bodies and chelipeds achieve higher reproductive success in porcellanid crabs (Decapoda Anomura), which exhibit sexual size dimorphism. However, the relationship between male size and reproductive success remains unexplored. This study tested whether male reproductive success is size-dependent in porcellanid crabs using Petrolisthes japonicus as a model species. We conducted 108 mating trials under laboratory conditions, each involving a female and two males of different sizes. During the trials, we observed mating behaviors and assessed spawning, fecundity, and offspring size of the females, all of which influence male reproductive success. Copulation occurred in 64 trials, with agonistic interactions occasionally observed between males. Larger males initiated more frequent contests against smaller males, suggesting a competitive advantage. In 59 trials, females copulated once with either a large male or a small male, while double matings occurred in five trials. Females copulated exclusively with large males in 44 trials and with small males in 18 trials, demonstrating a significant bias toward larger males. Copulation probability with the large male increased significantly when the small male was much smaller than the female. Mating approaches also varied: when mating with large males, approach patterns were equally distributed among male, female, and mutual approaches, while mating with small males occurred primarily via male-initiated approaches. Male size did not affect female spawning, fecundity, or offspring size, indicating that male reproductive success primarily depends on the number of females they copulate with and the fecundity of those females. Our results highlight the large-male mating advantage in P. japonicus, with both male-male competition and female mate choice likely contributing to this pattern. Further experimental studies are needed to clarify the relative importance of these mechanisms in shaping reproductive success in this species.

Seminal fluid proteins (Sfps) are essential for reproductive success and evolve fast on average, possibly driven by post-copulatory sexual selection (PCSS) originating from sperm competition and cryptic female choice. Counterintuitively, however, the coding region only in few Sfps evolves adaptively. Hence, additional genomic and functional factors must play a role in Sfp evolution independent of the protein coding region. To shed light on drivers of Sfp evolution we focus on those Sfps predominantly expressed in male accessory glands of Drosophila to examine their evolution in the tissue which produces the majority of Sfps. Unlike the testis, the accessory glands are known to develop normally in hybrids, allowing us to control for cellular environment differences arising during speciation. Here, we identify hotspots of rapid evolution in accessory gland protein genes (Acp) promoters, driven by base changes and insertions/deletions (indels). We further show that changes in promoter sequences are accompanied by gene expression divergence among closely related species. Using hybrids, we demonstrate that species-specific expression divergence is maintained for some Acps, while others exhibit dominance of one allele. These results indicate that regulatory evolution, rather than genome background variation, drives Acp expression changes and promotes their rapid evolution.

Polyandry influences sexual behaviour and supports sexual selection in moths, incl. Noctuids. In polyandry, a female mates with multiple males within a few days. Copulation is a multi-phase process, as sperm are packed into spermatophores and transferred to the female genitalia. This process is costly, since spermatophore production and transfer is time-consuming. When this process is repeated, it results in sexual selection and enhances heterozygosity. Pest and migrating Noctuids are often polyandrous. Polyandry is common in the "pest clade" of Noctuidae (e.g., Euxoa, Dichagyris). We hypothesised that the asymmetric allocation of sensory and stimulating functions on the genitalia enhance sexual selection and promote speciation. In polyandrous species of the subtribe Poliina, we show that the evolution of the secondary asymmetry of male external genitalia is connected to an increase in species diversity. Some species inhabiting temperate forests were found to be moderately (Orthosia) or highly (Conistra) polyandrous, in connection with their different life cycles and despite similar reproductive timing. Based on fecundity data of some common species, we hypothesised that habitat generalists should be more polyandrous than habitat specialists. However, our data are insufficient to determine whether ecology and/or phylogeny influence the level of polyandry. Other factors, such as the connection between the sex ratio and the level of polyandry, should be surveyed in future studies, considering their relevance to plant protection.

ABSTRACTMales of many frog and toad species advertise in leks. In these systems, female choice is based on male advertisement calls, which have been selected to convey information about an individual's quality. As such, calling behavior is an important aspect of reproductive fitness. Factors such as disease and infection can affect calling behavior, yet the direction and strength of these effects, as well as their underlying mechanisms, remain unclear. Calls are typically multicomponent displays, and traits within these displays can vary independently from one another both among and within individuals. It is important to understand the proximate infection‐imposed effects on signal production, as it allows us to make inferences about the downstream evolutionary consequences of such signaling. We studied the effects of Batrachochytrium dendrobatidis (Bd) infection on spring peeper (Pseudacris crucifer) advertisement calling behavior. We predicted that information about infection status would be present in dynamic traits (i.e., those that have high within‐individual variation) such that males with higher Bd infection loads would exhibit call traits less attractive to females. Overall, infection had little effect on male calling. There was no main effect of infection on call rate or dominant frequency. However, Bd infection did have body condition‐dependent effects on call duration, a trait with intermediate levels of within‐individual variation. As infection loads increased, males in better‐than‐average condition tended to have longer call durations, while males in worse‐than‐average conditions tended to have shorter call durations. Our results suggested that some males are more behaviorally tolerant than others when infected, meaning they invest more energy into current reproductive efforts (calling behavior) compared to future ones (fighting off disease). This may have interesting implications for the potential evolution of signals within these populations, as sexual selection for more attractive calls may also be selecting for more infected males.

The rate and chromosomal positioning of meiotic recombination significantly affects the distribution of the genetic diversity in eukaryotic genomes. Many studies have revealed sex-specific recombination patterns, with male recombination typically biased toward chromosome ends, while female recombination is more evenly distributed along chromosomes. It has been proposed that such pattern in females may counteract meiotic drive caused by selfish genetic elements associated with centromeres and should not occur in species devoid of clearly defined centromeres, but evidence for this expectation is scarce. Here, we constructed a sex-specific genetic map of a species with holocentric chromosomes, the bulb mite (Rhizoglyphus robini), a model organism for sexual selection studies with heritable alternative male reproductive phenotypes. We found a similar recombination landscape in both males and females, with a consistent pattern of increased rates towards both chromosome ends, and a higher recombination rate in females than in males. Recombination rate was positively correlated with repeat density and negatively with gene density. Our results are consistent with the meiotic drive hypothesis and suggest that the evolution of recombination patterns is closely linked to chromosome features.

Sexual selection potentially drives the evolution of exaggerated traits used in intrasexual contests. However, the extent to which mating systems influence weapon morphology remains unclear. In fiddler crab males, an exaggerated claw functions both as a weapon and a signaling tool, varying according to the species' mating system. We examined claw evolution in male fiddler crabs, differentiating between two main mating strategies: 1. males defend their mating burrows (= 'burrow'); 2. males do not mate in their own burrows (= 'surface'). We measured claw morphological traits and tested whether the mating system affects their evolutionary rates, expecting 'burrow' species to exhibit higher evolutionary rates. In general, claw size scales isometrically with body size across species. Both systems showed no correlation between claw elements and mechanical advantage, indicating the necessity of maintaining a conspicuous signaling tool alongside an efficient lever system for grip strength as body size increases. Contrary to predictions, however, 'burrow' males exhibited lower evolutionary rates in claw traits than 'surface' males, suggesting stronger stabilizing selection. These findings highlight the nuanced effects of sexual selection on male fiddler crab weapon evolution, suggesting that mating systems can modulate evolutionary trajectories, yet functional demands for dual weapon-signal roles constrain claw morphology.

The regime of selection acting on a trait is expected to shape its static allometry. Few studies, however, have quantified the form of sexual selection acting on a trait in the wild to test whether the trait allometrically scales as predicted. Even fewer studies have tested these predictions using males expressing weapon polymorphism as part of their alternative mating strategies. Here, I use field data to test how sexual selection shapes scaling allometries of male weaponry in the Wellington tree wētā (H. crassidens), a male-trimorphic and harem-polygynous insect endemic to New Zealand. Contrary to the prediction that 10th instar males' large weaponry would scale hyperallometrically because it is under direct sexual selection, I found that 10th instar weaponry is not subject to direct sexual selection and scales hypoallometrically. Similarly, neither 8th nor 9th instar male weaponry experiences direct sexual selection, and their weaponry scales hyperallometrically and hypoallometrically, respectively. My study suggests that disentangling competing hypotheses for the evolution of scaling patterns of sexually selected traits must go beyond a simple viability-sexual selection dichotomy by also considering weapon function and the ecological context within which the weapon is used.

Gamete dynamics (GD) theory for the evolutionary transition from isogamy to anisogamy relies on the biophysics of fertilisation, combining the dynamics of gamete limitation and gamete competition with the provisioning requirements of gametes and zygotes. A recent development by Siljestam & Martinossi-Allibert which incorporates competitive gamete traits (motility and target size) into anisogamy evolution reaches very different conclusions from previous GD predictions, and challenges current views on sexual selection. We develop models of these traits under conventional GD theory assumptions showing that (i) unless gamete limitation is strong and the trait is more efficient in the larger gamete, such traits tend to arise in the male gamete, complying with previous analyses predicting that sexually selected expenditures are most likely to arise in males, (ii) gamete trait evolution does not alter the conditions under which anisogamy evolves from isogamy, (iii) the differences between our results and those of Siljestam & Martinossi-Allibert arise from their specific function for zygote survival, which is arguably less plausible than those used previous GD theory, and (iv) as a novel finding, we show that the coevolution of gamete size with other gamete-level traits (motility, jelly coats, chemoattractants) can result in the evolution of slight anisogamy.