Does competition for nests affect genetic monogamy in Cory's shearwater Calonectris diomedea?

Most bird species are socially monogamous. However, extra‐pair copulations (EPCs), resulting in extra‐pair paternity (EPP), commonly occur. EPCs should allow females to adjust social mate choice and allow males that fail to obtain a nest a chance to avoid missing a breeding season, especially when poor nest supply constrains social mate choice. Procellariiformes (albatrosses and petrels) are socially monogamous seabirds which seldom divorce, even when nest availability constrains social mate choice. In Cory's shearwater Calonectris diomedea, a burrow‐nesting petrel, two studies conducted in the Mediterranean, where competition for nests is weak, detected no EPP. EPP remains to be investigated at localities where competition for nests is much stronger, such as Vila islet, Azores archipelago, Atlantic Ocean. We conducted a genetic (microsatellites) study over two successive years on Vila, involving the breeding pairs of the same 65 nests each year and their single chick. EPPs occurred each year, the overall rate being 11.6%. Coupling genetic analyses to a 7‐year demographic survey provided additional data on pair bonds and competition for nests. Overall, cuckoldry was unrelated to divorce, nest density and inbreeding avoidance, but was more frequent when the social male was small. Nest changes were more costly for males than for females, and some apparently unpaired males attempted to dislodge social males during within‐pair copulations. These results are compatible with the existence of a link between poor nest availability and EPP and confirm that even species considered strongly monogamous can adopt flexible mating strategies.     

Rapid assessment of female preference for male size predicts subsequent choice of spawning partner in a socially monogamous cichlid fish

Although size-assortative mating in convict cichlids, Amatitliana nigrofasciata, is supposed to result from mutual mating preference for larger individuals, female choice in relation to male size remains ambiguous. We revisited the evidence for directional preference for larger males in female convict cichlids using a classical two-way choice apparatus in which each female could decide to spend time in front of a small male or a large one. We found evidence for female preference for large males, as assessed from association preference during a 4-hour period following encounter. Furthermore, females decided to spawn in front of the initially preferred male more often than expected by chance. Our results thus confirm the existence of a directional preference for large males in female convict cichlids, and indicate that association preference measured over a short period of time can provide a quick and reliable proxy for reproductive preference in this species.     

Sex roles and mutual mate choice matter during mate sampling

The roles of females and males in mating competition and mate choice have lately proven more variable, between and within species, than previously thought. In nature, mating competition occurs during mate search and is expected to be regulated by the numbers of potential mates and same-sex competitors. Here, we present the first study to test how a temporal change in sex roles affects mating competition and mate choice during mate sampling. Our model system (the marine fish Gobiusculus flavescens) is uniquely suitable because of its change in sex roles, from conventional to reversed, over the breeding season. As predicted from sex role theory, courtship was typically initiated by males and terminated by females early in the breeding season. The opposite pattern was observed late in the season, at which time several females often simultaneously courted the same male. Mate-searching females visited more males early than late in the breeding season. Our study shows that mutual mate choice and mating competition can have profound effects on female and male behavior. Future work needs to consider the dynamic nature of mating competition and mate choice if we aim to fully understand sexual selection in the wild.     

Lonely hearts or sex in the city? Density-dependent effects in mating systems

Two very basic ideas in sexual selection are heavily influenced by numbers of potential mates: the evolution of anisogamy, leading to sex role differentiation, and the frequency dependence of reproductive success that tends to equalize primary sex ratios. However, being explicit about the numbers of potential mates is not typical to most evolutionary theory of sexual selection. Here, we argue that this may prevent us from finding the appropriate ecological equilibria that determine the evolutionary endpoints of selection. We review both theoretical and empirical advances on how population density may influence aspects of mating systems such as intrasexual competition, female choice or resistance, and parental care. Density can have strong effects on selective pressures, whether or not there is phenotypic plasticity in individual strategies with respect to density. Mating skew may either increase or decrease with density, which may be aided or counteracted by changes in female behaviour. Switchpoints between alternative mating strategies can be density dependent, and mate encounter rates may influence mate choice (including mutual mate choice), multiple mating, female resistance to male mating attempts, mate searching, mate guarding, parental care, and the probability of divorce. Considering density-dependent selection may be essential for understanding how populations can persist at all despite sexual conflict, but simple models seem to fail to predict the diversity of observed responses in nature. This highlights the importance of considering the interaction between mating systems and population dynamics, and we strongly encourage further work in this area.     

Mate competition and resource competition are inter‐related in sexual selection

Sexual selection can be affected by the competition for limited breeding resources and/or the competition for limited mates. Although there is ample evidence for each type of competition by itself, little is known about their relative importance and interaction. To address these questions, we established 48 experimental breeding populations of the two‐spotted goby (Gobiusculus flavescens), a substrate‐breeding fish with paternal care. In three experimental treatments, males were limited in the access to either nest sites or mates or were provided with both nests and mates in excess. We quantified male competition behaviour (agonistic and courtship), the opportunity for selection and selection on male body size. Limited access to nests and mates produced similar opportunities for selection, but only limited access to mates increased male competitive behaviours and caused positive selection on male body size. Selection on body size in the mate‐limited treatment was due both to larger males being more likely to take up nests and to larger males being more likely to mate once they had a nest. These findings demonstrate that resource and mate limitation can differ in their effects on sexual selection. The results also reveal that resource and mating competition can be highly inter‐related and not always separated in time, implying that methods to disentangle the two processes must be chosen with care. Future research should consider experimental and analytical approaches similar to those of the present study in attempts to elucidate the interaction of resource and mating competition in animals.     

Male mate preference and size-assortative pairing in the convict cichlid

Male convict cichlids Cichlasoma nigrofasciatum presented three females simultaneously were found to prefer larger females, even if a female's size exceeded their own. This indicates that some mechanism other than male choice, such as female choice or intrasexual competition, must contribute to male-larger assortative pairing in convict cichlids. Despite a preference for larger females, males continued to consort with smaller females when available, and a females attractiveness was a function of her size relative to other females.     

Opportunity for sexual selection arises at moderate densities in the frog Rana chensinensis: an experimental approach

Heavily male-biased operational sex ratio and high male density characterizing explosively breeding anurans are expected to enhance the opportunity for sexual selection through large-male mating advantage brought about by severe male–male competition. However, our previous study on the temperate frog Rana chensinensis suggested that these two factors might restrict the opportunity under natural conditions for females to be grasped by any male they meet first, and scramble competition between males occurred less frequently because amplectant pairs hid on the bottom of ponds. In the present experimental study, in which the operational sex ratios and male densities were much lower and individual interactions were sufficiently longer than in natural population, we found a large-male pairing advantage in female mate choice, multiple mate choice, and male displacement experiments. These findings lend circumstantial support for the predication, although there was no evidence of large-female mating advantage in a male mate choice experiment.     

Space,density and extra-pair matings have opposing impacts on male and female reproductive success

Many territorial species have a mating system characterized by males establishing home ranges in the breeding grounds prior to females, resulting in males competing for territories and females choosing a mate upon their arrival. It remains unknown, however, how the outcomes of decisions surrounding territory establishment and mate choice are influenced by the spatial configuration of the breeding grounds. We use a spatially explicit, individual-based model to investigate the sex-specific effects of these decisions on reproductive success. In our model, males that arrive earlier obtain higher quality territories and improve their chances for extra-pair copulations. Females can choose their mate to maximize the quality of the male or to attempt to minimize the density of other females near their nesting site to avoid competition. Females therefore face a tradeoff between high-density regions around high-quality males and low-quality males in areas of low competition. Our model predicts a negative correlation between male and female reproductive success under a wide range of conditions when the majority of the territories are on the margins of the breeding area. Most notably, this sexual conflict arises as an edge effect suggesting that fragmentation of breeding habitats could impact the consequences of mate choice in many species with territorial breeding habits.     

How is female mate choice affected by male competition?

The plethora of studies devoted to the topics of male competition and female mate choice belie the fact that their interaction remains poorly understood. Indeed, on the question of whether competition should help or hinder the choice process, opinions scattered throughout the sexual selection literature seem unnecessarily polarised. We argue, in the light of recent theoretical and empirical advances, that the effect of competition on mate choice depends on whether it results in the choosy sex attaining high breeding value for total fitness, considering both direct and indirect fitness benefits. Specifically, trade-offs may occur between different fitness benefits if some are correlated with male competitive ability whilst others are not. Moreover, the costs and benefits of mating with competitive males may vary in time and/or space. These considerations highlight the importance of injecting a life-history perspective into sexual selection studies. Within this context, we turn to the sexual selection literature to try to offer insights into the circumstances when competition might be expected to have positive or negative implications for pre-copulatory female choice. In this regard, we elaborate on three stages where competition might impact upon the choice process: (i) during mate detection, (ii) mate evaluation, and (iii) in dictating actual mating outcomes. We conclude by offering researchers several potentially rewarding avenues for future research.     

Sexual selection and assortative mating: an experimental test

Mate choice and mate competition can both influence the evolution of sexual isolation between populations. Assortative mating may arise if traits and preferences diverge in step, and, alternatively, mate competition may counteract mating preferences and decrease assortative mating. Here, we examine potential assortative mating between populations of Drosophila pseudoobscura that have experimentally evolved under either increased (‘polyandry’) or decreased (‘monogamy’) sexual selection intensity for 100 generations. These populations have evolved differences in numerous traits, including a male signal and female preference traits. We use a two males: one female design, allowing both mate choice and competition to influence mating outcomes, to test for assortative mating between our populations. Mating latency shows subtle effects of male and female interactions, with females from the monogamous populations appearing reluctant to mate with males from the polyandrous populations. However, males from the polyandrous populations have a significantly higher probability of mating regardless of the female's population. Our results suggest that if populations differ in the intensity of sexual selection, effects on mate competition may overcome mate choice.     

Mate preference for multiple cues: interplay between male and nest size in the sand goby, Pomatoschistus minutus

Female mating preferences are often based on more than one cue.In empirical studies, however, different mate choice cues aretypically treated separately ignoring their possible interactions.In the current work, we studied how male body size and sizeof the male's nest jointly affect mate preferences of femalesand gobies, Pomatoschistus minutus. The females were givena binary choice between males that differed either in body sizeor size of their nest or both. We found that neither body sizenor size of the nest alone affected male attractiveness, buttogether these 2 cues had a significant effect. Specifically,large males were more popular among females when they had alarge nest than when they occupied a small nest. The resultssuggest that if interaction effects between multiple mate choicecues are not considered, there is a danger of ignoring or underestimatingthe importance of these cues in sexual selection by female choice.     

Juvenile convict cichlids shoaling decisions in relation to shoal size and age

Many animals form groups and socialize in response to evolutionary pressures such as predation, food availability, and mate acquisition. Evidence of social choice based on various phenotypic characters (Group Phenotypic Composition [GPC]) has been observed in several animal species. In addition to the physical characteristics of the social group, it is also interesting to consider how decisions of who to socialize with might be expected to change for an individual over time. Younger individuals with limited life experience may discriminate differently between social groups than older conspecifics who have had the opportunity to learn and who may be faced with different ecological or environmental pressures. Here, we used a traditional two‐choice design to explore the shoaling behavior of juvenile convict cichlids and determine whether the number of fish and/or the size/life stage of the individuals within a shoal influenced social choices. We found that juvenile convict cichlids spent more time shoaling with similarly sized juvenile individuals and also preferred to shoal with larger shoals, but not when shoals were comprised of adult fish. The size of the individuals in a shoal was a more influential factor than the size of the shoal itself. Size of individual juveniles was correlated with tendency to visit shoals, but was not correlated with overall time spent shoaling, regardless of shoal composition. As juveniles, convict cichlids can make discriminatory choices that are influenced by specific aspects of shoal composition.     

MODE OF SEXUAL SELECTION DETERMINED BY RESOURCE ABUNDANCE IN TWO SAND GOBY POPULATIONS

We used field observations and experiments to show that sexual selection in two populations of sand gobies, Pomatoschistus minutus (Pisces, Gobiidae), was affected by differences in resource availability. Male sand gobies rely on empty mussel shells for nest building and spawning. The two populations differed considerably in nest-site abundance and sexual-selection regimes. In one population nest sites were scarce, leading to stronger male-male competition over nests, a higher nest site colonization rate and reduced potential for female choice compared with the other population that had a surplus of nests. In the high-competition population, males were larger than females, perhaps as a response to selection, whereas the other population was not sexually size dimorphic. The results from the field were confirmed in a pool experiment that demonstrated the effect of nest abundance on nest occupancy and male reproductive success. Larger males were more successful in obtaining nest sites in both high and low nest availability treatments. Larger males were also favored by females as mating partners, but only in the treatment with surplus nest sites. Nest shortage was associated with an increased potential for intrasexual selection (measured as the coefficient of variation), whereas the potential for intersexual selection was increased when nests were common. In conclusion, nest-site abundance can influence the relative contribution of intrasexual competition and mate choice in a population. Hence, resource availability can contribute to within-species variation in mating patterns.     

Nonlinear effects of female mate choice in wild threespine sticklebacks

Sexual selection by mate choice represents a very important selective pressure in many animal species and might have evolutionary impacts beyond exaggeration of secondary sexual traits. Describing the shape and strength of the relationships linking mating success and nonsexual traits in natural conditions represents a challenging step in our understanding of adaptive evolution. We studied the effect of behavioral (nest site choice), immunological (trematode level of infection), genetic diversity (measured by mean d2) and morphological (standard length and pectoral fin size) traits on male mating success in a natural population of threespine sticklebacks Gasterosteaus aculeatus. Male mating success was measured by microsatellite genotyping of embryos used to infer female genotypes. First, we analyzed all territorial males (full analysis) but also considered independently only males with a nonzero mating success (reduced analysis) because some of the males with no eggs could have been part of a later breeding cycle. Multiple linear regressions identified a significant negative effect of parasite load in the full analysis, whereas no linear effect was found in the reduced analysis. The quadratic analyses revealed that nest location and parasite load were significantly related to mating success by positive (concave selection) and negative (convex selection) quadratic coefficients respectively, resulting in a saddle-shaped fitness surface. Moreover, there were significant interactions between nest location, mean d2 and parasite load in the reduced analysis. The subsequent canonical rotation of the matrix of quadratic and cross-product terms identified two major axes of the response surface: a vector representing mostly nest site choice and a vector representing parasite load. These results imply that there exists more than one way for a male threespine stickleback to maximize its mating success and that such nonlinear relationships between male mating success induced by female mate choice and male characteristics might have been overlooked in many studies.     

Mating patterns of Minshan's toad (Bufo minshanicus) from three populations along an altitudinal gradient

The large-male mating advantage and size-assortative mating are two different size-based patterns, which deviate from random mating in toads. These two pairing patterns may arise due to female choice, male-male competition, male choice, or a combination of these. This study investigated the mating system of Minshan's toad (Bufo minshanicus) from three populations along an altitudinal gradient during two breeding reasons in the northeastern Tibetan plateau. Our study shows that males found in amplexus with females were larger on average than non-amplectant males in two sites with higher operational sex ratios. Similarly, in those sites, males and females found in amplexus maintained an optimal size ratio. These data suggest that male-male competition leads to size-assortative mating in the lack of mate choice (female and male mate choice) by Minshan's toad, as larger males performed higher frequencies for taking-over other low quality ones with amplectant females.     

Constraints on Size-assortative Mating in the Blister Beetle Tegrodera aloga (Coleoptera: Meloidae)

The mating system of Tegrodera aloga is similar to other blister beetles that have evolved sizeassortative mating in that males pass a cantharidin-rich spermatophore to their mates and females vary in size and fecundity. Despite this, previous studies found no assortative mating in this beetle. Results of this study suggest that nonassortative mating is not due to absolute constraints on mate choice. Males courted large females more frequently than small females, suggesting that males prefer big mates. Similarly, female choice is suggested by a large-male mating advantage in the absence of size-related male-male competition. In contrast to previous work, my results suggest that assortative mating may occur under certain conditions and may be due to large-phenotype mating advantages. The question remains, why does assortative mating occur only some of the time? One hypothesis is that assortative mating breaks down when sex ratios become male biased and males no longer discriminate between mates. However, although sex ratios can vary from day to day, assortative mating is not associated with periods when females outnumber males. Rather, the pattern appears to be associated with times of low overall population density. Hypotheses for density-dependent assortative mating are presented.     

Male mating constraints affect mutual mate choice: prudent male courting and sperm-limited females

Costs of sperm production may lead to prudence in male sperm allocation and also to male mate choice. Here, we develop a life history-based mutual mate choice model that takes into account the lost-opportunity costs for males from time out in sperm recovery and lets mate competition be determined by the prevailing mate choice strategies. We assume that high mating rate may potentially lead to sperm depletion in males, and that as a result, female reproduction may be limited by the availability of sperm. Increasing variation in male quality leads, in general, to increased selective mate choice by females, and vice versa. Lower-quality males may, however, gain access to more fecund higher-quality females by lowering their courting rate, thus increasing their sperm reserves. When faced with strong male competition for mates, low-quality males become less choosy, which leads to assortative mating for quality and an increased mating rate across all males. With assortative mating, the frequency of antagonistic interactions (sexual conflict) is reduced, allowing males to lower the time spent replenishing sperm reserves in order to increase mating rate. This in turn leads to lower sperm levels at mating and therefore could lead to negative effects on female fitness via sperm limitation.     

Seasonal sex role changes in the blenniid Petroscirtes breviceps, a nest brooder with paternal care

The plasticity of the sex roles in the blenniid fish Petroscirtes breviceps , a nest brooder with exclusive paternal care, was studied throughout an 8 month breeding season. Males performed most courtships early and late in the breeding season, whereas females performed most in the middle of the season. These results indicated that the sex of individuals initiating courtship changed seasonally, with courtship role reversal in the middle of the season. Intrasexual aggression in both sexes occurred much more frequently in mid-season than in the early and late seasons. Males frequently fought when available nest sites were limited, regardless of the presence of females, suggesting that males competed for nests in order to qualify to mate (resource competition). In contrast, courting females fought only in mid-season, when females' relative success in entering nests decreased, indicating that females competed for limited mating opportunities (mating competition). The reversed courtship roles and female mating competition in mid-season suggested that the sex roles in P. breviceps changed seasonally from the conventional roles to reversed roles and back again during one breeding season. This study provides the first empirical evidence of multiple changes in the sex roles of animals within a breeding season.     

Why is mutual mate choice not the norm? Operational sex ratios,sex roles and the evolution of sexually dimorphic and monomorphic signalling.

Biases in the operational sex ratio (OSR) are seen as the fundamental reason behind differential competition for mates in the two sexes, and as a strong determinant behind differences in choosiness. This view has been challenged by Kokko and Monaghan, who argue that sex-specific parental investment, mortalities, mate-encounter rates and quality variation determine the mating system in a way that is not reducible to the OSR. We develop a game-theoretic model of choosiness, signalling and parental care, to examine (i) whether the results of Kokko and Monaghan remain robust when its simplifying assumptions are relaxed, (ii) how parental care coevolves with mating strategies and the OSR and (iii) why mutual mate choice is observed relatively rarely even when both sexes vary in quality. We find qualitative agreement with the simpler approach: parental investment is the primary determinant of sex roles instead of the OSR, and factors promoting choosiness are high species-specific mate-encounter rate, high sex-specific mate-encounter rate, high cost of breeding (parental investment), low cost of mate searching and highly variable quality of the opposite sex. The coevolution of parental care and mating strategies hinders mutual mate choice if one parent can compensate for reduced care by the other, but promotes it if offspring survival depends greatly on biparental care. We argue that the relative rarity of mutual mate choice is not due to biases in the OSR. Instead, we describe processes by which sexual strategies tend to diverge. This divergence is prevented, and mutual mate choice maintained, if synergistic benefits of biparental care render parental investment both high and not too different in the two sexes.