Operational Sex Ratio Research Articles (Page 1)

In many species, demographic assessments of population viability require an estimate of the number or proportion of breeding adults in a population that are male (the breeding sex ratio). However, this estimate is often difficult to obtain directly in species with multiple paternity when males are difficult to sample. Parentage analysis of breeding females and offspring can produce this estimate by identifying the number of unique males that contribute genetic information to (i.e., sired) a given cohort. There is an added challenge of choosing a sample design with the desired level of confidence to identify all the fathers contributing to a cohort, either at the scale of individual clutches or an entire nesting season, given limited resources. Sampling effort can be defined as the number of offspring sampled per clutch, or the number of clutches sampled per breeding season, depending on the analysis. The minimum number of samples required may depend on the proportions of eggs that different fathers fertilize in a clutch (the paternal contribution mode), the total number of fathers fertilizing a clutch, the proportion of adults available for breeding that are male (the operational sex ratio), and population size. We conducted power analyses to quantify the confidence in identifying all fathers in animal populations with multiple paternity. We simulated sampling a theoretical sea turtle population with a range of population demographics, mating systems, and sampling effort, and used the proportion of 10,000 simulations in which all fathers were identified as a proxy for confidence. At the clutch level, confidence was strongly dependent on the paternal contribution mode, and when it was skewed, it also depended on the total number of fathers contributing and the number of offspring sampled. However, sampling about one third of a clutch was sufficient to identify all fathers with high confidence for most scenarios, unless the paternal contribution mode was extremely skewed and there were many contributing fathers, such that some fathers fertilized very few eggs and were difficult to detect. At the scale of an entire nesting season, confidence was more strongly affected by the operational sex ratio, the proportion of clutches sampled, and the presence or absence of polygyny than by the lesser effects of paternal contribution mode and within-clutch sample size. Sampling fewer offspring from more clutches increased confidence compared to sampling more offspring from fewer clutches. Relaxing the minimum required proportion of fathers identified from 100% to 90% led to high confidence while sampling 50% to a maximum of 75% of clutches, depending on the mating system, even as the population size increased by an order of magnitude. Our approach and results can be widely informative for sample design as well as quantifying uncertainty in existing and future estimates of the number of breeding males in populations with multiple paternity.

Abstract Many organisms with broad distributions show latitudinal variations in morphological phenotypes and life history traits, such as body size and phenology, in relation to environmental changes such as temperature along latitude. Such variations have usually been considered the result of natural selection, but sexual selection may also lead to these latitudinal patterns. Although a recent study has shown the latitudinal pattern in the strength of male–male competition in medaka fish, such a latitudinal pattern related to sexual selection is rarely known in other organisms. Here, we show the latitudinal pattern of a reproductive trait driven by sexual selection in the Japanese black salamander (Hynobius nigrescens), where snout‐vent length (SVL) in males predicts the outcome of male–male competition over egg sacs. First, we conducted phylogenetic analyses to examine the phylogenetic pattern along latitude. From the constructed phylogenetic tree, this species was split into five lineages that were roughly divided along latitude. We also used field surveys to examine whether the operational sex ratio (OSR: an index of the strength of male–male competition) varies across lineages with latitude. We found that the OSR was more biased toward males in a lineage distributed at lower latitudes due to its longer breeding period. We measured the SVLs of collected samples to determine if the latitudinal pattern also exists for SVL. Indeed, male SVLs were longer in lineages distributed at lower latitudes, whereas those in females did not differ among lineages. Our common garden experiment also showed that the individuals from a lineage distributed at lower latitudes had longer SVLs even when they grew under the same environmental conditions, suggesting that the latitudinal pattern in SVL is genetically determined. These results suggest that males at lower latitudes have evolved longer SVLs, driven by stronger male–male competition. Our study provides the first example, to the best of our knowledge, of a latitudinal pattern driven by sexual selection and its evolutionary determinant in detail in the wild.

Alternative reproductive tactics (ARTs) describe non-reversible or flexible alternative strategies that secure fertilization. For example, some male defend territories with females while others attempt sneaky matings. Often, ARTs are considered to be status-dependent and are explained by differences in mass or competitive ability. However, most studies on ARTs only approximate their fitness effect, ignore males that never reproduced and consider status (e.g. weight) as the sole mediator of ARTs. We used 244 male mice, Mus musculus domesticus, from semi-natural populations, to describe ARTs in Mus Musculus for the first time. We followed males throughout their life and categorized them as territorials or roamers over multiple monthly intervals, after validating our method of assigning tactics with detailed spatial data. We explored if tactic choice is repeatable, whether multiple social and/or intrinsic factors predict tactic choice and transitions between tactics, and tested for fitness and physiological differences between ARTs. Tactic choice was repeatable, but males switched flexibly between tactics. Tactic choice was associated with mass, age, the operational sex ratio and population size. Territorials had a higher probability of reproduction, but a lower gonadosomatic index. Our results reveal a personality component of ARTs, confirm equal mean fitness among tactics and suggest tactic choice as a multifaceted decision under various selective pressures.

Reproductive interactions between species could have negative effects on the fitness of the species involved, which can have important ecological and evolutionary consequences, such as population declines (including local extinction) or character divergence. Here, we report the courtship and attempted mating between two congeneric species of fireflies endemic to Mexico. The interactions involved males of the synchronous firefly Photinus palaciosi and females of the much larger, non-synchronous P. extensus. In the study site, the population density of P. palaciosi is much higher than that of P. extensus. Observations of marked P. extensus females throughout most of the mating season showed that 37.8% of their interactions with males were with P. palaciosi males. Although interspecific interactions were usually of shorter length, they frequently consumed a significant portion of the nightly mate-locating/courting period. These interspecific interactions are probably facilitated by the similarities in the mate location and courtship behavior of both species, which also share female brachyptery (elytra and wing reduction that makes females unable to fly). The simplest hypothesis to explain our behavioral observations is that P. palaciosi males mistakenly courted P. extensus females. The available evidence suggests that the operational sex ratio (OSR) of P. palaciosi is male-biased, as it seems to be the case in all synchronous fireflies studied to date. We hypothesize that the intense male competition for mates resulting from a male-biased OSR explains, at least in part, the “indiscriminate” sexual responses of P. palaciosi males. Another still not studied factor that could contribute to the frequent interspecific sexual interactions observed is the degree of similitude of the mating signals. The relatively high frequency of interspecific interactions and the significant amount of time invested in many of them (relative to the duration of the nightly mating period) indicate that the study of the potential fitness costs (and benefits?) of these interactions is a promising line of research.

Predicting the strength and direction of sexual selection is a challenge, as the effects of ecological factors, social environment and behavioural plasticity all need to be considered. The operational sex ratio (OSR) is a key variable, which has been shown to (i) affect the strength and direction of mating competition, as a social environment cue, and (ii) be affected itself by ecological conditions through sex‐specific environmental effects. Gaining a global view of (i) and (ii) in wild populations represents a necessary step for our understanding of sexual selection dynamics in the wild. Here, we address this challenge within the reaction norm framework.We conducted an extensive field study on the two‐spotted goby Pomatoschistus flavescens, monitoring six populations along a latitudinal gradient during an entire breeding season. We compared the temporal trajectories in social environment and sexual displays across populations, which is unprecedented.Using a reaction norm framework based on OSR theory, we show that what appears to be great variation in sexual displays across populations and sampling times, follows consistent rules. Sexual display behaviour followed behavioural reaction norms in response to the social environment that were consistent across populations, but social environment fluctuations were specific to each population.Recording behaviour not only over time, but also along a latitudinal gradient where ecological conditions vary and in turn affect OSR, was necessary to reveal the relationship between social environment and sexual displays, which in turn contributes to sexual selection dynamics.

Sexual selection promotes traits that enhance mating or fertilization success, but these traits can be very costly under harsh environmental conditions. The extent to which differential investment in costly traits under varying intensities of sexual selection is related to their susceptibility to environmental stress remains unclear. This study explored how experimental evolution under different operational sex ratios (OSRs) shapes traits and reproductive success of male Drosophila prolongata, and how developmental and/or adult heat stress affect the expression of these traits. We found males from even and slightly male-biased OSRs to be larger and display greater reduction in body size under developmental heat stress, suggesting pre-mating sexual selection on body size and condition-dependent thermal sensitivity. These populations also exhibited consistently high mating and fertilization success across temperatures, potentially indicating selection for robust phenotypes with "good genes" that perform well regardless of temperature. Conversely, males from strongly male-biased OSR populations experienced more pronounced decline in sperm competitiveness following exposure to developmental or adult heat stress. These results highlight how environmental stressors differentially impact populations, shaped by varying strengths of pre- and post-mating sexual selection. These observed patterns suggest potential interactions between past selection and the ability to adapt to changing environments.

Compared to our closest primate relatives, human life history involves greater longevity, which includes a distinctive postmenopausal life stage. Given mammalian reproductive physiology in which females build a finite stock of cells that can become oocytes early in life, which then continuously deplete mostly through cell death while males produce new sperm throughout adulthood, the postmenopausal stage makes the sex ratio in the fertile pool, called the adult sex ratio (ASR), male biased. Additionally, this affects a more fine-grained ratio, the operational sex ratio (OSR), defined as the ratio of males to females currently able to conceive. Here, we construct an ODE model in which males compete for paternities using either a multiple-mating or mate-guarding strategy. Our focus is on investigating the differences of strategy choice between populations with varying life histories, which include a distinct post-fertile stage for adult females. By simulating the system, we determine the dominant strategy and its dependence on various parameter combinations. Our results show that an increase in OSR and ASR correlates well with a change in the dominant strategy from multiple mating to guarding.

The intensity and direction of sexual selection are intricately linked to the social and ecological context. Both operational sex ratios (OSRs) and population densities can affect the ability of males to monopolize resources and mates, and thus the form and intensity of sexual selection on them. Here, we studied how the mating system of the promiscuous and strongly sexually dimorphic fruit fly Drosophila prolongata responds to changes in the OSR and population density. We recorded groups of flies over five days and quantified territory occupancy, mating success (MS) and competitive fertilization success. Although sexual selection was stronger under male-biased than even OSRs but unrelated to density, realized selection on morphological traits was higher under even OSRs and increased with density. Larger and more territorial males achieved both higher MS and competitive fertilization success, but only under even OSRs. Our combined results also support a shift in the mating system from territorial contest competition to scramble competition under male-biased OSRs and potentially at low density, where there was no clear contribution of the measured traits to reproductive success. Our study emphasizes the limitations of traditional selection metrics and the role of the socio-ecological context in predicting adaptation to a changing environment.

Body size is one of the most important traits in the life history of vertebrates. In this work, we analyzed the morphometric traits of breeding males of the eastern Pacific green sea turtle population known as the black sea turtle on the coast of Michoacan, Mexico. The morphometric analysis indicates that males have the smallest body size compared to other males in other Chelonia populations. The size of male black sea turtles is even smaller (by 15.2 cm in average CCL) in carapace curve length (70.5 cm CCL) than females (85.7 CCL) of the same population. This suggests that males reach maturity at an earlier age than females and would have implications for mating success. The assessment of the operational sex ratio was conducted over a multi-year period (2004, 2009, 2017, and 2023). From a total of 336 h of observation, we identified 653 mating groups containing 1986 males and 669 females. The estimated operational sex ratio (OSR) during the study period was 2.96 males for every female. Among the mating groups, 34.3% consisted of only one male, while 65.7% included two or more males, with the number of males per female ranging from 2 to 17. The analysis revealed that there are interannual and monthly variations in OSR values, with observations showing a range from 2.3:1 (male to female) in 2023 to 4.3:1 (male to female) in 2009. The OSR variations show an evolving trend that can inform future strategies. In September, the ratio was 2.8:1, which changed to 3:1 in October and November, and then reached 3.3:1 in December (male/female). This gradual increase provides a clear opportunity to delve deeper into these dynamics and take proactive measures to address them positively.

Abstract The parental roles of males and females differ remarkably across the tree of life, and several studies suggest that parental sex roles are associated with biased sex ratios. However, there is considerable debate on the causal relationship between sex roles and sex ratios and on the relative importance of the operational sex ratio (OSR), the adult sex ratio (ASR), and the maturation sex ratio (MSR). Here, we use individual-based evolutionary simulations to investigate the joint evolution of sex-specific parental behavior and the various sex ratios in several life history scenarios. We show that typically, but not always, the sex with lower mortality or faster maturity tends to provide most of the care. The association of parental sex roles with the various sex ratios is more intricate. At equilibrium, the OSR is typically biased toward the less caring sex, but the direction and strength of OSR biases may change considerably during evolution. When the MSR or ASR is biased, a broad spectrum of parental care patterns can evolve, although the overrepresented sex generally does most of the caring. We conclude that none of the sex ratios is a driver of parental sex roles; they rather coevolve with care biases in a subtle manner.

ABSTRACTAedes aegpyti mosquitoes are vectors of several viruses of major public health importance, and many new control strategies target mating behaviour. Mating in this species occurs in swarms characterised by male scramble competition and female choice. These mating swarms have a male‐biased operational sex ratio, which is expected to generate intense competition among males for mating opportunities. However, it is not known what proportion of swarming males successfully mate with females, how many females each male is able to mate with, and to what extent any variation in the male mating success phenotype can be explained by genetic variation. Here, we describe a novel assay to quantify individual male mating success in the presence of operational sex ratios characteristic of Ae. aegypti. Our results demonstrate that male mating success is skewed. Most males do not mate despite multiple opportunities, and very few males mate with multiple females. We compared measures of male mating success between fathers and sons and between full siblings to estimate the heritability of the trait in the narrow h2 and broad H2 sense, respectively. We found significant broad sense heritability estimates but little evidence for additive genetic effects, suggesting a role for dominance or epistatic effects and/or larval rearing environment in male mating success. These findings enhance our understanding of sexual selection in this species and have important implications for mass‐release programmes that rely on the release of competitive males.

Commonly used two-sex discrete-time population projection models rely on mating functions developed for continuous-time frameworks that overestimate the number of unions between reproductive individuals. This has important consequences for our understanding of the evolution and demography of two-sex populations and consequently for management and conservation. Here, we propose a novel mating function that is robust by obeying all properties necessary to be ecologically valid and flexible by accommodating all mating systems and efficiency in mating encounters. We illustrate the usefulness of this novel function with an application to the sexually size-dimorphic and polygynous wild boar (Sus scrofa). We show that the population growth rate depends on the harem size, the operational sex ratio, and the mating efficiency. This novel function can be applied to all mating systems and tactics and is highly relevant in the context of global changes under which mating systems and mating efficiency are expected to change.

The floral biology of Calamus is relatively unknown except for several species. In this study, Calamus lobbianus and Calamus pygmaeus were selected to represent the non-climbing rattan of the Sundaland’s flagellate group. Observations on phenology, floral rewards and floral visitors as well as experiments on the breeding mechanism and operational sex ratio were performed for both species. For both species, anthesis started in the early morning for pistillate and staminate plants, ended by late noon (staminate plants) but lasted till the next day in pistillate plants for both species. Although both species appeared to be aseasonal in flowering, C. lobbianus exhibited a male-biased population while C. pygmaeus did not exhibit any sex bias. Nectar was observed to be extruded from the base of the petals of C. lobbianus of the staminate flowers and sterile staminate flowers of the pistillate plants. The concentration and volume of the nectar of the staminate flower and sterile staminate flower of C. lobbianus peaked at c. 11% and 9 μL around 1100 (Day 1) and c. 13% and 8 μL around 0930 (Day 1), respectively, but only appeared as a layer of glistening exudate in C. pygmaeus. Floral scent was not detected in any of the inflorescences of both species. Several insect visitors were observed to be the primary visitors of both Calamus species which include two species of Tetragonula, a species of Liostenogaster sp., and Stenodyneriellus sp. Experiments on breeding mechanism of C. lobbianus and C. pygmaeus indicate that both species are most likely apomictic. C. pygmaeus is capable of vegetative propagation through the rooting at the tip of inflorescence.

Sexual size dimorphism theory predicts biased operational sex ratios (OSRs) and an uneven distribution of males among certain females. We studied this phenomenon through a field census of the giant wood spider Nephila pilipes (family Nephilidae) in Singapore, a species where females are, on average, 6.9 times larger than males. Specifically, we tested two hypotheses concerning male distribution, given their tendency to aggregate in certain female webs. The optimal female size hypothesis predicts that males would predominantly occupy webs of intermediate-sized females. The web clustering hypothesis posits that more males would be found in webs closer together compared to those farther apart. Our snapshot census revealed a female-biased OSR (females: males = 1.85) with an uneven distribution of males in female webs. Most males were found in webs of intermediate-sized females aligning with the optimal female size hypothesis. Proximity among female webs was indicative of male presence, lending support to the web clustering hypothesis. While our study's limited sample size warrants caution, we conclude that in N. pilipes, male occupation of female webs is facilitated by the clustering of webs, and males prefer to cohabit with optimally sized, receptive females.

A size-based, histological analysis of the reproductive life history of the blacktip grouper, Epinephelus fasciatus (Forsskål 1775), was conducted in Indonesia to evaluate the error rate associated with macroscopic reproductive analysis. Histological results indicated that E. fasciatus was protogynous with female L50 at 13.4 cm total length (LT) and a size at sexual transition of 22.0 cm LT. The weight-length relationship for the species was W = 0.011 L3.13. Overall sex ratios were significantly female biased, operational sex ratios were significantly male biased, and sex ratios of mature individuals varied predictably with length from female to male dominance as size increased. No significant relationship between length and batch fecundity was found. The population has a spawning period from February to August. Overall, 54.4% of macroscopic evaluations were incorrect compared to histological results. Of the errors, 14.8% were a failure to detect ovotestes, 12.7% were classifying non-gonadal tissue as ovary or testis, 12.2% were misclassifying sex, and 12.7% were misclassifying maturity status. However, the largest source of error (47.7%) was from misclassifying both sex and maturity status. Of these, 92.9% were macroscopically classified as immature females, but were histologically confirmed to be mature males. Compared to histological results, the only accurate macroscopic results were the absence of a sex-based difference in weight-length relationship and spawning seasonality estimated by a gonadosomatic index (February-June). The use of macroscopic methods to estimate reproductive life-history parameters for sex-changing reefes fish may introduce significant inaccuracies and misinterpretations. Of the parameters estimated by histological methods, size at maturity, size-specific sex ratios, and spawning seasonality have the greatest potential to inform local fishery management policy.

Gender inequality in workloads explained by operational sex ratio

P. euphratica stands as the pioneering and dominant tree within desert riparian forests in arid and semi-arid regions. The aim of our work was to reveal why dioecious P. euphratica in natural desert riparian forests in the lower Tarim River exhibits sexual spatial distribution differences combined with field investigation, tree ring techniques, isotope analysis techniques, and statistical analyses. The results showed that P. euphratica was a male-biased population, with the operational sex ratio (OSR) exhibiting spatial distribution differences to variations in drought stress resulting from groundwater depth change. The highest OSR was observed under mild drought stress (groundwater depth of 6-7 m), and it was reduced under non-drought stress (groundwater depth below 6 m) or severe drought stress (groundwater depth exceeding 7 m). As drought stress escalated, the degradation and aging of the P. euphratica forest became more pronounced. Males exhibited significantly higher growth rates and WUEi than females under mild drought stress. However, under severe drought stress, males' growth rates significantly slowed down, accompanied by significantly lower WUEi than in females. This divergence determined the sexual spatial segregation of P. euphratica in the natural desert riparian forests of the lower Tarim River. Furthermore, the current ecological water conveyance project (EWCP) in the lower Tarim River was hard to fundamentally reverse the degradation and aging of the P. euphratica forest due to inadequate population regeneration. Consequently, we advocated for an optimized ecological water conveyance mode to restore, conserve, and rejuvenate natural P. euphratica forests.

Factors that increase reproductive variance among individuals act to reduce effective population size (Ne), which accelerates the loss of genetic diversity and decreases the efficacy of purifying selection. These factors include sexual cannibalism, offspring investment and mating system. Pre-copulatory sexual cannibalism, where the female consumes the male prior to mating, exacerbates this effect. We performed comparative transcriptomics in two spider species, the cannibalistic Trechaleoides biocellata and the non-cannibalistic T. keyserlingi, to generate genomic evidence to support these predictions. First, we estimated heterozygosity and found that genetic diversity is relatively lower in the cannibalistic species. Second, we calculated dN/dS ratios as a measure of purifying selection; a higher dN/dS ratio indicated relaxed purifying selection in the cannibalistic species. These results are consistent with the hypothesis that sexual cannibalism impacts operational sex ratio and demographic processes, which interact with evolutionary forces to shape the genetic structure of populations. However, other factors such as the mating system and life-history traits contribute to shaping Ne. Comparative analyses across multiple contrasting species pairs would be required to disentangle these effects. Our study highlights that extreme behaviours such as pre-copulatory cannibalism may have profound eco-evolutionary effects.

Abstract Increased female age at mating is considered a detrimental factor on reproductive output and fitness in insects, even if the impact is rather species specific. The effect of delayed mating on reproductive output has been widely studied in pest species controlled with mating disruption, as if the method is not fully effective in suppressing matings, it could still delay them, limiting female fitness and pest damage. Female mating delay, however, may also occur in natural habitats without invoking mating disruption. We studied the effect of female delayed mating in Cerambyx welensii (Küster) (Coleoptera: Cerambycidae), an oak‐living sapro‐xylophagous longhorn beetle considered a critical factor in oak decline. Several life history and ecological traits may potentially delay mating, including low abundance, (re)colonisation processes, sexual communication channel, host‐produced kairomones, operational sex ratio, reproductive interference and adverse weather conditions. We conducted laboratory tests to assess the impact of mating delays (0‐, 10‐, 20‐ and 30‐day post‐emergence) on reproductive output. Data showed that mating delay until at least 20 days of age had a limited effect on mating success, lifetime fecundity, longevity and fertility. The daily fecundity pattern depended on mating delay, and virgin females showed ovarian retention. We conclude that C. welensii females have evolved physiological adaptations to overcome mating delays and optimise fitness. We discuss our results from an evolutionary perspective, considering specifically the risk of early predation and egg‐laying time limitation. We hypothesise that unpredictable recurrent stochastic variation in male availability could act as an additional driver selecting for synovigeny in this longhorn species.

The mating system affects the animal’s mating behavior, operational sex ratio (OSR), and the genetic information of paternity. It is generally known that many species in the order Serpent accept polyandry. We observed the mating behavior of steppe rat snakes three times from April 1 until September 30, 2020. In all cases, the sex ratio participating in mating was one female and two or more males. Moreover, in two cases, two males were observed to insert their hemipenes into one female’s cloaca at the same time, which was a unique finding. It was concluded that steppe rat snakes would be highly likely to adopt polyandry, a common mating system of snakes. It will be possible to understand the exact mating system of steppe rat snakes by conducting behavioral genetic studies on them.