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In most fishes, the number of offspring increases with maternal body size. Although this size‐fecundity relationship often varies among species as a result of the coevolution of life‐history traits, the genetic basis of such size‐fecundity relationships remains unclear. We explored the genetic basis underlying this size‐fecundity relationship in two small medaka species, Oryzias latipes and O. sakaizumii. Our findings showed that O. sakaizumii has a higher fecundity than O. latipes, and quantitative trait locus analysis using interspecific F2 hybrids showed that chromosome 23 is linked to the size‐fecundity relationship. In particular, the genes igf1 and lep‐b in this region are known to be associated with life‐history traits, including somatic growth, gonad maturation, and progeny numbers in various taxa. Because O. sakaizumii is distributed at higher latitudes and has a shorter spawning season than O. latipes in the wild, we propose that the relatively high fecundity observed in O. sakaizumii is an adaptation to high latitudes. We also discuss the potential ecological ramifications associated with the evolution of increased fecundity in this species. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 pL around 1100 (Day 1) and c. 13% and 8 pL 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. [ABSTRACT FROM AUTHOR]

The operational sex ratio (OSR) is a key component influencing the magnitude of sexual selection driving the evolution of male sexual traits, but males often also retain the ability to plastically modulate trait expression depending on the current environment. Here we employed an experimental evolution approach to determine whether the OSR affects the evolution of male calling effort in decorated crickets, a costly sexual trait, and whether plasticity in calling effort is altered by the OSR under which males have evolved. Calling effort of males from 2 selection regimes maintained at different OSRs over 18–20 generations (male vs. female biased) was recorded at 2 different levels of perceived competition, in the absence of rivals or in the presence of an experimentally muted competitor. The effect of the OSR on the evolution of male calling effort was modest, and in the opposite direction predicted by theory. Instead, the immediate competitive environment strongly influenced male calling effort as males called more in the presence of a rival, revealing considerable plasticity in this trait. This increased calling effort came at a cost, however, as males confined with a muted rival experienced significantly higher mortality. [ABSTRACT FROM AUTHOR]

The strength of precopulatory sexual selection may vary within a breeding season depending on fluctuations in the social environment faced by active individuals. Here, we investigated within-season variations in precopulatory sexual selection in the river lamprey, Lampetra fluviatilis. We determined individual lifetime mating success in a group of 20 females and 15 males through constant video recording in an experimental aquarium. Among each sex, we calculated the potential strength of sexual selection and selection on identified traits (body size and temporal spawning patterns) at two different timescales: a fine-grained scale in which we considered mating events at daily time steps and a global scale in which we pooled all mating events that occurred during the breeding season. We found a predictable change in the operational sex ratio that corresponded with a change in the opportunity for sexual selection in males. Selection on both male and female body size was stronger when active competitors were larger. In addition, the timing (onset and span) of both the male's and the female's reproductive activity affected the number of matings performed over the season with a strength dependent on individual body size. Overall, our study documented that river lampreys experience within-season changes in precopulatory sexual selection. Such temporal dynamics are important to consider, and we advocate for longitudinal observation of mating behaviour to complement traditional time-integrated estimation of mating success based on genetic parentage analysis or cross-sectional analysis. • The opportunity for sexual selection changes throughout a breeding season. • The advantage conferred by body size depends on the operational sex ratio. • The timing of activity affects male and female mating success. • Instantaneous estimates can misrepresent the actual strength of sexual selection. [ABSTRACT FROM AUTHOR]

Intense male competition for access to females has often led to the evolution of alternative reproductive tactics (ARTs) such as sneaking, female mimicking, and satellite behavior. In many cases, which tactic a male expresses will depend upon intrinsic condition and/or environmental factors. Parental effects are also expected to play a critical role in ART expression when the parental environment is predictive of the offspring environment. Such parental effects on ARTs have remained largely unexplored. Here, we investigated maternal effects associated with operational sex ratio (OSR) on male ARTs (fighter and sneaker) in two-spotted spider mites Tetranychus urticae. In this mite species, mothers can anticipate the future male competition intensity (the future OSR) based upon the present OSR. To adjust local mate competition (LMC) among sons, mothers produce proportionally more sons under female-biased OSR and vice versa. We manipulated the maternal OSR and found that changes in maternal OSR did not alter the sneaker: fighter ratio among sons, but sneaker sons from mothers that experienced female-biased OSR started to guard females more quickly. Sneakers stay motionless on the dorsum of molting females, which is an advantageous position for mating with the emerging adult, and appear to deceive rival fighter males because they are not challenged. Earlier guarding by sneakers is advantageous for securing matings under intense male competition—as would be anticipated from the female-biased maternal OSR. We conclude that mothers anticipate the level of male competition in the next generation and influence their sons' mating behavior accordingly. Significance statement: In response to intense male competition for access to females, males often evolve alternative reproductive tactics (ARTs) such as sneaking behavior. Parental effects are widespread and expected to play an important role in the determination of offspring's ARTs when the parental environment is predictive of the offspring environment. We focus on operational sex ratio (OSR) as a predictor of male competition intensity in the next generation and found that female-biased OSR in the parental generation induces earlier guarding by sneaker sons in two-spotted spider mites. Mothers produce proportionally more sons under female-biased OSR, resulting in intense male competition in the next generation, and improve the odds of early guarding sneakers. Overall, our study suggests that spider mite mothers adjust their sons' mating behavior by anticipating the intensity of male competition in the next generation from the present OSR. [ABSTRACT FROM AUTHOR]

Researchers have long puzzled over suicidal behavior. In this paper, we posit that when people are unable to attract mates given unfavorable sex ratios, suicide rates increase. Sex ratio, the proportion of males in a population, is linked to a variety of behaviors, including marriage stability, violence, depression, and infidelity. We test whether suicide rates are associated with county-level sex ratios utilizing data from 1999 to 2018, controlling for a variety of factors known to be associated with suicide risk. We find that sex ratio is associated with suicide rates, where a greater proportion of males in a county (age 35–74) is associated with an increased rate of suicide for these males. Mediation analyses show that these effects are mediated by male marriage rates. Counter to predictions, male-biased sex ratios also tend to be associated with increased female suicide rates for women aged 35 to 74, and this effect is mediated by the unmarried sex ratio (i.e., when there are more unmarried men compared to unmarried women in a county, there is increased female suicide). Overall, these results suggest that male-biased sex ratios are associated with suicide rates for both men and women, but the mediators vary. [ABSTRACT FROM AUTHOR]

Unlike most cephalopods, the firefly squid Watasenia scintillans is highly monoandrous. We investigated this unusual phenomenon by considering the rare occurrence of polyandrous mating and the extremely male-biased operational sex ratio (OSR) at the beginning of the mating season. Theoretically, male-biased OSR can intensify competition for mates, leading to increased polyandry. We estimated OSR (male/female) as 32.7 and 9.1 at the beginning and end of the mating period, respectively. Next, we estimated the rate of polyandry in a population on a weekly basis, based on our new finding that the rate of polyandry can be estimated stochastically from the total number of spermatangia attached to one of the two seminal receptacles within a female. We found that the female squid sustains their highly monoandrous mating regime despite the fact that the OSR is extremely male-biased. Furthermore, there were no significant changes in the polyandry rate during the reproductive season. These results suggest that the squid mating system was not influenced by seasonal changes in OSR. Here, we discuss the evolutionary mechanism of how monoandry persists in this species. [ABSTRACT FROM AUTHOR]

In recent years, researc in subjective well-being has found several ecological factors that may underpin societal differences in happiness. The adult sex ratio, the number of males relative to females in an environment, influences many behaviours in both humans and non-human animals. However, the possible influence of the sex ratio on subjective well-being has received little attention. I investigated the relationship between the adult sex ratio and subjective well-being in over 29,000 respondents from 133 regions of Europe. I find that women report lower subjective well-being in areas with more female-biased sex ratios, but males' well-being was unaffected. I did not find that the sex ratio influences the sex specific probability of marriage, or marriage rates overall. I also find that increased population density is associated with lower well-being. Drawing from sociological and evolutionary theories, I suggest that results may be due to females' decreased bargaining power in the dating market. [ABSTRACT FROM AUTHOR]

The operational sex ratio (OSR), the ratio of sexually active males to sexually receptive females, is one of the main measures used to predict the intensity and direction of mating competition, influencing the opportunity for sexual selection. Here, we conducted the first experimental study to investigate how OSR and male age impacts the intensity of mating competition in reindeer (Rangifer tarandus), under semi-natural conditions during the rut. We manipulated OSR on two levels in two enclosures, a female biased treatment (3♂:6♀ = OSR 0.5) and a sex balanced treatment (3♂:3♀ = OSR 1), over 2 years with males from two age groups, and with females of various ages. We found some support for prevailing OSR theory, notably with female intrasexual competition occurring at lower frequencies in OSR 1 than OSR 0.5, and male intrasexual competition occurring at higher frequencies in the older male age group. Courtship behaviour was found to occur at higher frequencies in OSR 1 than OSR 0.5; however, there was no effect of male age. To successfully pass on genes to the next generation, one needs access to mates and winning a competitive bout is not always indicative of successfully accomplishing gene flow. Studies on OSR have the potential to help us understand the drivers behind sexual competition and how best to predict breeding outcomes during a rut. [ABSTRACT FROM AUTHOR]

Wildlife management requires reliable demographic information to assess the status of a population and its vulnerability to threats. This study calculated age class‐ and sex‐specific demographic parameters and assessed the viability of a community of Indo‐Pacific bottlenose dolphins (Tursiops aduncus) resident to the Peel‐Harvey Estuary in Western Australia. Boat‐based photo‐identification surveys (n = 483) were conducted between 2016 and 2019. A population viability analysis (PVA) was used to assess the community status and evaluate the effects of adult female and calf mortality, and reproduction on population growth rate. The community comprised 88 (SD = 4.43) individuals with a sex ratio close to parity in all but the adult age class where it was skewed towards females. Demographic changes in this community are driven by births, deaths, and the likely permanent emigration of juvenile males. No immigration was observed. The population is stable (r = −0.004, SD = 0.062) given the current demographic rates. To maintain a community of ~90 individuals, management should consider action to lower adult female and calf mortality. This should involve aiming for zero human caused mortality and ensuring adverse impacts to the population are considered in future development planning. [ABSTRACT FROM AUTHOR]

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., (© Penerbit Universiti Sains Malaysia, 2024.)

Bateman gradients, the slope of the regression of reproductive success on mating success, are among the most commonly reported measures of sexual selection. They are particularly insightful in species with reversed sex roles, where females are expected to be under sexual selection. We measured Bateman gradients in replicate experimental populations of the spermatophore gift‐giving bushcricket Kawanaphila nartee (Orthoptera: Tettigoniidae). In this species, the operational sex ratio (OSR) and thus the sex competing for mates varies depending on the availability of pollen food resources: under pollen‐limited regimens females are more competitive, whereas under pollen‐rich regimens males are more competitive. We maintained populations in enclosures with either limited or supplemented pollen and calculated Bateman gradients for males and females under both conditions. Bateman gradients were significantly positive in males, and the slope was steeper in pollen‐supplemented populations where the OSR was more male‐biased. Bateman gradients for females were shallow and nonsignificant regardless of pollen availability. Our results show that the strength of sexual selection on males can depend on environmental context. The lack of significant gradients among females may reflect experimental limitations on our ability to estimate Bateman gradients in female K nartee. [ABSTRACT FROM AUTHOR]

In many animal species, an increase in the operational sex ratio (OSR), density or a combination of both should lead to more intensive competition among individuals of the more abundant sex. To test this, we examined pairing patterns of Minshan's toad (Bufo minshanicus) from six populations between 2008 and 2015 along the eastern Tibetan Plateau in south-west China. OSRs in breeding aggregations of Minshan's toad are normally male biased and males actively compete with each other for acquisition and retention of mates. We found evidence that deviations from random mating by size varied between populations and between years according to the magnitude of the OSR and male density. Larger males were generally more successful in pairing than smaller males when the OSR was slightly male biased and male density was high. However, the resulting size-disproportionate mating was more evident when OSR was closer to 1.99, indicating a positive correlation with the intensity of aggressive scramble competition. Thus, the intensity of male-male competition may partly explain variation in size-disproportionate mating among populations. [ABSTRACT FROM AUTHOR]

In a warming climate, male sea turtles may become increasingly rare due to temperature-dependent sex determination with females being produced at warmer temperatures. Hence there is widespread concern that a lack of adult males may impact population viability. However, there is controversy over this scenario and here we review aspects of the biology of male sea turtles that will help mitigate female-biased hatchling sex ratios. In particular, there is strong evidence that males generally breed more frequently than females (i.e. have a shorter remigration interval) and that individual breeding males actively search for females and may mate with multiple females from different nesting sites. These aspects of the biology of male turtles will cause female-biased hatchling sex ratios to translate into more balanced adult sex ratios on the breeding grounds (i.e. operational sex ratios). Sexual dimorphism is widespread with adult male turtles generally being smaller than females. In freshwater turtles, this sexual dimorphism is linked to earlier age at maturity for males, although this possibility has not been examined widely in sea turtles. We make a forward-looking horizon-scanning prediction for key changes that might be expected at sea turtle breeding grounds if female-biased sex ratios become so extreme that male turtles start to become limiting and start driving populations to extinction. In particular, as the numbers of adult males on the breeding grounds become limiting there may be changes in egg fertility, multiple paternity and hatching success within clutches. [ABSTRACT FROM AUTHOR]

Life-history theory suggests that male anurans living in harsh and unpredictable environmental conditions (e.g., low temperature and/or high seasonality) should decrease reproductive investment because of short periods of activity and poor productivity. Although geographic variations in female life-history traits have been investigated intensively across a wide range of taxa, similar studies in males have been scarce. Here, we examined geographic variation in male testis mass of the brown frog (Rana kukunoris) across seven different altitudes on the eastern Tibetan plateau, and tested the hypothesis that relative size of testis mass should increase with increasing temperature and/or decreasing seasonality. In this study, we found male body condition, age and testis mass did not increase with temperature and temperature seasonality, which do not conform to our previous hypothesis, revealing that differences in temperature and temperature seasonality across populations could not drive the evolution of testis size. However, we found a positive correlation between testis mass and body condition or age when controlling for population and the effect of temperature and temperature seasonality, indicative of a condition-dependent expression of testis size. In addition, a correlation between testis mass and operational sex ratio was not significant, suggesting that male–male competition did not lead to increasing levels of sperm competition. [ABSTRACT FROM AUTHOR]

Abstract Chance plays a critical but underappreciated role in determining mating success. In many cases, we tend to think of chance as background noise that can be ignored in studies of mating dynamics. When the influence of chance is consistent across contexts, chance can be thought of as background noise; in other cases, however, the impact of chance on mating success can influence our understanding of how mates are acquired and how sexual selection operates. In particular, when the importance of chance covaries with biological or ecological factors in a systematic manner—that is, when chance becomes consistently more or less important under certain conditions—then chance is important to consider if we want to fully understand the operation of mate acquisition and sexual selection. Here, we present a model that explores how chance covaries with factors such as sex ratio, adult population size, and mating regime in determining variation in mating success. We find that in some cases, chance covaries with adult population size and the operational sex ratio to create variation in mating success. We discuss how chance can influence our more general understanding of the operation of mating dynamics and sexual selection.

Researchers have long puzzled over suicidal behavior. In this paper, we posit that when people are unable to attract mates given unfavorable sex ratios, suicide rates increase. Sex ratio, the proportion of males in a population, is linked to a variety of behaviors, including marriage stability, violence, depression, and infidelity. We test whether suicide rates are associated with county-level sex ratios utilizing data from 1999 to 2018, controlling for a variety of factors known to be associated with suicide risk. We find that sex ratio is associated with suicide rates, where a greater proportion of males in a county (age 35–74) is associated with an increased rate of suicide for these males. Mediation analyses show that these effects are mediated by male marriage rates. Counter to predictions, male-biased sex ratios also tend to be associated with increased female suicide rates for women aged 35 to 74, and this effect is mediated by the unmarried sex ratio (i.e., when there are more unmarried men compared to unmarried women in a county, there is increased female suicide). Overall, these results suggest that male-biased sex ratios are associated with suicide rates for both men and women, but the mediators vary.

Sexual dimorphism is regarded as the consequence of differential responses by males and females to selection pressures. Limb muscle plays a very important role during amplexus, which is likely to be under both natural and sexual selection in anurans. Here, we studied the effects of natural and sexual selection on limb muscle mass in males and females across 64 species of anurans. The results showed that there were non-significant differences in relative limb muscle mass between the sexes among species, exhibiting no sexual dimorphism in limb muscle. Absolute and relative limb muscle mass positively displayed correlations with snout-vent length (SVL)for both sexes. However, neither male-biased operational sex ratio (OSR) nor environmental harshness [e.g., coefficient of variation (CV) in temperature and CV in rainfall] can explain relative limb muscle mass (e.g., forelimb, hindlimb, and total limb muscle) within each sex. The findings suggest that environmental harshness and sexual selection cannot play important roles in promoting variations in limb muscle among anuran species.

The operational sex ratio (OSR) is used as a predictor for the intensity of mating competition. While many factors affect the OSR, there tends to be a high male bias in primate species with long interbirth intervals and non-seasonal breeding, such as hominid apes. However, the OSR of bonobos (Pan paniscus) is lower than that of chimpanzees (Pan troglodytes), which is thought to reduce competitive and aggressive male behaviors. The low OSR of bonobos is considered to result from the early resumption of female sexual receptivity during postpartum infertility and the receptivity that they continue to show until the late stage of pregnancy. In this study, we aimed to examine the early resumption of sexual receptivity by providing quantitative data on the resumption of maximal swelling (MS) in sexual skin and copulation, and changes in urinary estrone conjugate (E1C) concentrations during postpartum infertility in wild bonobos at Wamba in the Luo Scientific Reserve, Democratic Republic of the Congo. An analysis of 9 years of data revealed that females showed the first MS at 225.4 ± 132.7 days after parturition and performed the first copulation at 186.8 ± 137.5 days after parturition, both of which were in the early stage of postpartum infertility. The proportion of days with MS and the frequency of copulation steadily increased subsequently; however, the rate of increase gradually slowed approximately 42–48 months after parturition. There was a significant correlation between the proportion of days with MS and the frequency of copulation in each period for each female. We confirmed that E1C concentrations were significantly higher during the MS phase than during the non-MS phase. Data collected over 15 months on the E1C concentration during MS showed that it increased linearly from the early stage of lactation to the next conception. These results suggest that, although female bonobos do not usually conceive until 49.7 months after parturition, they resume MS and receptivity at a low level of E1C concentration during an early stage of postpartum infertility. This study of female bonobo receptivity and sex hormone changes during the postpartum non-fertile period provides important insights for examining the evolution of low OSR, which has been considered to contribute to peaceful social relationships among bonobos. [ABSTRACT FROM AUTHOR]

The operational sex ratio (OSR), the ratio of sexually active males to sexually receptive females, is one of the main measures used to predict the intensity and direction of mating competition, influencing the opportunity for sexual selection. Here, we conducted the first experimental study to investigate how OSR and male age impacts the intensity of mating competition in reindeer (Rangifer tarandus), under semi-natural conditions during the rut. We manipulated OSR on two levels in two enclosures, a female biased treatment (3♂:6♀ = OSR 0.5) and a sex balanced treatment (3♂:3♀ = OSR 1), over 2 years with males from two age groups, and with females of various ages. We found some support for prevailing OSR theory, notably with female intrasexual competition occurring at lower frequencies in OSR 1 than OSR 0.5, and male intrasexual competition occurring at higher frequencies in the older male age group. Courtship behaviour was found to occur at higher frequencies in OSR 1 than OSR 0.5; however, there was no effect of male age. To successfully pass on genes to the next generation, one needs access to mates and winning a competitive bout is not always indicative of successfully accomplishing gene flow. Studies on OSR have the potential to help us understand the drivers behind sexual competition and how best to predict breeding outcomes during a rut. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Male mating harassment may occur when females and males do not have the same mating objectives. Communal animals need to manage the costs of male mating harassment. Here, we demonstrate how desert locusts in dense populations reduce such conflicts through behaviors. In transient populations (of solitarious morphology but gregarious behavior), we found that nongravid females occupied separate sites far from males and were not mating, whereas males aggregated on open ground (leks), waiting for gravid females to enter the lekking sites. Once a male mounted a gravid female, no other males attacked the pair; mating pairs were thereby protected during the vulnerable time of oviposition. In comparison, solitarious locusts displayed a balanced sex ratio in low-density populations, and females mated irrespective of their ovarian state. Our results indicate that the mating behaviors of desert locusts are density dependent and that sex-biased behavioral group separationmay minimize the costs of male mating harassment and competition. [ABSTRACT FROM AUTHOR]

The short-lived polygamous moth Grapholita molesta (Busck) is an important fruit pest worldwide. Trapping males by synthetic female sex pheromones is not an effective reproductive control strategy. It is important to improve this technology by understanding the mating system of G. molesta. This study investigated mating opportunities and fertile egg production by altering the operational sex ratio, mating age, and male mating history in repeated single mating and multiple mating in the two sexes. Our results showed that the mating and reproductive parameters of virgin males were affected by the number and age of virgin females. Males preferred a female number ≤three-fifths of the male number or ≤2-day-old females, while they discriminated against a female number ≥three times of the male number or ≥5-day-old females. On the other hand, the mating and reproductive parameters of virgin females were affected by repeated single mating and especially multiple mating under different male mating histories. Females preferred once-mated males and discriminated against virgin males. These results indicated that mating systems including more and older virgin females for virgin males and different virgin males for virgin females may be suitable for suppressing G. molesta populations. Hence, these results revealed that preventing mating of virgin adults by synthetic female sex pheromones should be most effective in controlling G. molesta. [ABSTRACT FROM AUTHOR]

Chance plays a critical but underappreciated role in determining mating success. In many cases, we tend to think of chance as background noise that can be ignored in studies of mating dynamics. When the influence of chance is consistent across contexts, chance can be thought of as background noise; in other cases, however, the impact of chance on mating success can influence our understanding of how mates are acquired and how sexual selection operates. In particular, when the importance of chance covaries with biological or ecological factors in a systematic manner—that is, when chance becomes consistently more or less important under certain conditions—then chance is important to consider if we want to fully understand the operation of mate acquisition and sexual selection. Here, we present a model that explores how chance covaries with factors such as sex ratio, adult population size, and mating regime in determining variation in mating success. We find that in some cases, chance covaries with adult population size and the operational sex ratio to create variation in mating success. We discuss how chance can influence our more general understanding of the operation of mating dynamics and sexual selection. [ABSTRACT FROM AUTHOR]

Exploring the mechanisms that affect mating pattern with respect to body size has implications for understanding the evolution of sexual selection. Theory predicts that the absence of a relationship between female body size and fecundity, unbiased operational sex ratio, and a short breeding season will lead to random mating by body size in anuran amphibians. We tested these predictions in the Himalayan toad Duttaphrynus himalayanus inhabiting southeastern Tibet. Our study did not detect any correlation between female body size and number of eggs laid, nor was there a significant difference in the sex ratio of toads captured from the breeding site. In addition, the toads were reproductive for only a short period, from late April to early May (typical of an explosively breeding species). As expected, we detected a weak but not significant relationship between body size of amplexing males and females. Our results revealed no apparent size-assortative pairing in the study population of the Himalayan toad and may contribute to an increasing body of literature on mating patterns in relation to body size in animals with indeterminate growth. [ABSTRACT FROM AUTHOR]

Post‐copulatory sexual selection (PSS) is an important selective force that determines fitness in polyandrous species. PSS can be intense in some cases and can drive the evolution of remarkable ejaculate properties. In males, investment in ejaculate plays an important role in the outcome of PSS. Thus, males are expected to adaptively tailor their ejaculate according to the perceived competition in their vicinity. Plastic responses in ejaculate investment to variation in intrasexual competition are disparate and widespread in males. We investigated the evolution of plasticity in reproductive traits using Drosophila melanogaster populations evolving for more than 150 generations under male‐ or female‐biased sex ratios. When exposed to different numbers of competitors early in their life, males from these two regimes responded differently in terms of their copulation duration and sperm competitive ability. In addition, the effect of this early life experience wore off at different rates in males of male‐biased and female‐biased regimes with increasing time from the removal of competitive cues. Furthermore, our study finds that males change their reproductive strategies depending upon the identity of rival males. Together, our results provide evidence of the evolution of male reproductive investment that depends on socio‐sexual cues experienced early in life. [ABSTRACT FROM AUTHOR]

Competition over mates is a powerful force shaping trait evolution. For instance, better cognitive abilities may be beneficial in male-male competition and thus be selected for by intrasexual selection. Alternatively, investment in physical attributes favoring male performance in competition for mates may lower the resources available for brain development, and more intense male mate competition would coincide with smaller brains. To date, only indirect evidence for such relationships exists, and most studies are heavily biased toward primates and other homoeothermic vertebrates. We tested the association between male brain size (relative to body size) and male-male competition across N = 30 species of Chinese anurans. Three indicators of the intensity of male mate competition—operational sex ratio (OSR), spawning-site density, and male forelimb muscle mass—were positively associated with relative brain size, whereas the absolute spawning group size was not. The relationship with the OSR and male forelimb muscle mass was stronger for the male than for the female brains. Taken together, our findings suggest that the increased cognitive abilities of larger brains are beneficial in male-male competition. This study adds taxonomic breadth to the mounting evidence for a prominent role of sexual selection in vertebrate brain evolution. [ABSTRACT FROM AUTHOR]

In most animals, females are larger than males. Paradoxically, sexual size dimorphism is biased towards males in most mammalian species. An accepted explanation is that sexual dimorphism in mammals evolved by intramale sexual selection. I tested this hypothesis in primates, by relating sexual size dimorphism to seven proxies of sexual selection intensity: operational sex ratio, mating system, intermale competition, group sex ratio, group size, maximum mating percentage (percentage of observed copulations involving the most successful male), and total paternity (a genetic estimate of the percentage of young sired by the most successful male).I fitted phylogenetic generalised least squares models using sexual size dimorphism as the dependent variable and each of the seven measures of intensity of sexual selection as independent variables. I conducted this comparative analysis with data from 50 extant species of primates, including Homo sapiens, Pan troglodytes, and Gorilla spp.Sexual dimorphism was positively related to the four measures of female monopolisation (operational sex ratio, mating system, intermale competition, and group sex ratio) and in some cases to group size, but was not associated with maximum mating percentage or total paternity. Additional regression analyses indicated that maximum mating percentage and total paternity were negatively associated with group size.These results are predicted by reproductive skew theory: in large groups, males can lose control of the sexual behaviour of the other members of the group or can concede reproductive opportunities to others. The results are also consistent with the evolution of sexual size dimorphism before polygyny, due to the effects of natural, rather than sexual, selection. In birds, the study of molecular paternity showed that variance in male reproductive success is much higher than expected by behaviour. In mammals, recent studies have begun to show the opposite trend, i.e. that intensity of sexual selection is lower than expected by polygyny.Results of this comparative analysis of sexual size dimorphism and sexual selection intensity in primates suggest that the use of intramale sexual selection theory to explain the evolution of polygyny and sexual dimorphism in mammals should be reviewed, and that natural selection should be considered alongside sexual selection as an evolutionary driver of sexual size dimorphism and polygyny in mammals. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Surveying the breeding population of a given species can be difficult for many logistic reasons. Marine turtles are a challenging taxon for the study of reproductive ecology and breeding strategies, because turtles aggregate off-shore and males remain exclusively at sea. For successful management of sea turtle populations, determining operational sex ratios (OSRs) on a continuing basis is critical for determining long-term population viability, particularly in the context of changing hatchling sex ratios due to temperature-dependent sex determination in a warming climate. To understand how survey technique and stage of the breeding season might influence the ability to detect turtles and determine OSRs, we surveyed the presence and identified the sex of adult male and female green sea turtles (Chelonia mydas) using a boat and small commercial unoccupied/unmanned aerial vehicle (UAV), at the start (October) and peak (December) of a nesting season at an important breeding site at Heron Island, Great Barrier Reef, Australia. The ratio of males to females within the breeding ground detected by both survey methods changed from being male-biased in October to heavily female-biased in December, indicating that most males cease their reproductive effort and depart before the peak of the nesting season. Surveying with a UAV more than doubled the rate of turtles seen per minute of survey effort compared with surveying solely from the boat and allowed surveys to be conducted at times and/or places unsafe or inaccessible for boats. The sex of a slightly greater proportion of turtles seen could not be identified by observers using a UAV versus a boat, although more turtles were detected using the UAV. The departure of many males during the peak of the nesting season is likely due to an increasing biological cost of residency in the area because males encounter fewer receptive females as the season progresses and the limited foraging opportunity is insufficient to support the number of males present. Overall, we found that UAVs are an effective tool for studying important but difficult to observe aspects of sea turtle biology.

Spatial memory is crucial for mating success because it enables males to locate potential mates and potential competitors in space. Intraspecific competition and its varying intensity under certain conditions are potentially important for shaping spatial memory. For example, spatial memory could enable males to know where competitors are (contest competition), it could help males find mating partners (scramble competition) or both. We manipulated the intensity of intraspecific competition in two distinct contexts by altering the operational sex ratio of prairie voles (Microtus ochrogaster) living in outdoor enclosures by creating male- and female-biased sex ratios. After living freely under these contexts for four weeks, we compared males' performance in a laboratory spatial memory test. Males in the male-biased context demonstrated better spatial memory performance than males in the female-biased context. Notably, these data show that in spite of experiencing equally complex spatial contexts (i.e. natural outdoor enclosures), it was the social context that influenced spatial cognition, and it did so in a manner consistent with the hypothesis that spatial memory is particularly relevant for male–male interactions.

In polygynandrous species, males face the trade-off between the pursuit of increased mating opportunities and securing paternity. Within such systems, males need to accurately assess the social composition of the local environment to maximize fitness. Here, we investigated this capability in the water strider (Aquarius remigis), a semi-aquatic insect known to exhibit a broad spectrum of mating behaviors and inhabit a socially diverse and changing environment. Using a combination of methodological designs to track both within- and between-subject effects, individuals remained in same-sex housing prior to being exposed to slightly biased operational sex ratios (2:1 vs. 1:2) with or without prior cohabitation to determine the effects on mating duration. Results show that males were sensitive to these subtle differences in social conditions, mating for longer periods within male-biased environments, but this was true only under conditions with prior cohabitation. In particular, when individuals could acclimate to the testing environment, mating duration dropped precipitously in female-biased conditions. These findings do not support the view that male water striders have consistent behavioral syndromes, and instead show that individuals are able to differentiate between, and adaptively respond to, small changes in the local sex ratio. In addition to improving our understanding of the plasticity of male mating behavior in this species, this study offers new insights for future laboratory research studying reproductive competition across a diverse range of polygynandrous animals.

Given that sperm production can be costly, theory predicts that males should optimally adjust the quantity and/or quality of their sperm in response to their social environment to maximize their paternity success. Although experiments demonstrate that males can alter their ejaculates in response to manipulations of the social environment and studies show that ejaculate traits covary with social environment across populations, it is unknown whether individual variation in sperm traits corresponds to natural variation found within wild populations. Using an island population of brown anole lizards (Anolis sagrei), we tested the prediction that sperm traits (sperm count, sperm morphology, sperm velocity) respond to natural variation in the risk of sperm competition, as inferred from the local density and operational sex ratio (OSR) of conspecifics. We found that males living in high-density areas of the island produced relatively larger sperm midpieces, smaller sperm heads, and lower sperm counts. Sperm traits were unrelated to OSR after accounting for the covariance between OSR and density. Our findings broaden the implications of sperm competition theory to intrapopulation social environment variation by showing that sperm count and sperm morphology vary with fine-scale differences in density within a single wild population. [ABSTRACT FROM AUTHOR]

While the importance of male mate choice is increasingly recognized, it continues to be an understudied aspect of sexual selection. Here, the effect of female body size on male mate choice was evaluated in the annual killifish Austrolebias reicherti, a species with marked sexual dimorphism in which males are more conspicuous than females. In dual-choice mate choice trials with unequal sized females we found that males spent significantly more time with larger females. Furthermore, larger females spawned more and larger eggs when spawning was allowed in non-choice trials. Therefore, male selection is probably attributable to the higher reproductive success of larger females. To our knowledge, this is the first study that provides evidence of both male mate choice in annual fishes and its possible fitness advantages. The implications of male mate choice for sexual selection in these fish highlight the need for further studies. [ABSTRACT FROM AUTHOR]

The operational sex ratio (OSR) is one of the most important ecological factors affecting the structure of mating systems and individual mating investment. Bush cricket males produce a highly costly nuptial gift during mating, so it is expected that OSR would affect this investment. In this study, we confronted the bush cricket Isophya rizeensis with three different OSR environments, measuring effects on the males' spermatophore, spermatophylax, ampulla weight, and absolute sperm transfer. As expected, male I. rizeensis showed a decrease in the transferred sperm in a female-biased environment. When mating in different OSR environments, males did not modify their behavioral strategies with respect to spermatophore transfer and copulation duration, which was largely influenced by male body size, likely due to the high costs of nuptial gifts. Although we dismiss the "longer copulation durations function as a mate assessment period" hypothesis, longer copulations positively affect spermatophore size and sperm number. [ABSTRACT FROM AUTHOR]

The competition for fertilization among sperm from different males can drive variation in male reproductive investments. However, the mechanisms shaping reproductive allocation and the resulting variations in reproductive investment relative to environmental variables such as resource availability and male-male competition remain poorly known in frogs. Here, we investigated inter-population variation in male somatic condition and testis mass across four populations of the swelled vent frog Feirana quadranus along an altitudinal gradient. We found that relative testis mass did not increase with altitude, which was inconsistent with previous predictions that an increase in latitude and/or altitude should result in decreased sperm production in anurans due to shortened breeding seasons and the decline in resource availability. We also found no increase in somatic condition and male/female operational sex ratio with altitude. However, the somatic condition exhibited a positive correlation with testis mass, which indicated the condition-dependent testis size in F. quadranus. Moreover, an increase of testis mass with increasing male/female operational sex ratio suggest that male-male competition can result in an increased intensity of sperm competition, thereby increasing testis mass. [ABSTRACT FROM AUTHOR]

Our understanding of sexual selection has greatly improved during the last decades. The focus is no longer solely on males, but also on how female competition and male mate choice shape ornamentation and other sexually selected traits in females. At the same time, the focus has shifted from documenting sexual selection to exploring variation and spatiotemporal dynamics of sexual selection, and their evolutionary consequences. Here, I review insights from a model system with exceptionally dynamic sexual selection, the two-spotted goby fish Gobiusculus flavescens. The species displays a complete reversal of sex roles over a 3-month breeding season. The reversal is driven by a dramatic change in the operational sex ratio, which is heavily male-biased at the start of the season and heavily female-biased late in the season. Early in the season, breeding-ready males outnumber mature females, causing males to be highly competitive, and leading to sexual selection on males. Late in the season, mating-ready females are in excess, engage more in courtship and aggression than males, and rarely reject mating opportunities. With typically many females simultaneously courting available males late in the season, males become selective and prefer more colorful females. This variable sexual selection regime likely explains why both male and female G. flavescens have ornamental colors. The G. flavescens model system reveals that sexual behavior and sexual selection can be astonishingly dynamic in response to short-term fluctuations in mating competition. Future work should explore whether sexual selection is equally dynamic on a spatial scale, and related spatiotemporal dynamics. [ABSTRACT FROM AUTHOR]

Detecting and monitoring illegal harvesting pressure on wild populations is challenging due to the cryptic nature of poaching activities. Although change in population age structure has been suggested as an indicator of harvesting pressure, few studies have tested its validity when based on short-term field surveys. Using data from rapid demographic assessment surveys carried out in 2009 at six sites in Tanzania, we examined whether African elephant populations experiencing contrasting levels of poaching pressure showed significant differences in their age structure, operational sex ratio (i.e. adult males to adult females), dependent individual to adult female ratio at the group level, and proportion of tuskless individuals. We also compared similar metrics between the population sampled in Ruaha National Park in 2009 and again in 2015 following a suspected increase in poaching. Elephant populations experiencing medium and high levels of poaching in 2009 were characterised by fewer calves and old individuals, a reduced number of adult males relative to adult females, and a lower ratio of calves to adult females within groups. We also found a higher proportion of tuskless individuals in poached populations (>6%). Changes in age structure in the Ruaha population between 2009 and 15 were similar to those observed across sites in 2009. Our findings are consistent with previous work documenting how the loss of older individuals – targeted for their larger tusks – decreases recruitment and survival of elephant calves. Illegal killing for ivory is a huge threat to the survival of African elephants. In this context, the present study contributes towards validating the use of age structure as an indicator of poaching pressure in elephant populations, but also in other wildlife populations where illegal offtake is targeted at specific age classes. [ABSTRACT FROM AUTHOR]

Abstract: The operational sex ratio (OSR) has long been assumed to be a key ecological factor determining the opportunity and direction of sexual selection. However, recent theoretical work has challenged this view, arguing that a biased OSR does not necessarily result in greater monopolisation of mates and therefore stronger sexual selection in the mate‐limited sex. Hence, the role of the OSR for shaping animal mating systems remains a conundrum in sexual selection research. Here we took a meta‐analytic approach to test whether OSR explains interspecific variation in sexual selection metrics across a broad range of animal taxa. Our results demonstrate that the OSR predicts the opportunity for sexual selection in males and the direction of sexual selection in terms of sex differences in both the opportunity for sexual selection and the Bateman gradient (i.e. the selection differential of mating success), as predicted by classic theory. [ABSTRACT FROM AUTHOR]

In biology, the term 'mating market' has been fashionable for a few decades only, but sexual selection theory was implicitly based on economic principles from the start. I regard mating individuals explicitly as traders on markets and distinguish 'global mating markets', consisting of all reproducing members of a population, from 'local mating markets' (LMMs) that are disconnected in space and/or time. I focus on ways in which individuals make the best of variation among LMMs by adapting their mating strategy to each local market they enter. The 'operational sex ratio' (OSR; Emlen ST, Oring LW (1977) Science 197:215-223) gives a first approximation of the balance of power between the two trader classes: males and females ready to reproduce. The parameter I use is the local OSR (LOSR), the OSR of a single LMM. The balance of power is dependent not only on the LOSR, however, but also on the production costs and exchange values of 'crucial commodities', which often vary locally and over time. Distinguishing LMMs is most useful for species with strong variation in the LOSR. Human mating markets distorted by war, selective abortion and sex-biased migration are among the best-documented local markets with aberrant OSRs. Traders on LMMs may strategically adjust to supply and demand ratios and changes in their own market value but also attempt to change local market conditions or transfer to another local market with better conditions. Fine-tuning can result not only from conditional strategies, evolved under natural and sexual selection, but also from learning processes as far as species-specific cognitive constraints allow. Significance statement: Biological market theory (BMT), which deals with cooperation among unrelated agents in general, is combined with sexual selection theory (SST), which deals with reproductive cooperation, to focus on an aspect that received little attention in the vast SST literature: adaptations that improve mating success when the market value of an individual varies considerably from one local mating market (LMM) to the next. An individual's market value on an LMM is determined not only by the local operational sex ratio (LOSR) but also by the value of the goods and services that both sexes invest in their mates and/or the communal offspring. Case studies of both humans and non-human animals are used to illustrate the difference between global and local markets and to evaluate predictions based on the LMM-hypothesis. [ABSTRACT FROM AUTHOR]