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22 results for “operational sex ratio”
Data from: Deer indirectly alter the reproductive strategy and operational sex ratio of an unpalatable forest perennial
Environmental conditions impose restrictions and costs on reproduction. Multiple reproductive options exist when increased reproductive costs drive plant populations toward alternative reproductive strategies. Using four 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 non-trophic indirect effects by overabundant deer on this perennial result in delayed female sex expression to unsustainably large plant sizes and leads 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.
Does operational sex ratio influence relative strength of purging selection in males vs. females?
<p>According to theory, sexual selection in males may efficiently purge mutation load of sexual populations, reducing or fully compensating "the cost of males". For this to occur, mutations not only need to be deleterious to both sexes, they also must affect males more than females. A frequently overlooked problem is that relative strength of selection on males vs. females may vary between environments, with social conditions being particularly likely to affect selection in males and females differently. Here we induced mutations in red flour beetles (Tribolium castaneum) and tested their effect in both sexes under three different operational sex ratios (1:2, 1:1, 2:1). Induced mutations decreased fitness of both males and females, but their effect was not stronger in males. Surprisingly, operational sex ratio did not affect selection against deleterious mutations nor its relative strength in the sexes. Thus, our results show no support for the role of sexual selection in the evolutionary maintenance of sex.</p> <p> </p>
Data from: The effect of operational sex ratio and density on the strength of sexual selection against mutant males in Drosophila melanogaster
<p>Higher male:female operational sex ratio (OSR) is often assumed to lead to stronger sexual selection on males. Yet, this premise has been directly tested by very few studies, with mixed outcomes. We investigated how OSR affects the strength of sexual selection against two deleterious alleles, a natural <em>ebony</em> mutant and a transgenic <em>GFP</em> insertion, in <em>Drosophila melanogaster. </em>To this end, we estimated the relative paternity share of homozygous mutant males competing against wildtype males under different OSRs (1:2, 1:1, 2:1). We also manipulated the mating pool density (18, 36 or 54 individuals), and assessed paternity over three consecutive days, during which the nature of sexual interaction changed. The strength of sexual selection against the <em>ebony</em> mutant increased with OSR, became weaker after the first day and was little affected by density. In contrast, sexual selection against the <em>GFP</em> transgene was markedly affected by density: at the highest density it increased with OSR, but at lower densities it was strongest at 1:1 OSR, remaining strong throughout the experiment. Thus, while OSR can strongly affect the strength of sexual selection against "bad genes", it does not necessarily increase monotonically with male:female OSR. Furthermore, the pattern of relationship between OSR and the strength of sexual selection can be locus-specific, likely reflecting the specific phenotypic effects of the mutation.</p>
Data from: The effect of operational sex ratio and density on the strength of sexual selection against mutant males in Drosophila melanogaster
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Operational sex ratio bias due to sex-specific cohort splitting in response to predation
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Data from: Deer indirectly alter the reproductive strategy and operational sex ratio of an unpalatable forest perennial
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Does operational sex ratio influence relative strength of purging selection in males vs. females?
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Sex-specific sterility caused by extreme temperatures is likely to create cryptic changes to the operational sex ratio in Drosophila virilis
<p>Climate change is increasing the frequency and severity of short-term heat shocks that threaten the persistence of natural populations. The effect of thermal stress on natural selection is a common topic of debate, but high temperatures can also influence sexual selection. Typically, males and females of a species can survive at similar extreme temperatures, but males have been shown to lose fertility at lower temperatures than females. Here, we examine how a brief exposure of pupae to high temperatures in the fruit fly Drosophila virilis affects adult fertility in both males and females. We find strong sexual dimorphism in temperature-induced sterility. This has the potential to quickly and unpredictably create populations composed of mostly sterile males and fertile females, resulting in changes to the operational sex ratio (OSR). These disruptions are likely to be cryptic and difficult to measure in the wild, especially considering that males can eventually recover fertility and that sterile males of some species can still copulate. Changes to the OSR in this way are likely to influence sexual selection by favouring females that can discriminate between fertile and sterilised males, and possibly leading to female-female conflict over a limited pool of fertile males. Further research on how cryptic disruptions to the OSR affect sexual selection dynamics is critical for understanding the impact of environmental change on biodiversity.</p>
Data from: Adult sex ratio and operational sex ratio exhibit different temporal dynamics in the wild
Adult sex ratio (ASR, the proportion of adult males in the adult population) and operational sex ratio (OSR, the proportion of sexually active males in the mating pool) are fundamental properties of breeding populations and they are often linked to mating systems and sexual selection. However, ASR and OSR emerge via different routes in a population and may exhibit different temporal patterns. Here, we use data from a well-monitored polygamous snowy plover Charadrius nivosus population sampled over 3 consecutive breeding seasons to investigate whether temporal changes in ASR relate to changes in OSR. We found that snowy plovers exhibited male-biased ASR and OSR. Consistent with theoretical expectations, OSR was more variable than ASR. However, there was no consistent relationship between OSR and ASR: in only 1 of the 3 study years we found a weak positive relationship (r = 0.22). The lack of association was corroborated by time series analyses and sensitivity tests. Our work therefore suggests that ASR and OSR exhibit different temporal patterns in a polygamous population, and we call for further theoretical and empirical studies that analyze their relationship across a range of different breeding systems.
Data from: Detecting elusive aspects of wildlife ecology using drones: new insights on the mating dynamics and operational sex ratios of sea turtles
Offspring and breeding (operational) sex ratios (OSR) are a key component of demographic studies. While offspring sex ratios are often relatively easy to measure, measuring OSRs is often far more problematic. Yet highly skewed OSRs, and a lack of male-female encounters, may be an important extinction driver. Using loggerhead sea turtles (Caretta caretta) as a case study, we showed the utility of drones, i.e. unmanned aerial vehicles (UAVs) to distinguish adult males and females in a marine breeding area, using a combination of morphological characteristics (tail length) and behavioural differences (active mating, courting and searching by males versus resting by females). Through repeated surveys, we documented seasonal changes in the OSR. While the number, and ratio, of males and females on the breeding grounds changed massively, the ratio of receptive females (derived from the rate of influx of new individuals to the area) to breeding males remained close to 1:1 for much of the period before nesting commenced. Hence, we show how large imbalances in the number of adult males and females may translate into relatively balanced OSRs. Our results suggest that the departure of males from the breeding grounds is linked to a decline in female receptivity, with female sea turtles being known to store sperm to ensure high clutch fertility throughout the nesting season. In conclusion, while we detected up to three times more females than males at the breeding ground, at present, OSRs appear stable. However, because most males breed annually (versus biannually by females), there might only be ˜100 males in the adult population (i.e. adult sex ratio of 1:7.5), which might become further skewed under expected climate change scenarios; thus, we need to identify the minimum number of males required to prevent extinction. Finally, we highlight the use of UAVs for assessing the mating dynamics of other marine, terrestrial or avian species, in which adults might exhibit visually detectable differences, such as sexual dimorphism, external body characteristics or grouping tendencies.
Operational sex ratio predicts the opportunity and direction of sexual selection across animals
<p>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 selec- tion 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.</p>
Data from: Mate choice and the operational sex ratio: an experimental test with robotic crabs
The operational sex ratio (OSR) (sexually active males: receptive females) predicts the intensity of competition for mates. It is less clear, however, under what circumstances the OSR predicts the strength of sexual selection – that is, the extent to which variation in mating success is attributable to traits that increase the bearer's attractiveness and/or fighting ability. To establish causality experiments are required that manipulate OSR. Furthermore, if it is possible to control for any OSR-dependent changes in the chosen sex (e.g. changes in male courtship), we can directly test whether the OSR affects the behaviour of the choosing sex (e.g. female choice decisions). We conducted female mate choice experiments in the field using robotic models of male fiddler crabs (Uca mjoebergi). We used a novel design with two females tested sequentially per trial. As in nature, the choice of the first female to mate therefore affected the mates available to the next female. In general we detected significant sexual selection due to female choice for 'males' with larger claws. Importantly, the strength of sexual selection did not vary across five different OSR/density treatments. However, as the OSR decreased (hence the number of available males declined), females chose the 'males' with the largest claws available significantly more often than expected by chance. Possible reasons for this mismatch between the expected and observed effects of the OSR on the strength of sexual selection are discussed.
Data from: Opposing effects of mortality factors on progeny operational sex ratio may thwart adaptive manipulation of primary sex ratio
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Data from: Mate choice and the operational sex ratio: an experimental test with robotic crabs
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Data from: Adult sex ratio and operational sex ratio exhibit different temporal dynamics in the wild
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Sex-specific sterility caused by extreme temperatures is likely to create cryptic changes to the operational sex ratio in Drosophila virilis
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Data from: Detecting elusive aspects of wildlife ecology using drones: new insights on the mating dynamics and operational sex ratios of sea turtles
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Divergence of responses to variable socio-sexual environments in laboratory populations of Drosophila melanogaster evolving under altered operational sex-ratios
<p class="BodyA">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 intra-sexual competition are disparate and widespread in males.</p> <p class="BodyA"><span>We investigated the evolution of plasticity in reproductive traits using <i>Drosophila melanogaster</i> populations evolving for more than 150 generations under male- or female-biased sex ratios.</span> <span>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. </span></p>
Data from: Sperm competitive ability evolves in response to experimental alteration of operational sex ratio
In naturally polygamous organisms such as Drosophila, sperm competitive ability is one of the most important components of male fitness and is expected to evolve in response to varying degrees of male–male competition. Several studies have documented the existence of ample genetic variation in sperm competitive ability of males. However, many experimental evolution studies have found sperm competitive ability to be unresponsive to selection. Even direct selection for increased sperm competitive ability has failed to yield any measurable changes. Here we report the evolution of sperm competitive ability (sperm defense-P1, offense-P2) in a set of replicate populations of Drosophila melanogaster subjected to altered levels of male–male competition (generated by varying the operational sex ratio) for 55–60 generations. Males from populations with female-biased operational sex ratio evolved reduced P1 and P2, without any measurable change in the male reproductive behavior. Males in the male-biased regime evolved increased P1, but there was no significant change in P2. Increase in P1 was associated with an increase in copulation duration, possibly indicating greater ejaculate investment by these males. This study is one of the few to provide empirical evidence for the evolution of sperm competitive ability of males under different levels of male–male competition.
Data from: Operational sex ratio but not density affects sexual selection in a fish
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