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303 results for “Sex ratio”
Data from: The spatial ecology of sex ratios in a dioecious plant: relations between ramet and genet sex ratios
1. In clonal dioecious plants, the frequency and spatial distribution of flowering ramets contains information on the underlying genet sex ratio. These measures can also provide insight on potential ecological mechanisms causing variation and bias in sex ratios among populations. 2. We used a novel likelihood-based approach and spatial clustering model to estimate the genet sex ratios from flowering ramet data collected from 32 populations of dioecious Thalictrum pubescens, a clonal species from eastern N. America that occupies moist wetland and forested environments. We investigated sex ratios of seed families, clone size, patterns of flowering and plant height to determine potential causes of sex ratio bias. 3. Flowering ramet sex ratios varied considerably among populations but were significantly male-biased. Seed families grown to flowering also exhibited the same degree of male bias. Both models predicted close correspondence between ramet and genet sex ratios. The likelihood model revealed that gender differences in ramet production could not account for biased sex ratios. The spatial clustering model indicated that ramets were significantly clustered at two spatial scales and estimated similar cluster sizes and densities for both sexes. There was no evidence for spatial segregation of the sexes. Both sexes were equally likely to flower in consecutive years and repeated bouts of flowering had no effect on ramet height. 4. Synthesis. Our analyses suggest that the widespread occurrence of male-biased sex ratios in T. pubescens is unlikely to result from sexual differences in clonal growth or habitat preferences. The bias appears to become established early in the life cycle, perhaps at the seed stage as consequence of local resource competition.
Data from: Sex ratio effects on reproductive strategies in humans
Characterizations of coy females and ardent males are rooted in models of sexual selection that are increasingly outdated. Evolutionary feedbacks can strongly influence the sex roles and subsequent patterns of sex differentiated investment in mating effort, with a key component being the adult sex ratio (ASR). Using data from eight Makushi communities of southern Guyana, characterized by varying ASRs contingent on migration, we show that even within a single ethnic group, male mating effort varies in predictable ways with the ASR. At male-biased sex ratios, men's and women's investment in mating effort are indistinguishable; only when men are in the minority are they more inclined towards short-term, low investment relationships than women. Our results support the behavioural ecological tenet that reproductive strategies are predictable and contingent on varying situational factors.
Data from: Testing sex ratio theory with the malaria parasite Plasmodium mexicanum in natural and experimental infections
The malaria parasite (Plasmodium) life history accords well with the assumptions of Local Mate Competition (LMC) of sex ratio theory. Within a single meal of the blood-feeding vector, sexually dimorphic gametocyte cells produce gametes (females produce 1, males several) that mate and undergo sexual recombination. The theory posits several factors drive the Plasmodium sex ratio: male fecundity (gametes/ male gametocyte), number and relative abundance of parasite clones, and gametocyte density. We measured these traits for the lizard malaria parasite, P. mexicanum, with a large sample of natural infections and infections from experiments which manipulated clonal diversity. Sex ratio in single-clone infections was slightly female-biased, but matched predictions of theory for this low-fecundity species. Sex ratio was less female-biased in clonally diverse infections as predicted by LMC for the experimental, but not natural infections. Gametocyte density was not positively related to sex ratio. These results are explained by the P. mexicanum life history of naturally low clonal diversity and high gametocyte production. This is the first study of a natural malaria system that examines all traits relevant to LMC in individual vertebrate hosts and suggests a striking example of sex ratio theory having significance for human public health.
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.
Data from: Positive selection of deleterious alleles through interaction with a sex-ratio suppressor gene in African buffalo: a plausible new mechanism for a high frequency anomaly
Although generally rare, deleterious alleles can become common through genetic drift, hitchhiking or reductions in selective constraints. Here we present a possible new mechanism that explains the attainment of high frequencies of deleterious alleles in the African buffalo (Syncerus caffer) population of Kruger National Park, through positive selection of these alleles that is ultimately driven by a sex-ratio suppressor. We have previously shown that one in four Kruger buffalo has a Y-chromosome profile that, despite being associated with low body condition, appears to impart a relative reproductive advantage, and which is stably maintained through a sex-ratio suppressor. Apparently, this sex-ratio suppressor prevents fertility reduction that generally accompanies sex-ratio distortion. We hypothesize that this body-condition-associated reproductive advantage increases the fitness of alleles that negatively affect male body condition, causing genome-wide positive selection of these alleles. To investigate this we genotyped 459 buffalo using 17 autosomal microsatellites. By correlating heterozygosity with body condition (heterozygosity-fitness correlations), we found that most microsatellites were associated with one of two gene types: one with elevated frequencies of deleterious alleles that have a negative effect on body condition, irrespective of sex; the other with elevated frequencies of sexually antagonistic alleles that are negative for male body condition but positive for female body condition. Positive selection and a direct association with a Y-chromosomal sex-ratio suppressor are indicated, respectively, by allele clines and by relatively high numbers of homozygous deleterious alleles among sex-ratio suppressor carriers. This study, which employs novel statistical techniques to analyse heterozygosity-fitness correlations, is the first to demonstrate the abundance of sexually-antagonistic genes in a natural mammal population. It also has important implications for our understanding not only of the evolutionary and ecological dynamics of sex-ratio distorters and suppressors, but also of the functioning of deleterious and sexually-antagonistic alleles, and their impact on population viability.
Data from: Is biasing offspring sex ratio adaptive? a test of Fisher's principle across multiple generations of a wild mammal in a fluctuating environment
Fisher's principle explains that population sex ratio in sexually reproducing organisms is maintained at 1:1 due to negative frequency-dependent selection, such that individuals of the rare sex realize greater reproductive opportunity than individuals of the more common sex until equilibrium is reached. If biasing offspring sex ratio towards the rare sex is adaptive, individuals that do so should have a higher number of grandoffspring. In a wild population of North American red squirrels (Tamiasciurus hudsonicus) that experiences fluctuations in resource abundance and population density, we show that overall across 26 years, the secondary sex ratio was 1:1; however, stretches of years during which adult sex ratio was biased did not yield offspring sex ratios biased towards the rare sex. Females that had litters biased towards the rare sex did not have more grandoffspring. Critically, adult sex ratio was not temporally autocorrelated across years, thus the population sex ratio experienced by parents was independent of the population sex ratio experienced by their offspring by the time of their primiparity. Expected fitness benefits of biasing offspring sex ratio may be masked or negated by fluctuating environments across years, which limit the predictive value of the current sex ratio.
Extraordinarily precise nematode sex ratios: adaptive responses to vanishingly rare mating opportunities
<p><span>Sex ratio theory predicts both mean sex ratio and variance under a range of population structures. Here, we compare two genera of phoretic nematodes (<i>Parasitodiplogaster </i>and <i>Ficophagus </i>spp.) associated with twelve fig-pollinating wasp species in Panama. The host wasps exhibit classic Local Mate Competition: only inseminated females disperse from natal figs, and their offspring form mating pools that consist of scores of the adult offspring contributed by one or a few foundress mothers. In contrast, in both nematode genera only sexually undifferentiated juveniles disperse and their mating pools routinely consist of ten or fewer adults.</span> <span>Across all mating pool sizes, the sex ratios observed in both nematode genera are consistently female-biased (~0.34 males), which is markedly less female-biased than is often observed in the host wasps (~0.10 males). In further contrast with their hosts, variances in nematode sex ratios are also consistently precise (significantly less than binomial). The constraints associated with predictably small mating pools within highly subdivided populations appear to select for precise sex ratios that contribute both to the reproductive success of individual nematodes, and to the evolutionary persistence of nematode species. We suggest that some form of environmental sex determination underlies these precise sex ratios.</span></p>
Spatiotemporal variation in hatching success and nestling sex ratios track rapid movement of a songbird hybrid zone
<p>Hybridization often occurs at the parapatric range interface between closely related species, but fitness outcomes vary: hybrid offspring exhibit diverse rates of viability and reproduction when compared to their parental species. The mobile hybrid zone between two chickadee congeners ( Poecile atricapillus x P. carolinensis ) has been well studied behaviorally and genetically but the viability of hybrids, as well as the underlying mechanisms contributing to hybrid fitness, have remained unclear. To better characterize the fitness costs of hybridization in this system, we analyzed 21 years of data from four sites, including over 1,400 breeding attempts by the two species, to show that rates of hatching success changed substantially as the zone of hybridization moved across the landscape. Admixture-associated declines in hatching success correlated with reduced proportions of heterogametic (female) offspring as predicted by Haldane's rule. Our data support an underlying mechanism implicating genetic admixture of the homogametic (male) parent as the primary determinant of offspring sex ratio, via incompatibilities on the hemizygous Z chromosome. Our long-term study is the first to directly measure changes in fitness costs as a vertebrate hybrid zone moves, and it shows that changes in these costs are a way to track the distribution of a hybrid zone across the landscape.</p>
Density dependence of clutch size and offspring sex ratio in starling colonies
<p><span>Optimal life-history theory predicts that individuals should adjust both the number and the sex of their offspring to maximize fitness in response to environmental and social factors such as breeding density. While reductions in optimal clutch size are well-studied in birds, the evidence for sex ratio adjustments is still equivocal and, so far, we lack a thorough understanding of how these strategies interact to maximize fitness. Here, we investigate how breeding density simultaneously affects brood sex ratio and clutch size in a sexually dimorphic and polygynous bird. We tested the prediction that mothers breeding at a higher density lay smaller clutches and overproduce daughters, the sex with less variable fitness returns and that disperses further away from their natal territory. We distributed nest boxes at either a high (HD) or a low density (LD) and monitored clutch sizes and sex ratios during five years in a wild breeding colony of spotless starlings. While mothers breeding in HD nests produced more daughters than those breeding in LD nests, the density dependence of clutch size varied among years, with a tendency to lay smaller clutches in HD nests. Our results suggest that mothers consistently adjust offspring sex ratio in response to breeding density, whereas adjustments in clutch size varied in a more complex way. These results support the role of sex allocation strategies in response to density and show that further theoretical and empirical research is required to understand the interaction between clutch size and sex ratio adjustments in animals.</span></p>
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>
Figure 2 in Sex ratios of oribatid mite assemblages differ among microhabitats
Figure 2. Average sex ratio [% females] of sexual oribatid mite species found in a respective microhabitat. Different letters indicate significant differences (Tukey pairwise contrast; P ≤ 0.05). Circles represent means; lines are standard errors.
Figure 1 in Sex ratios of oribatid mite assemblages differ among microhabitats
Figure 1. Summarized numbers of sexual vs parthenogenetic species (A) and individuals (B), respectively, and percentages (C) of sexually reproducing individuals (light bars) and sexual species (dark bars) in the microhabitats dead wood, grass sod, lichen, litter, moss, and tree bark.
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: Gynodioecy in structured populations: understanding fine-scale sex ratio variation in Beta vulgaris ssp. maritima
Natural selection, random processes, and gene flow are known to generate sex ratio variations among sexually polymorphic plant populations. In gynodioecious species, in which hermaphrodites and females coexist, the relative effect of these processes on the maintenance of sex polymorphism is still up for debate. The aim of this study was to document sex ratio and cytonuclear genetic variation at a very local scale in wind-pollinated gynodioecious Beta vulgaris ssp. maritima, and attempt to elucidate which processes explained the observed variation. The study sites were characterized by geographically distinct patches of individuals and appeared to be dynamic entities, with recurrent establishment of distinct haplotypes through independent founder events. Along with substantial variation in sex ratio and unexpectedly low gene flow within study sites, our results showed a high genetic differentiation among a mosaic of genetically distinct demes, with isolation by distance or abrupt genetic discontinuities taking place within a few tens of meters. Overall, random founder events with restricted gene flow could be primary determinants of sex structure, by promoting the clumping of sex-determining genes. Such high levels of sex structure provide a landscape for differential selection acting on sex-determining genes, which could modify the conditions of maintenance of gynodioecy in structured populations.
Brain size affects responsiveness in mating behavior to variation in predation pressure and sex-ratio
<p>Despite ongoing advances in sexual selection theory, the evolution of mating decisions remains enigmatic. Cognitive processes often require simultaneous processing of multiple sources of information from environmental and social cues. However, little experimental data exist on how cognitive ability affects such fitness-associated aspects of behavior. Using advanced tracking techniques, we studied mating behaviors of guppies artificially selected for divergence in relative brain size, with known differences in cognitive ability, when predation threat and sex-ratio was varied. In females, we found a general increase in copulation behavior in when the sex-ratio was female biased, but only large-brained females responded with greater willingness to copulate under a low predation threat. In males, we found that small-brained individuals courted more intensively and displayed more aggressive behaviors than large-brained individuals. However, there were no differences in female response to males with different brain size. These results provide further evidence of a role for female brain size in optimal decision-making in a mating context. In addition, our results indicate that brain size may affect mating display skill in male guppies. We suggest that it is important to consider the association between brain size, cognitive ability and sexual behavior when studying how morphological and behavioral traits evolve in wild populations.</p>
Queen-worker conflict can drive the evolution of social polymorphism and split sex ratios in facultatively eusocial life-cycles
<p>Hamilton's idea that haplodiploidy favors the evolution of altruism – the haplodiploidy hypothesis -- relies on the relatedness asymmetry between the sexes, caused by the sex-specific ploidies. Theoretical work on the consequences of relatedness asymmetries has significantly improved our understanding of sex-allocation and intra-colony conflicts, but the importance of haplodiploidy for the evolution of altruism came to be seen as minor. However, recently it was shown that haplodiploidy can strongly favor the evolution of eusociality, provided additional "preadaptations" are also present, such as the production of multiple broods per season and maternal ability to bias offspring sex ratios. These results were obtained assuming no influence of workers on the sex ratio, even though worker control of the sex ratio is known to occur. Here we model the evolution of sex-specific fratricide as a mechanism of worker control over the sex ratio. We show that fratricide can facilitate the initial evolution of helping. However, fratricide can also hamper the evolution of unconditional help. Instead, social polymorphism evolves, a mixture of helping and dispersing offspring. Finally, we show that the co-evolution of sex-allocation strategies of workers (fratricide) and queens leads to a split production of the sexes, with some colonies specializing in males and others in females. Thus, the model predicts that fratricide spawns a diversity of co-existing life cycles that strongly vary in degree of sociality and sex ratios.</p>
Figure 2 in Population dynamics, seasonality and sex ratio of twig-girdling beetles (Coleoptera: Cerambycidae: Lamiinae: Onciderini) of an Atlantic rain forest in south-eastern Brazil
Figure 2. Mean (± SE) number of adult beetles of the tribe Onciderini found at the Serra do Japi during the period of adult activity (between October and May) from 2002 to 2006. (A) Species with peak abundance between December and February. (B) Species with peak abundance between January and March. (n = 5). An asterisk indicates significance among monthly abundances after Bonferroni correction (Friedman's ANOVA; p ≤ 0.013).
Figure 1 in Population dynamics, seasonality and sex ratio of twig-girdling beetles (Coleoptera: Cerambycidae: Lamiinae: Onciderini) of an Atlantic rain forest in south-eastern Brazil
Figure 1. Rank-abundance pattern of adult beetles of the tribe Onciderini found at the Serra do Japi between December 2002 and December 2006. (n = 1113 individuals).
Figure 3 in Population dynamics, seasonality and sex ratio of twig-girdling beetles (Coleoptera: Cerambycidae: Lamiinae: Onciderini) of an Atlantic rain forest in south-eastern Brazil
Figure 3. (A) Population fluctuation (number of individuals) of adult beetles of the tribe Onciderini found at the Serra do Japi between December 2002 and December 2006 (n = 1113 individuals). (B) Climate diagram for the Biological Reserve of the Serra do Japi at the same time period (data from a meteorological station approximately 8 km from the study site). The mean monthly temperature (◦ C) and rainfall (mm) are scaled to represent the potential evapotranspiration. Values inside each diagram indicate mean temperature (◦C) and total rainfall (mm) for each sampling year. Dry months are represented by dotted areas; humid months by the vertical lines and months with rain in excess of 100 mm are in solid black.
Influence of local density and sex ratio on pollination in an ambophilous flowering plant
<p class="MSbody"><span><b>PREMISE OF THE STUDY:</b> Variation in local density and sex ratio in dioecious plants can affect mating success through the actions of pollen vectors, principally generalist insects or wind. Increased density and male-biased sex ratios should promote pollen transfer and seed production, but their combined effects have not been investigated for ambophilous species, which exhibit both insect and wind pollination. </span></p> <p class="MSbody"><span><b>METHODS: </b>We manipulated density (low vs. high) and sex ratio (1:1 vs. 3:1 male-biased) in arrays of dioecious ambophilous <i>Thalictrum pubescens</i>. We quantified visitation rates and foraging time to examine whether pollinators exhibited sex-specific preferences and determined the seed set of arrays.<b> </b></span></p> <p class="MSbody"><b>KEY RESULTS:</b> Pollinators visited more plants per foraging bout at high than low density. Visitation rates and foraging times of visitors were greater for male than female plants but did not depend on the density or sex ratio of arrays. However, whereas solitary bees displayed a strong preference for males, hover flies were indifferent to plant sex phenotype. Solitary bees also visited significantly more plants per foraging bout than hover flies. There was a significant interaction between density and sex ratio on seed set. At low density, seed set was greater for 1:1 than 3:1 arrays, but at high density the opposite pattern occurred. </p> <p class="MSbody"><b>CONCLUSIONS: </b>The demographic factors we investigated had complex influences on pollinator foraging behavior and patterns of seed set. Several factors may explain our results including the influence of density and sex ratio on pollen export from arrays, grooming by pollinators and the contribution of wind pollination.</p>
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