Pattern Of Sex Ratio Variation Research Articles

Paucity and Disparity of Publicly Available Sex-Disaggregated Data for the COVID-19 Epidemic Hamper Evidence-Based Decision-Making.

COVID-19 has joined the long list of sexually dimorphic human disorders. Higher lethality in men, evident in the first reports from China, was confirmed in the subsequent Italian outbreak. Newspapers and scientific journals commented on this finding and the preexisting conditions, biological processes, and behavioral differences that may underlie it. However, little appeared to be released about sex differences in severity of disease, comorbidities, rate of recovery, length of hospital stay, or number of tests performed. Systematic analysis of official websites for 20 countries and 6 US states revealed a wide disparity in sex-disaggregated data made available to the public and scholars. Only a handful reported cases by sex. None of the other characteristics, including deaths, were stratified by sex at the time. Beyond suboptimal sex disaggregation, we found a paucity of usable raw data sets and a generalized lack of standardization of captured data, making comparisons difficult. A second round of data capture in April found more complete, but even more disparate, information. Our analysis revealed a wide range of sex ratios among confirmed cases. In countries where a male bias was initially reported, the proportion of women dramatically increased in 3 weeks. Analysis also revealed a complex pattern of sex ratio variation with age. Accurate, peer-reviewed, analysis of harmonized, sex-disaggregated data for characteristics of epidemics, such as availability of testing, suspected source of infection, or comorbidities, will be critical to understand where the observed disparities come from and to generate evidence-based recommendations for decision-making by governments.Electronic supplementary materialThe online version of this article (10.1007/s10508-020-01882-w) contains supplementary material, which is available to authorized users.

Variation and evolution of sex ratios at the northern range limit of a sexually polymorphic plant

Gender strategies involve three fundamental sex phenotypes - female, male and hermaphrodite. Their frequencies in populations typically define plant sexual systems. Patterns of sex-ratio variation in a geographical context can provide insight into transitions among sexual systems, because environmental gradients differentially influence sex phenotype fitness. Here, we investigate sex-ratio variation in 116 populations of Sagittaria latifolia at the northern range limit in eastern N. America and evaluate mechanisms responsible for the patterns observed. We detected continuous variation in sex phenotype frequencies from monoecy through subdioecy to dioecy. There was a decline in the frequency and flower production of females in northerly populations, whereas hermaphrodite frequencies increased at the range limit, and in small populations. Tests of a model of sex-ratio evolution, using empirical estimates of fitness components, indicated that the relative female and male contribution of males and hermaphrodites to fitness is closer to equilibrium expectations than female frequencies. Plasticity in sex expression and clonality likely contribute to deviations from equilibrium expectations.

Patterns of sex ratio variation and genetic diversity in the dioecious forest perennial Mercurialis perennis

In small populations of plant species with separate sexes, it can be expected that besides the local environment also stochastic events influence population sex ratios. Biased sex ratios may in turn negatively affect genetic diversity due to increased genetic drift and, in clonal plants, due to reduced sexual reproductive output. Empirical evidence for these processes is scarce, however. We investigated the pattern of sex ratio variation and the distribution of genetic variation of the dioecious clonal forest herb Mercurialis perennis using AFLP markers. Analysis of molecular variance indicated a pronounced genetic structure. Overall within-population genetic diversity was moderate and local sex ratios were slightly male biased. The proportion of male to female plants in large populations slightly increased with increasing light penetration to the herb layer. Small populations, on the contrary, displayed high variability in sex ratios, unrelated to the local light environment. Genotypic diversity decreased with more male-biased sex ratios. We conclude that stochastic events related to small population size and the local forest environment, related to canopy closure, affect the proportion of female plants and indirectly influence local genotypic diversity, likely through the degree of sexual reproduction. This is one of the first studies to report a clear association between gender proportions and genetic diversity of a dioecious plant species in a fairly large survey.

Sex ratio schedules in a dynamic game: the effect of competitive asymmetry by male emergence order

Studies of sex allocation have provided some of the most successful tests of theory in behavioral and evolutionary ecology. For instance, local mate competition (LMC) theory has explained variation in sex allocation across numerous species. However, some patterns of sex ratio variation remain unexplained by existing theory. Most existing models have ignored variation in male competitive ability and assumed all males have equal opportunities to mate within a patch. However, in some species experiencing LMC, males often fight fiercely for mates, such that male mating success varies with male fighting ability. Here, we examine the effect of competitive ability on optimal sex allocation schedules using a dynamic programming approach. This model assumes an asymmetric competitive ability derived from different mortalities according to the timing of male emergence. If the mortality of newly emerging males is larger than that of already emerged males, our model predicts a more female-biased sex ratio than expected under traditional LMC models. In addition, females are predicted to produce new males constantly at a low rate over the offspring emergence period. We show that our model successfully predicts the sex ratios produced by females of the parasitoid wasp Melittobia, a genus renowned for its vigorously fighting males and lower than expected sex ratios. Key words: dynamic game model, lethal male combat, local mate competition, Melittobia, parasitoid wasps, sex allocation. [Behav Ecol]

Offspring sex ratio variation in relation to brood size and mortality in a promiscuous species: the Aquatic Warbler Acrocephalus paludicola

We analyse nestling sex ratio variation in the Aquatic Warbler Acrocephalus paludicola to test for predictions from sex allocation theory that the brood sex ratio is close to parity. We also tested Fiala's (1980) prediction that there is no difference in sex ratio between broods affected and not affected by mortality, and whether a shift in primary sex ratios or simple differential mortality by sex underlies that difference. Furthermore, we explore additional analytical possibilities for inferring proximate mechanisms through simulation modelling. In the Aquatic Warbler, which is promiscuous, the overall sex ratio determined by molecular sexing of nestlings at 8–11 days of age did not deviate significantly from parity (proportion of females 0.509), nor did we find any predictive effect of brood size, maternal body mass, fat condition, wing and bill length, laying date, mean daily temperature, and multiple‐male mating. However, extensive simulation suggested that the whole pattern of sex ratio variation is unlikely to arise purely by chance: (1) there is a diverging sex ratio between complete and partial broods, (2) large broods tended to be female‐biased and small broods male‐biased, and (3) low ambient temperature prior to the laying period seemed to increase the proportion of female offspring in complete broods. We conclude that most variation in nestling sex ratio is non‐adaptive in nature, and results from variation in female nestlings mortality dependent on brood size and sex ratio.

Patterns of Sex Ratio Variation in House Sparrows

AbstractPCR amplification of a sex-linked gene was used to assay the sex ratio of nestling House Sparrows (Passer domesticus) from five consecutive breeding seasons. We tested several predictions from sex ratio theory, including that at the population level, sparrows should produce a 1:1 sex ratio. Of 1162 nestlings, 53% were male, which is not significantly different from 50%. We did find a significant skew toward males in two of the five years, and significant heterogeneity in sex ratio among seasons. There was no evidence that brood sex ratios were skewed from a binomial distribution, despite a modest excess of all male broods. We found that male nestlings weighed significantly more (0.5 g) than their female siblings. We tested the possibility that females produce males when conditions are good. Sex ratio was not associated with nest attempt, despite some evidence that conditions varied seasonally. Clutch size was negatively associated with date of first egg, but neither nestling weight nor sex ratio showed any correlation with date. We also tested an array of female characteristics; sex ratio was not associated with female body size, condition, or age. Females that hatched a larger proportion of eggs produced more males, a consistent pattern in all five seasons. However, the female's mate's characteristics (size, condition, age, or size of throat patch) did not influence sex ratio. These data contribute to a growing number of studies that provide an inconsistent picture of sex ratio variation in birds.Patrones de Variación en la Proporción de Sexos de Passer domesticusResumen. Un gen ligado al sexo, amplificado por PCR, fue usado para examinar la proporción de sexos de pichones de Passer domesticus provenientes de cinco temporadas de apareamiento consecutivas. Se probaron varias predicciones de la teoría de proporción de sexos, incluyendo que a nivel poblacional, los gorriones deberían producir una proporción de sexos 1:1. De 1162 pichones, 53% fueron machos, lo cual no difiere significativamente de 50%. Se encontró un sesgo significativo hacia machos en dos de los cinco años y una heterogeneidad significativa en la proporción de sexos entre estaciones. No hubo evidencia de que la proporción de sexos de la camada se desviara de una distribución binomial, a pesar de un moderado exceso de crías macho. Se encontró que los pichones machos pesaron significativamente más (0.5 g) que sus hermanas. Se examinó la posibilidad de que las hembras producen machos cuando las condiciones son benignas. La proporción de sexos no estuvo asociado con el momento de anidación dentro de la temporada, a pesar de que algunas evidencias sugieren que las condiciones variaron estacionalmente. El tamaño de la camada estuvo negativamente asociado con la fecha de puesta del primer huevo, pero ni el peso del pichón ni la proporción de sexos mostró correlación con dicha fecha. Se examinaron además un serie de características de la hembra; la proporción de sexos no estuvo asociada con el tamaño del cuerpo, la condición o la edad de la hembra. Las hembras que empollaron una mayor cantidad de huevos produjeron más machos, un patrón consistente en las cinco temporadas. Las características de las parejas de las hembras (tamaño, condición o tamaño de la mancha del cuello) no influyeron la proporción de sexos. Estos datos contribuyen al creciente número de estudios que presentan una descripción contradictoria de la variación de la proporción de sexos en aves.

Fledgling sex ratio variation and future reproduction probability in Montagu's harrier, Circus pygargus

I studied patterns of sex ratio variation in 543 fledglings from 192 broods of Montagu's harriers Circus pygargus (1992–1998) in Madrid, central Spain, to determine whether offspring sex ratio was adjusted according to variables influencing future reproduction. Fledgling sex ratio variation was related to estimates of food availability during the pre-laying period, with more females produced in years of higher food availability. This variation was related to the probability that females, but not males, were recruited to the breeding population and were recruited at younger ages, if fledged in good food years. Furthermore, the relationship between food and sex ratio was only significant for the first two nestlings in each brood, and only nestlings from first and second ranks benefited from good breeding conditions to advance their age of first breeding. Results presented here differ from other published data for the same species. Geographical variation in observed sex ratio might be explained by differences between populations in factors influencing age of first breeding or other demographic variables.

Sex-specific dispersal in spatially varying environments leads to habitat-dependent evolutionary stable offspring sex ratios

When the environment varies spatially, so that some habitats are more favorable to reproduction than others, an individual should attempt to increase the number of offspring establishing in high-quality habitats. Hence, if male and female dispersal behavior differ, it may be adaptive to produce more offspring of the more dispersing sex in low-quality habitats, since these offspring are likely to disperse to another patch, and more offspring of the most philopatric sex in high-quality habitats, since these offspring are likely to remain in that patch. Such a strategy is shown to be evolutionarily stable provided that male and female dispersal rates are different and that reproductive success varies between habitats (lack of ideal free distribution). Highly biased sex ratios are predicted (1) in rare habitats, (2) in poor habitats, (3) when difference between habitat quality is large, (4) when at least one sex disperses at a rate close to random with respect to habitat availability, (5) when both sexes disperse at a high rate, (6) when individuals are unable to select their reproducing habitat, and, presumably, (7) with moderate temporal variation of habitat quality. The model appears to be a good candidate to explain the pattern of sex ratio variation in a variety of species: phytophagous arthropods, species with environmental sex determination, and territorial passerines. Key words: environmental sex determination, ideal free distribution model, phytophagous arthropods, reaction norm, territorial passerines. [Behav Ecol 11:421–428 (2000)]

Sex Ratios in Extant Ungulates: Products of Contemporary Predation or Past Life Histories?

Variation in mammalian adult sex ratios (ASR) is striking both within and among species. Darwin (1871) originally suggested that causes of variation included competition among males for females and predation. He also recognized that intensity of competition might be greater in species adorned with secondary sexual traits. Assuming that sexual dimorphism is a reasonable indicator of intrasexual competition, we predicted that ASR would become increasingly skewed among dimorphic species and this pattern would be exacerbated by the intensity of predation. With effects of common ancestry removed by computing phylogenetically independent contrasts, we failed to detect strong relationships between ASR and either sexual dimorphism or body mass of males or females. Presence of predators also had no consistent effect on these broader patterns, and even within Cervidae and Bovidae, there was not a solid relationship between dimorphism and ASR. The only generalized pattern that emerged was that males were killed disproportional to their abundance (74% of 31 species). However, differences between live and killed sex ratios in a population were not correlated with female or male body mass or sexual dimorphism. Our results suggest that relationships between sexual dimorphism and mortality are not as straightforward as presumed. Nevertheless, at a proximate level, predation directly affects patterns of sex ratio variation among adult ungulates, but differences in survival of sexes may arise as a direct consequence of greater age-specific mortality among males, for which the ultimate cause is likely to be selection operating differently on males and females. A challenge for the future lies not so much in the separation of proximate from ultimate factors but in evaluating what, if any, life-history traits predispose sexes to differential mortality and the extent to which predation may shape these characteristics.

Contradictory findings in studies of sex ratio variation in roe deer ( Capreolus capreolus )

Patterns of sex ratio variation and maternal investment reported in the literature are often inconsistent. This could be due to intra- and inter-specific variation in social systems, but may also be a result of the a posteriori nature of much of this type of analysis or the testing of models which are inappropriate. Two recent papers reported directly opposed results concerning variation in offspring sex ratios in relation to maternal condition in roe deer, interpreting the results as support for the Trivers and Willard model and for the local resource competition hypothesis, respectively. In this paper, we present data on offspring sex ratios and early juvenile body weight from two long-term studies of this species to test predictions arising from these two models concerning sex biases in litter composition and maternal care. First, we observed no consistent pattern of sex differences in an index of weaning weight or body weight at 1 month old in either population, indicating a lack of sex bias in maternal care. However, in one population, higher maternal body weight was associated with higher juvenile body weight of daughters, but not of sons. Secondly, we found a negative, but not statistically significant, relationship between maternal body weight and litter sex ratio such that heavier females tended to produce more daughters and lighter females to produce more sons. These results indicate that roe females which have additional investment potential available do not invest it in sons, as predicted by the Trivers and Willard model. Our results may provide some support that roe deer are subject to local resource competition acting at the level of the individual mother; however, the fact that particular trends in sex ratio data can be explained in functional terms provides no indication that they are actually adaptive.

Sex ratio variation as a function of host size in Pseudacteon flies (Diptera: Phoridae), parasitoids of Solenopsis fire ants (Hymenoptera: Formicidae)

Some Pseudacteon (Diptera: Phoridae) flies are parasitoids of Solenopsis (Hymenoptera: Formicidae) ant workers in North and South America. Laboratory studies of sex allocation revealed a pattern of sex ratio variation as a function of host size, with more females arising from larger hosts. Environmental sex determination is a possible mechanism for the observed pattern, and examination of Pseudacteon life history reveals several traits assumed to be important in models predicting conditions under which environmental sex determination is favoured. Sex allocation patterns of Pseudacteon are compared with theoretical predictions and empirical data from better-studied hymenoptcran parasitoids that have haplodiploid sex determination. The pattern of sex ratio variation observed has important implications for biocontrol efforts of imported Solenopsis fire ants by the introduction of Pseudacteon parasitoids.

Sex ratio variation as a function of host size inPseudacteonflies (Diptera: Phoridae), parasitoids ofSolenopsisfire ants (Hymenoptera: Formicidae)

SomePseudacteon(Diptera: Phoridae) flies are parasitoids ofSolenopsis(Hymenoptera: Formicidae) ant workers in North and South America. Laboratory studies of sex allocation revealed a pattern of sex ratio variation as a function of host size, with more females arising from larger hosts. Environmental sex determination is a possible mechanism for the observed pattern, and examination ofPseudacteonlife history reveals several traits assumed to be important in models predicting conditions under which environmental sex determination is favoured. Sex allocation patterns ofPseudacteonare compared with theoretical predictions and empirical data from better-studied hymenopteran parasitoids that have haplodiploid sex determination. The pattern of sex ratio variation observed has important implications for biocontrol efforts of importedSolenopsisfire ants by the introduction ofPseudacteonparasitoids.