Variation In Sex Ratio Research Articles

Sex-ratio variation and the function of staminodes inAralia nudicaulis1This article is part of a Special Issue entitled “Pollination biology research in Canada: Perspectives on a mutualism at different scales”.

Aralia nudicaulis L. is a clonal dioecious herb common to forested ecosystems in eastern North America. Across 15 sites, the frequency of female ramets ranged between 5% and 95%. This variation was used to examine hypotheses involving the life history and pollination biology of A. nudicaulis. Because female reproductive function is thought to incur greater resource costs than male function, we predicted that female ramets should (i) be more common where resources (and light in particular) are more abundant and (ii) have lower rates of survival than male ramets. However, there was (i) no significant association between light availability (measured as canopy closure) and female frequencies across sites and (ii) ramet ages were skewed towards older ages for females, not males. Female A. nudicaulis produce staminodes that render female inflorescences visually similar to male inflorescences. We experimentally manipulated the presence–absence of staminodes and found that pollen receipt by females without staminodes was substantially lower than for plants with intact staminodes. Finally, pollinators tend to prefer visiting male over female flowers. We were interested in evaluating whether such preferences might scale up to the population level, with higher pollinator abundances at male-dominated sites. However, we instead found a strong positive association between pollinator abundances and female frequencies. Although there is unlikely to be a causal relationship between them, the identification of a common variable underlying variation in female frequencies and pollinator abundances might reveal key insights into the factors regulating the life history and pollination of forest understory plants.

Population Sex Ratios of Pleurocerid Snails (Leptoxisspp.): Variability and Relationships with Environmental Contaminants and Conditions

Abstract. Anthropogenic introduction of steroidal estrogens and synthetic estrogenic compounds to aquatic environments has increased interest in assessment of the endocrine-disrupting effects of these compounds on aquatic organisms, including mollusks. For gonochoristic freshwater snails in the family Pleuroceridae, little is known about the spatial and temporal variation in population sex ratios or potential relationships between sex ratios and exposure to estrogenic compounds. Population sex ratios of Leptoxis (Rafinesque, 1819) spp., were evaluated within the Shenandoah River watershed (Virginia, U.S.A.), where agricultural operations are a source of estrogenic compounds to streams, and in four other river basins in Virginia. Proportions of females varied among streams within the Shenandoah River watershed, with overall mean proportions from four sampling periods ranging from 0.46–0.87. Sex ratios were consistently female-biased at nine of fifteen stream sampling sites. There was little within-site var...

Variation in adult sex ratio alters the association between courtship, mating frequency and paternity in the lek‐forming fruitfly Ceratitis capitata

The intensity with which males deliver courtship and the frequency with which they mate are key components of male reproductive success. However, we expect the strength of the relationship between these traits and a male's overall paternity to be strongly context dependent, for example to be altered significantly by the extent of post-mating competition. We tested this prediction in a lekking insect, Ceratitis capitata (medfly). We examined the effect of manipulating the sex ratio from male- to female-biased (high and low male competition, respectively) on courtship behaviour, mating frequency and paternity of focal males. Under high male competition, focal males delivered significantly more courtship but gained lower paternity than under lower competition. Paternity was positively associated with mating frequency and small residual testes size. However, the association between mating frequency and paternity was significantly stronger under low competition. We conclude that manipulation of sex ratio significantly altered the predictors of mating success and paternity. The relationship between pre- and post-mating success is therefore plastic and alters according to the prevailing level of competition. The results highlight the importance of post-copulatory processes in lekking species and illuminate selection pressures placed on insects such as medflies that are mass reared for pest control.

Influences of orange spot patterns and operational sex ratio on male mating behaviors in the guppy Poecilia reticulata

The operational sex ratio (OSR) is an important factor that influences male mating behaviors in many animals. Several previous studies have demonstrated the effect of the OSR on mating behaviors in the guppy Poecilia reticulata, but the results are often inconsistent. In this study, we examined the effects of the OSR and exaggeration of a male sexual ornament (orange spot patterns) on mating behaviors in male guppies. We used a pair of males, one with large orange spots (bright male), an important criterion of female mate choice, and another with small orange spots (dull male). The OSR was adjusted based on the number of females. Both bright and dull males performed courtship displays and sneaking attempts more frequently under a female-biased OSR than those under a male-biased OSR. Bright males performed courtship displays more often than dull males, regardless of sex ratio. Under a male-biased OSR, females frequently responded to courtships from bright males, and bright males obtained more cooperative copulations than dull males. However, under a female-biased OSR, dull males frequently received female responses and copulated with females. These results indicate that mating behaviors of male guppies were affected by the OSR and males with large orange spots behaved differently than those with small orange spots. The results also imply that variations in local sex ratios may play a role in maintaining the diversity of orange spot patterns in male guppies by enhancing the mating opportunities of males with small orange spots in a female-biased environment.

Nest Temperatures and Hatchling Sex Ratios from Loggerhead Turtle Nests Incubated Under Natural Field Conditions in Georgia, United States

Abstract We examined loggerhead nest temperatures and hatchling sex ratios in an effort to more accurately predict hatchling sex ratios produced from 2 barrier islands in the northern management unit (Blackbeard Island National Wildlife Refuge and Wassaw National Wildlife Refuge, Georgia, United States) from 2000 to 2004. Temperature data loggers were placed into 169 nests to monitor incubation temperatures. Average critical period temperatures ranged from 26.3°C to 33.2°C (mean ± SE, 29.2° ± 0.1°C) and indicated seasonal variation in sex ratios. The sex of 669 hatchlings found dead in nests was histologically evaluated (n = 212 nests; 14–90 nests/yr). The sex ratios varied from 0% to 100% female per nest (n = 1–53 hatchlings/nest) and average sex ratio for all nests ranged from 55.5% female in 2003 to 85.4% female in 2002. In addition to monitoring nest temperature, 10 hatchlings per nest were euthanized to verify sex during 2003 on Blackbeard Island National Wildlife Refuge (n = 10 nests) and 2004...

Hatchling sex ratio and female mating status in the great reed warbler, Acrocephalus arundinaceus (Aves, Passeriformes): further evidence for offspring sex ratio manipulation

Offspring sex in birds is maternally determined, thus females are expected to vary brood sex ratio adaptively in response to various environmental and/or genetic conditions preferring one sex to the other sex. However, recent studies provide controversial evidence showing mixed support for adaptive sex manipulation theory. One problem may lie in the fact that only sex ratios at fledging have been used in many of these studies, ignoring post-hatching sex-biased mortality. To validate these results, some authors call for replication of previous researches. Here we examined hatchling sex ratio variation in relation to female mating status in a local population of facultatively polygynous great reed warblers Acrocephalus arundinaceus in Central Europe. In agreement with results of previous works, we found that monogamous and primary females had a higher proportion of sons in their broods than secondary females. These results provide further evidence that great reed warbler females manipulate the sex ratio of their offspring in response to their social status.

Male fecundity and optimal gametocyte sex ratios for Plasmodium falciparum during incomplete fertilization

A mathematical model of the within-vector dynamics of Plasmodium falciparum in an Anopheles mosquito with unbiased random mating and incomplete fertilization is used to investigate the effects of varying fecundity and population size on the gametocyte sex ratio when strains maximize their individual fitnesses. Previous studies considered either the effects of variable fecundity or the effects of incomplete fertilization. Here we investigate the simultaneous effects of variable fecundity, incomplete fertilization, and variable number of ingested gametocytes per strain on the optimal gametocyte sex ratio. Our model results agree with others in the case of two identically sized populations in that large differences in fecundities lead to female-biased total population sex ratios. When the assumption of identically sized populations is relaxed in our model, population sex ratios vary from highly female-biased to slightly male-biased, depending on relative strain fecundities and population sizes. Our results provide a plausible explanation for the high variation in gametocyte sex ratios of P. falciparum observed in nature.

An equal sex ratio followed by differential sex mortality causes overestimation of females in gall midges: no evidence for sex ratio regulation

Monogeny, the production of unisexual broods by individual females, has been recognized for nearly 80 years. The genetic nature of gall midges' sex determination predicts an equal numbers of male-producing and female-producing females in the populations such that the overall sex ratio is expected to be nearly 1:1. However, observations of some strictly monogenous populations with biased sex ratio, mainly toward females, have raised the question of whether gall midges are able to adjust their offspring sex ratio in response to changes in environmental conditions, and some authors have even considered sex ratio regulation as a strong force in the course of the evolution of monogeny. In this paper, first, by studying the sex ratio variations of the predatory gall midge, Aphidoletes aphidimyza within a generation, we showed that adult males emerge up to 1 day earlier and have shorter life span than females (less than 4 days and up to 6 days, respectively). Although, the sex ratio of A. aphidimyza at the time of emergence was nearly 1:1 (52.41% males), a simple population simulation indicated that the differential mortality of sexes can lead to a female-biased sex ratio estimation (57.88% females) under random sampling in the natural environments. Our results imply that the primary sex ratio of monogenous gall midges is nearly 1:1 and that the arrhenogenic/thelygenic gall midges are not able to alter the number of their male/female progenies in response to changes in environmental conditions.

Variable Variation: Annual and Seasonal Changes in Offspring Sex Ratio in a Bat

Many organisms produce offspring with sex-ratios that deviate from equal numbers of males and females, and numerous adaptive explanations have been proposed. In some species, offspring sex-ratio varies across the reproductive season, again with several explanations as to why this might be adaptive. However, patterns for birds and mammals are inconsistent, and multiple factors are likely involved. Long-term studies on a variety of species may help untangle the complexity. I analyzed a long-term data set on the variation in offspring sex-ratio of the big brown bat, Eptesicus fuscus, a temperate-zone, insectivorous species. Sex ratio varied seasonally, but only in some years. Births early in the season were significantly female biased in years in which parturition occurred relatively early, but not in years with late parturition. Survival of female pups increased with earlier median birth date for the colony, and early-born females were more likely to survive and reproduce as one-year olds, compared to later-born pups. I argue that, due to the unusual timing of reproductive activities in male and female bats that hibernate, producing female offspring early in the year increases their probability of reproducing as one year olds, but this is not the case for male offspring. Thus, mothers that can give birth early in the year, benefit most by producing a female pup. The relative benefit of producing female or male offspring varies depending on the length of the growing season and thus the time available for female pups to reach sexual maturity. This suggests that not only does sex-ratio vary seasonally and among years, depending on the condition of the mother and the environment, but also likely varies geographically due to differences in season length.

Status of and threats to the Black-winged Lovebird (Agapornis taranta) in Entoto Natural Park and Bole Sub-City, Addis Ababa, Ethiopia

A study of the population status of and threats to the Black-winged Lovebird (Agapornis taran- ta) was carried out from September 2008 to February 2009 in Entoto Natural Park (ENP) and in Bole Sub- City, Addis Ababa, Ethiopia. A point transect count technique was employed to examine the status of the lovebirds. An average total of 58 birds was recorded of which 46 were counted in Bole Sub-City and 12 in Entoto Natural Park areas. The maximum number recorded during our census was 57 in the dry season and 15 in the wet season in Bole Sub-City and Entoto Natural Park, respectively. The ratio of adult males to adult females in Entoto Natural Park was 1:0.71 and 1:0.60, and 1:0.76 and 1:0.78 in Bole Sub-City dur- ing both wet and dry seasons, respectively. The seasonal variation and difference in sex ratios between males and females was not significant ( p > 0.05) at either of the study sites. The threat for the Black- winged Lovebird or other birds in Entoto Natural Park is due to the fragmentation of natural habitats, thus decreasing food sources, owing to the domination of eucalyptus (Eucalyptus globulus) trees in the area. Grazing by livestock, intensive farming practices and deforestation by the local community have contrib- uted greatly to the degradation of the land resulting in erosion, thus devastating the habitat of the Natural Park. The vegetation cover of Entoto Natural Park and urban gardens should be managed effectively for the survival of the avian fauna of the area.

Brood sex ratio varies with diet composition in a generalist raptor

While sex allocation has been investigated productively at both population and family levels, as yet no general theory has been developed that is capable of linking processes at these two ecological scales, and very few empirical studies have examined cross-scale patterns. In Finnish northern goshawks (Accipiter gentilis), nestling sex ratio of local subpopulations is related to the spatial and temporal variation in the abundance of their principal avian prey, woodland grouse. Using data from an urban breeding population in Hamburg, Germany, I investigated: (1) whether brood sex ratio of goshawks varies with diet composition at the family level; (2) whether such variation could reflect adaptive adjustment; and (3) how family-level allocation can drive population-level patterns, such as those observed in Finland. Feral pigeons (Columba livia) were the most important prey species, with a pooled contribution to total diet of 36%. Brood sex ratio varied significantly with the proportion of pigeons in the breeding-season diet of pairs (increasing male bias). However, there was no evidence for sex-differential effects of diet composition, so it remains unclear whether the observed sex-ratio variation was an adaptive response. As all study pairs inhabited an (urban) environment where pigeons were unusually abundant, family-level sex-ratio adjustment caused a marked male bias in offspring sex ratio at the population level (male-biased nestling sex ratio in four of five years; pooled data: 60% males). This suggests that the large-scale variation observed in Finnish goshawk populations mirrors sex-ratio adjustment shown by individual families in response to small-scale environmental conditions. Apart from linking patterns empirically across ecological scales, this study is, to my knowledge, the first to demonstrate that family-level brood sex ratio varies with realized resource use (diet composition) in a raptor species. Previous studies either failed to find significant associations or, more commonly, violated theoretical assumptions by measuring environmental prey abundance (often integrated over large areas) rather than realized prey use of individual breeding pairs. I conducted a meta-analysis of offspring sex-ratio data from 17 goshawk populations across Europe to put my results into perspective. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105, 937‐951.

Spatio‐temporal variation in mating success of female bagworms

AbstractThe study investigated spatio‐temporal variation in the mating success of female bagworms, Thyridopteryx ephemeraeformis Haworth (Lepidoptera: Psychidae), across a broad latitudinal range in Indiana, USA, between 2007 and 2009. A series of interconnected equations based on estimates of demographic parameters at different intervals was used to derive the punctual sex ratio and female mating success early and late in the season. Both the mating success of females and the relative abundance of bagworms declined with latitude, which provides indirect support to the mate encounter Allee hypothesis. However, the late emergence of females at northern locations combined with the consistently low mating success of late‐emerging females suggests that the impact of latitude on mating probability is indirectly mediated by emergence time. A variable level of protandry was observed each year, and the low ratio of males per female late in the season was associated with low female mating success. The reduced level of activity of males at temperatures below 18 °C may also account for the low mating probability of late‐emerging females. The weak, inconsistent effect of local variation in sex ratio on female mating success suggests that males commonly disperse from their natal patch, a behavior that may have evolved to reduce inbreeding in local populations of bagworms. Altogether, these results suggest that temporal variation in sex ratio has a greater impact on the mating success of female bagworms than spatial variation in sex ratio.

The Influence of Pollination Intensity on Fertilization Success, Progeny Sex Ratio, and Fitness in a Wind-Pollinated, Dioecious Plant

Variation in pollination intensity can influence the degree of gametophytic competition with diverse consequences for plant fitness. Such effects should be particularly important in wind-pollinated plants because they commonly possess uniovulate flowers. In several dioecious species, competition between female- and male-determining microgametophytes has been proposed as a cause of female-biased sex ratios. Here, we examine functional relations among pollination intensity, seed set, sex ratios, and sporophyte vigor in Rumex hastatulus, a wind-pollinated, dioecious plant. Using controlled pollinations, we applied a range of pollen grain numbers (1–71) to female flowers. Pollination intensity significantly predicted percent seed set, which was >80% when more than 15 pollen grains were added to stigmas. We estimated that the proportion of pollen grains participating in fertilization was 0.28 and the proportion of fertilized ovules developing into seed was 0.89. Overall, progeny sex ratios were female biased (0.62), with an increased probability of female offspring over the range of 1–15 pollen grains. We detected no effect of pollination intensity on sporophytic vigor. Our study is the first to investigate the relation between pollination intensity and female fertility in a wind-pollinated species and provides support for the contribution of gametophytic competition to sex ratio variation in plants.

Sex ratios in the arctic—do man‐made chemicals matter?

The objective was to analyze the variation of secondary sex ratios across the Arctic and to estimate the time trend. The rationale for this was claims in news media that, in the Arctic, sex ratios have become reduced due to exposure to anthropogenic contaminants in the environment. Data was collected from 27 circumpolar jurisdictions from public websites of the eight Arctic countries. Sex ratios at birth were calculated for each jurisdiction and each available year. Linear regression models of the sex ratios across time were fit within each jurisdiction to estimate the change in sex ratio over time. All male:female sex ratios were close to 1.05 with time trends close to 0. In a Bayesian hierarchical model overall sex ratio was estimated at 1.054 (95% confidence interval 1.048, 1.058). The estimate for the 10-year slope across all jurisdictions was 0.0010 (95% confidence interval -0.0021, 0.0046). Separate analyses of indigenous populations in Alaska and Greenland gave similar results and similar sex ratios were found among Greenland Inuit in 1900 and today. The absence of deviation of the secondary sex ratio in any of the Arctic jurisdictions indicates that the contaminants that are present are not disrupting endocrine systems to the extent that sex ratios are being affected.

FACTORS AFFECTING SEX RATIO AT BIRTH IN CROATIA 1998–2008

This investigation aims to contribute to the existing literature on demographic and ecological factors affecting the sex ratio at birth, by analysing the births in Croatia from 1998 to 2008. Data from birth certificates for all Croatian births for the investigated period (n=420,256) were used to establish the link between parental ages, birth order, region of birth, parental occupation and parental education level, and sex of the child. The χ² test and t-test were used to assess the significance of each of the factors, along with multiple logistic regression to control for possible confounding effects. The results suggest that a joint higher age of both parents significantly lowers the sex ratio at birth. There is also a regional variation in sex ratio at birth, the lowest value being in Central Croatia and the highest in the City of Zagreb. Changes in the reproductive physiology of older parents are most probably responsible for the lower sex ratio, although the limited sample size warns against widespread generalizations. The causes of the regional variation in sex ratio at birth are most likely the different regional levels of obesity and physical inactivity.

Reproductive biology of the tub gurnard Trigla lucerna (linnaeus, 1758), in the Libyan eastern coast of Mediterranean Sea.

The reproductive biology of Trigla lucerna, (family: Triglidae) inhabiting Libyan eastern coast was studied during the period from January to December 2007. There were monthly variation in sex ratio, and a tendency for more females (731 fish, 52.2%) than males (669 fish, 47.8%) for the collected population. Overall sex ratio was 1: 1.09 for males to females. The male of Trigla lucerna attained its first maturity at the total length of 13.9 cm, while the first sexual maturity of female was attained at 15.3 cm. The gonado-somatic indices of males were lower than that of females. Males and females have a definite breeding season which extends from January till March. An increase in oocyte diameter was evident in November (with an average of 225 µ) and this increase continued in the following months till March, with an average value of 498µ. The absolute fecunncldity ranged from 678 to 10047 eggs for fish length ranging from 13.5 to 22.6 cm. Also, the fecundity increased from 755 to 10055 eggs with the increase of weight from 16.7 to 108.2 g.

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 metres. 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.

Adult Sex Ratio of a Population of the Japanese Salamander, Hynobius tokyoensis (Caudata: Hynobiidae)

Adult sex ratio is considered to be a key factor in understanding sexual selection, mating behavior, life history and population dynamics. Following Fisher (1930), a general consensus was formed that sex ratio tends to be largely balanced. However, gainsaying this, large variations in sex ratio have been reported among populations and species of amphibians. There is little information on sex ratio in natural populations of Japanese hynobiid salamanders. To help fill this gap, we conducted a long-term census of a population of Hynobius tokyoensis from 1976 to 1985 and estimated the adult sex ratio (the proportion of males within the population) at 0.578 from data gathered during eight non-breeding seasons. The bootstrap 95% confidence interval was 0.516–0.641. The study showed that the adult sex ratio was significantly biased toward males in the population studied; it was nearly 1.5:1 (males:females). Age at first reproduction is 1 year later in females, which suggests that the most likely factor driving the unbalanced sex ratio is the differential maturation rate between the sexes.

The origin and population structure of Purple Sandpipers Calidris maritima in northern France

Biometrics of Purple Sandpipers captured in Brittany, northern France, indicated that they belonged to a long-billed population, so may have originated in Canada or Russia. There was a preponderance of males (81%). An uneven sex ratio in favour of males is proving to be the norm for winter populations of Purple Sandpiper. A high percentage of captures were of first-year birds. This was probably biased by the catching methods (mist-netting and dazzle-netting), because a lower value was obtained by observing free-living birds.

Inbreeding and Allele Retention for Lake Sturgeon Populations under Different Supplementation Strategies

AbstractLake sturgeon Acipenser fulvescens once were abundant throughout the Great Lakes basin but have been reduced to less than 1% of historical levels because of habitat degradation and overexploitation. Current management plans suggest stocking as a tool to increase abundance, but stocking also has genetic implications. The objectives of this study were to determine the supplementation level that would maintain long‐term population persistence with the smallest genetic impact and to determine whether different supplementation strategies would be required for different starting conditions. An individual‐based model that incorporated demographics and genetics was used to explore scenarios that included three initial population sizes, two different supplementation time frames, varying sex ratios, variance in family size, and different percentages of the adult population contributing progeny for supplementation. As expected, all supplementation scenarios reduced extinction risk, increased population sizes, increased allele retention, and reduced inbreeding over time compared with scenarios involving no supplementation. Supplementation over long time frames was only necessary when supplementing large population sizes. Contrary to expectations, a skewed sex ratio and an unequal family size had little impact on the genetics of populations. When the percentage of adults contributing progeny for supplementation was reduced, allele retention decreased and inbreeding increased. Supplementing larger populations over longer time frames and capturing the greatest number of adults from the population for supplementation are the most important considerations for maintaining alleles and reducing inbreeding under the scenarios we explored.Received December 22, 2010; accepted July 28, 2011