Variation In Sex Ratio Research Articles

Secondary sex ratio covaries with demographic trends and ecological conditions in the barn swallow

Parents are expected to invest more in the sex that benefits most from the local environment. When the quality of breeding sites varies spatially and natal dispersal of males and females differs, parents in high-quality habitats should skew their progeny sex ratio in favor of the less dispersing sex. We tested this prediction in the barn swallow (Hirundo rustica L.), by relating the proportion of male offspring around fledging (secondary sex ratio) of first and second broods to the ecological quality (presence of livestock farming and relative surface of hayfields in the foraging range) and local demographic trends of the farms where the colonies were located. Consistent with our predictions, the proportion of male offspring, which are more philopatric than females, increased with the extent of hayfields, which are high quality, preferred foraging habitats. Moreover, the proportion of male offspring in second broods was smaller in colonies with positive demographic trends, possibly indicating density-dependent effects on sex ratio. Independent of the mechanism generating uneven sex ratio (zygote sex ratio adjustment or sex-related pre-fledging mortality), barn swallows breeding under favorable conditions overproduced the sex that is more likely to benefit from such conditions.

Breeding behavior and sex ratio variation of Pleuroncodes monodon (Crustacea: Galatheidae) off the Peruvian coast

The breeding behavior and sex ratio variation of the carrot squat lobster Pleuroncodes monodon were analyzed. Specimens were collected from experimental hauls in three acoustic pelagic resource assessment surveys during 2001 off the Peruvian coast. A total of 8517 individuals were differentiated according to sex. Males showed clear sexual dimorphism, while females were divided into ovigerous or nonovigerous individuals. Size did not differ significantly between sexes (Kolmogorov-Smirnov test, P > 0.05); average sizes were smaller than 20 mm cephalothoraxic length. The sex ratio indicated that in winter males were more predominant than females (χ2 test, P < 0.05). Ovigerous females were observed during all the period of study, with major activity in spring, and population dispersion occurred during the breeding season.

Sex ratio dependent dispersal when sex ratios vary between patches

Female biased sex ratios reduce competition between brothers when mating takes place within local patches. Male dispersal prior to mating is another strategy that reduces competition between brothers. One may thus expect these two traits to co-evolve and this is partially met in that sex ratios becomes less female biased as dispersal increases. However, the evolutionary stable degree of dispersal is unaffected by the sex ratio. The analytical models developed to reach these conclusions ignored variance in sex ratios, since this increases the structural complexity of models. For similar reasons finite clutch sizes are also routinely ignored. To overcome these shortfalls, we developed individual based simulations that allowed us to incorporate realistic clutch sizes and binomial variance in sex ratios between patches. We show that under variable sex ratios, males evolve to more readily disperse away from patches with higher sex ratios than lower sex ratios. We show that, while the dispersal rate is insensitive to the sex ratio when sex ratios are precise, it is affected by the number of males with dispersal decreasing as the number of males decreases.

Environmental factors influencing adult sex ratio in Poecilia reticulata: laboratory experiments

The potential causes of adult sex ratio variation in guppies Poecilia reticulata were tested in laboratory experiments that evaluated the mortality rates of male and female P. reticulata exposed to potential predators (Hart's rivulus Rivulus hartii and freshwater prawns Macrobrachium crenulatum) and to different resource levels. Poecilia reticulata mortality increased in the presence of R. hartii and M. crenulatum, and low resource levels had an effect on mortality only in the presence of M. crenulatum. Rivulus hartii preyed more often on male than on female P. reticulata, and this sex-biased predation was not simply the result of males being smaller than females. In contrast, no sex-biased mortality was attributable to M. crenulatum or low resource levels.

Sex Ratio at Birth and Mortality Rates Are Negatively Related in Humans

Evolutionary theory posits that resource availability and parental investment ability could signal offspring sex selection, in order to maximize reproductive returns. Non-human studies have provided evidence for this phenomenon, and maternal condition around the time of conception has been identified as most important factor that influence offspring sex selection. However, studies on humans have reported inconsistent results, mostly due to use of disparate measures as indicators of maternal condition. In the present study, the cross-cultural differences in human natal sex ratio were analyzed with respect to indirect measures of condition namely, life expectancy and mortality rate. Multiple regression modeling suggested that mortality rates have distinct predictive power independent of cross-cultural differences in fertility, wealth and latitude that were earlier shown to predict sex ratio at birth. These findings suggest that sex ratio variation in humans may relate to differences in parental and environmental conditions.

Does predation result in adult sex ratio skew in a sexually dimorphic insect genus?

Theory proposes that sexually dimorphic, polygynous species are at particularly high risk of sex-biased predation, because conspicuous males are more often preyed upon compared to females. We tested the effects of predation on population sex ratio in a highly sexually dimorphic insect genus (Hemideina). In addition, introduction of a suite of novel mammalian predators to New Zealand during the last 800 years is likely to have modified selection pressures on native tree weta. We predicted that the balance between natural and sexual selection would be disrupted by the new predator species. We expected to see a sex ratio skew resulting from higher mortality in males with expensive secondary sexual weaponry; combat occurs outside refuge cavities between male tree weta. We took a meta-analytic approach using generalized linear mixed models to compare sex ratio variation in 58 populations for six of the seven species in Hemideina. We investigated adult sex ratio across these populations to determine how much variation in sex ratio can be attributed to sex-biased predation in populations with either low or high number of invasive mammalian predators. Surprisingly, we did not detect any significant deviation from 1 : 1 parity for adult sex ratio and found little difference between populations or species. We conclude that there is little evidence of sex-biased predation by either native or mammalian predators and observed sex ratio skew in individual populations of tree weta is probably an artefact of sampling error. We argue that sex-biased predation may be less prevalent in sexually dimorphic species than previously suspected and emphasize the usefulness of a meta-analytic approach to robustly analyse disparate and heterogeneous data.

Spatial and seasonal variation in a target fishery for spotted eagle ray Aetobatus narinari in the southern Gulf of Mexico

The target fishery for the spotted eagle ray Aetobatus narinari in the southern Gulf of Mexico is little known. The landings of four small-scale vessels at two fishing localities were sampled and fishermen were interviewed in 2009. Rays landed at Campeche [mean ± standard deviation (SD) 1204 ± 225.3 mm disc width (DW)], fished at 30–50 km from the shore, were larger than rays landed at Seybaplaya (924 ± 206.5 mm DW), fished at 8–15 km from the shore. Ray catches were male biased off Campeche and female biased off Seybaplaya. Catch rate off Campeche was 6.6 (±4.9) rays per vessel trip and off Seybaplaya was 3.0 (±2.9) rays per vessel trip. Fishermen stated that catches of A. narinari are positively influenced by winter cold fronts, turbidity, low sea temperature, and new moon phase, and negatively influenced by the presence of cownose rays Rhinoptera bonasus. Spatial variation in size composition, and sex and maturity ratios of A. narinari were evident between sites. Catch rates of A. narinari varied with individual fisherman and seasonally between months with winter cold fronts versus warmer months. Fishermen reported a general decline in catches of A. narinari over recent decades in this region.

Low sex ratio in children of professional basketball players in Spain

The aim of this study was to ascertain the variations in the reproductive sex ratio (number of men to number of women) among male professional basketball players in Spain. This retrospective, cross-sectional study is based on a survey conducted in the Spanish professional basketball leagues during the season 2009-2010. A total of 172 professional basketball players completed an anonymous survey. Forty-seven of the respondents had offspring, with a total of 61 children: 70% girls and 30% boys, with a sex ratio value of 0.42. Thirty-three basketball players were Caucasian (CAU), with 44 children, nine boys and 35 girls (sex ratio = 0.26). Fourteen were black, of African heritage (AFR), with 17 children, nine boys and eight girls, (sex ratio = 1.12). Differences (P < 0.01) were found in offspring sex ratio values for all basketball players (sex ratio = 0.42) and for CAU group (sex ratio = 0.26) when compared with the general Spanish population (sex ratio = 1.06). Moreover, a significant seasonal variation was observed in CAU offspring sex ratio during the first quarter compared with the rest of the year (0.66 versus 0.12) (P < 0.03). In conclusion, a significant increase in the sex ratio value in favour of female offspring was observed in the group of CAU professional basketball players.

Lack of teratological effects in rats exposed to 20 or 60 kHz magnetic fields.

BACKGROUND: A risk assessment of magnetic field (MF) exposure conducted by the World Health Organization indicated the need for biological studies on primary hazard identification and quantitative risk evaluation of intermediate frequency (300 Hz–100 kHz) MFs. Because induction heating cookers generate such MFs for cooking, reproductive and developmental effects are a concern due to the close proximity of the fields' source to a cook's abdomen. METHODS: Pregnant Crl:CD(SD) rats (25/group) were exposed to a 20 kHz, 0.2 mT(rms) or 60 kHz, 0.1 mT(rms) sinusoidal MF or sham-exposed for 22 hr/day during organogenesis, and their fetuses were examined for malformations on gestation day 20. All teratological evaluations were conducted in a blind fashion, and experiments were duplicated for each frequency to confirm consistency of experimental outcomes. RESULTS: No exposure-related changes were found in clinical signs, gross pathology, or number of implantation losses. The number of live fetuses and low-body-weight fetuses as well as the incidence of external, visceral, and skeletal malformations in the fetuses did not indicate significant differences between MF-exposed and sham-exposed groups. Although some fetuses showed isolated changes in sex ratio and skeletal variation and ossification, such changes were neither reproduced in duplicate experiments nor were they common to specific field frequencies. CONCLUSIONS: Exposure of rats to MFs during organogenesis did not show significant reproducible teratogenicity under experimental conditions. Present findings do not support the hypothesis that intermediate frequency MF exposure after implantation carries a significant risk for developing mammalian fetuses. Birth Defects Res (Part B) 92:469–477, 2011. © 2011 Wiley Periodicals, Inc.

Mixed sex allocation strategies in a polytocous mammal, the house mouse (Mus musculus)

The issue of adaptive adjustment of offspring sex ratio (proportion of male births) in polytocous mammals, producing several offspring per litter, is controversial because females of these species can maximize their fitness mainly by adjusting offspring number. To address this issue, we examined the effect of maternal condition at mating, experimentally decreased by pre-mating food restriction, on the sex ratio variation in 137 female mice. We tested two basic sex allocation hypotheses plausible for polytocous mammals: (1) the Myers hypothesis, predicting that cheaper sex should be favored in poor environmental conditions to maximize offspring number; and (2) the Williams hypothesis, predicting maximum fitness returns by adjusting size- and sex-specific composition of the litter according to the maternal condition. The food-restricted mothers produced larger litters with a higher proportion of cheaper daughters than the control mothers. By contrast, the control mothers optimized size and sex composition of the litter according to their weight at mating. In addition, the offspring of the food-restricted mothers suffered less from pre-weaning mortality than those of the control mothers. Therefore, when comparing the groups, the Myers hypothesis had a general significance while the Williams hypothesis was plausible only for the control mothers. Furthermore, some of the food-restricted mothers partly coped with the pre-mating food restriction and increased the proportion of sons in the litter with the increasing maternal weight loss (during the period of food restriction). The sex ratio variation was thus a result of three sex allocation strategies depending on the maternal condition at mating.

Life Cycle and Population Structure of Aegla paulensis (Decapoda: Anomura: Aeglidae)

We describe growth, longevity, sex ratio, reproductive period, and recruitment of Aegla paulensis from JaraguaState Park, Sao Paulo, Brazil (23u27927.90S; 46u45932.30 W). The population was sampled monthly (September 2007 through August 2009) with the aid of traps. Over five thousand individuals were captured, sexed, measured (carapace length 5 CL) and inspected for reproductive traits (females only), and then released back to the sampling site. The pattern of the reproductive cycle was strongly seasonal (austral mid autumn through late winter), with a single recruitment pulse per year. The obtained von Bertalanffy growth equations were CL 5 21.25(1-e 20.041(t + 1.250) ) and CL 5 16.52(1-e 20.049(t + 1.823) ) for males and females, respectively. Males (mean CL 6 SD 5 11.86 6 2.79 mm) attain larger sizes than females (mean CL 6 SD 5 10.84 6 2.36 mm). Aegla paulensis reproduces twice during an estimated life span of 40.2 months for females and 33.9 months for males. Temporal variation of sex ratio showed a distinctive pattern characterized by a sequence of three distinct periods that repeated from one year to another, and which suggested that a behavioral component influence the proportion of sex in adult specimens sampled with traps during reproductive and non-reproductive periods.

Between-Year Variation in Population Sex Ratio Increases with Complexity of the Breeding System in Hymenoptera

While adaptive adjustment of sex ratio in the function of colony kin structure and food availability commonly occurs in social Hymenoptera, long-term studies have revealed substantial unexplained between-year variation in sex ratio at the population level. In order to identify factors that contribute to increased between-year variation in population sex ratio, we conducted a comparative analysis across 47 Hymenoptera species differing in their breeding system. We found that between-year variation in population sex ratio steadily increased as one moved from solitary species, to primitively eusocial species, to single-queen eusocial species, to multiple-queen eusocial species. Specifically, between-year variation in population sex ratio was low (6.6% of total possible variation) in solitary species, which is consistent with the view that in solitary species, sex ratio can vary only in response to fluctuations in ecological factors such as food availability. In contrast, we found significantly higher (19.5%) between-year variation in population sex ratio in multiple-queen eusocial species, which supports the view that in these species, sex ratio can also fluctuate in response to temporal changes in social factors such as queen number and queen-worker control over sex ratio, as well as factors influencing caste determination. The simultaneous adjustment of sex ratio in response to temporal fluctuations in ecological and social factors seems to preclude the existence of a single sex ratio optimum. The absence of such an optimum may reflect an additional cost associated with the evolution of complex breeding systems in Hymenoptera societies.

Interpopulation Variation in Sex Ratio of the Japanese Pond Turtle Mauremys japonica (Reptilia: Geoemydidae)

We investigated sex ratio in seven populations of the Japanese pond turtle, Mauremys japonica, inhabiting ponds in the central region of Aichi Prefecture, Japan. The sex ratio, calculated as a numerical proportion (%) of males to total individuals sexed, was highly variable among the populations, ranging from no more than 20.0% to no less than 92.9%. Since M. japonica shows a temperature-dependent sex determination (TSD) with eggs producing more males under a relatively low incubation temperature and more females under a higher temperature, we hypothesized that the observed remarkable inter-population variation in sex ratio reflects that in the nest temperature. Our field observations indicated that the nesting sites of male-biased populations were largely shaded by wood, thus supposedly under a relatively low ambient temperature, while those of female-biased populations were located in open environments with direct exposure to solar heat. These results offer a circumstantial support to the above hypothesis. Remarkably skewed sex ratios observed in some populations might reflect the effect of substantial habitat fragmentation and severe nesting site limitation caused by recent artificial land development and changes in agricultural land use. If this is actually the case and the effect continues to operate, many of the M. japonica populations may eventually disappear.

Accuracy for determining sex of white‐tailed deer fetuses

AbstractExplanations for variation in fetal sex ratio are needed to better understand the white‐tailed deer (Odocoileus virginianus) life‐history strategy. Sex determination in fetal white‐tailed deer is often based on external morphology and previous studies indicate that fetal sex does not become morphologically distinct until 63–69 days postconception. Until the advent of molecular techniques, there was no means to evaluate observer accuracy or to determine whether fetal sexing is feasible at &lt;63 days postconception. We collected fetuses from wild deer in Mississippi, USA during spring 2008 (n = 55) and 2009 (n = 88) and measured fetal sex classification among 3 groups of observers with differing levels of experience: an inexperienced group (n = 31), an experienced group (n = 3), and a wildlife biologist group (n = 12). We then determined fetal sex using sex‐linked genetic markers and modeled observer accuracy using logistic regression. The inexperienced group required fetuses aged 63 days, 67 days, and 75 days old to correctly classify with 90%, 95%, and 99% accuracy, respectively. The experienced group correctly sexed fetuses at 54 days, 55 days, and 56 days old with 90%, 95%, and 99% accuracy, respectively. The wildlife biologists correctly sexed fetuses at 55 days, 57 days, and 59 days old with 90%, 95%, and 99% accuracy, respectively. Our study was the first to use known‐sex fetuses to estimate observer accuracy of fetal white‐tailed deer at various ages. Our results suggest training observers to recognize fetal tissue development can improve classification accuracy. © 2011 The Wildlife Society.

How much better are females? The occurrence of female advantage, its proximal causes and its variation within and among gynodioecious species.

Gynodioecy is a reproductive system of interest for evolutionary biologists, as it poses the question of how females can be maintained while competing with hermaphrodites that possess both male and female functions. One necessary condition for the maintenance of this polymorphism is the occurrence of a female advantage, i.e. a better seed production or quality by females compared with hermaphrodites. Theoretically, its magnitude can be low when sterility mutations are cytoplasmic, while a 2-fold advantage is needed in the case of nuclear sterility. Such a difference is often thought to be due to reduced inbreeding depression in obligatory outcrossed females. Finally, variation in sex ratio and female advantage occur among populations of some gynodioecious species, though the prevalence of such variation is unknown. By reviewing and analysing the data published on 48 gynodioecious species, we examined three important issues about female advantage. (1) Are reduced selfing and inbreeding depression likely to be the major cause of female advantage? (2) What is the magnitude of female advantage and does it fit theoretical predictions? (3) Does the occurrence or the magnitude of female advantage vary among populations within species and why? It was found that a female advantage occurred in 40 species, with a magnitude comprised between 1 and 2 in the majority of cases. In many species, reduced selfing may not be a necessary cause of this advantage. Finally, female advantage varied among populations in some species, but both positive and negative correlations were found with female frequency. The role of reduced selfing in females for the evolution of gynodioecy, as well as the various processes that affect sex ratios and female advantage in populations are discussed.

The dissimilar costs of love and war: age-specific mortality as a function of the operational sex ratio

Lifespan and ageing are strongly affected by many environmental factors, but the effects of social environment on these life-history traits are not well understood. We examined effects of social interaction on age-specific mortality rate in the sexually dimorphic neriid fly Telostylinus angusticollis. We found that although interaction with other individuals reduced longevity of both sexes, the costs associated with variation in operational sex ratio were sex specific: males' early-life mortality rate increased, and lifespan decreased, with increasing male bias in the sex ratio, whereas surprisingly, the presence of males had no effect on early-life mortality or lifespan of females. Intriguingly, early-life (immediate) mortality costs did not covary with late-life (latent) costs. Rather, both sexes aged most rapidly in a social environment dominated by the opposite sex. Our findings suggest that distinct reproductive activities, such as mating and fighting, impose different age-specific patterns of mortality, and that such costs are strongly sex specific.

Sex ratio and subdioecy inFragaria virginiana: the roles of plasticity and gene flow examined

Here we examined the roles of sex-differential plasticity (SDP) and gene flow in sex ratio evolution of subdioecious Fragaria virginiana. We assessed whether female frequency varied with resource availability in 17 natural populations and then characterized plasticity and mean investment in allocation to female function at flower and plant levels in the sex morphs in the glasshouse. We estimated patterns of population divergence using five microsatellite markers. We reveal SDP in fruit production substantial enough to translate into a higher equilibrium female frequency at low resources. Thus SDP can account, in part, for the strong negative relationship between female frequency and resources found in the field. Pollen-bearing morphs varied in plasticity across populations, and the degree of plasticity in fruit number was positively correlated with in situ variation in nitrogen (N) availability, suggesting an adaptive component to sex-allocation plasticity. Low neutral genetic differentiation, indicating high gene flow or recent divergence, may contribute to the absence of population differentiation in fruit-setting ability of pollen-bearing morphs despite considerable sex ratio variation. We consider how these processes, in addition to other features of this system, may work in concert to influence sex ratios and to hinder the evolution of dioecy in F. virginiana.

Disentangling the effect of genes, the environment and chance on sex ratio variation in a wild bird population

Sex ratio theory proposes that the equal sex ratio typically observed in birds and mammals is the result of natural selection. However, in species with chromosomal sex determination, the same 1 : 1 sex ratio is expected under random Mendelian segregation. Here, we present an analysis of 14 years of sex ratio data for a population of song sparrows (Melospiza melodia) on Mandarte Island, at the nestling stage and at independence from parental care. We test for the presence of variance in sex ratio over and above the binomial variance expected under Mendelian segregation, and thereby quantify the potential for selection to shape sex ratio. Furthermore, if sex ratio variation is to be shaped by selection, we expect some of this extra-binomial variation to have a genetic basis. Despite ample statistical power, we find no evidence for the existence of either genetic or environmentally induced variation in sex ratio, in the nest or at independence. Instead, the sex ratio variation observed matches that expected under random Mendelian segregation. Using one of the best datasets of its kind, we conclude that female song sparrows do not, and perhaps cannot, adjust the sex of their offspring. We discuss the implications of this finding and make suggestions for future research.

Reproduction reduces photosynthetic capacity in females of the subdioecious Honckenya peploides

As a consequence of the different reproductive functions performed by the sexes, sexually dimorphic/polymorphic plants may exhibit gender-related variations in the energy and resources allocated to reproduction, and in the physiological processes that underlie these differences. This study investigated whether the sexes of the subdioecious plant Honckenya peploides differ in ecophysiological traits related to photosynthetic capacity and whether possible differences depend on reproductive status and on the plant’s position (edge or centre) in the population. We registered in three sites in NW Spain, the sex and density of shoots of two segregated clumps of plants. These clumps represent an extreme case of sex-ratio variation across space, with separated single-sex clumps of plants. In two of these sites we measured photosynthetic efficiencies, chlorophyll content, and specific leaf areas. In females, reproduction reduced photochemical efficiency, chlorophyll content and increased the specific leaf area, which is a key leaf trait related to photosynthetic capacity. In males, no differences due to reproduction were detected. The position within the clump affected the specific leaf area of the shoots, with shoots growing at the edge having the lowest values, regardless of the sex. Finally, the effects of position in photosynthetic efficiency and chlorophyll content where highly variable among clumps. We conclude that the differential effects of reproduction on sexes may entail different costs that could be crucial in the outcome of interactions between them, contributing to their spatial segregation.

Brood sex ratio variation in a cooperatively breeding bird

In cooperatively breeding species, the fitness consequences of producing sons or daughters depend upon the fitness impacts of positive (repayment hypothesis) and negative (local competition hypothesis) social interactions among relatives. In this study, we examine brood sex allocation in relation to the predictions of both the repayment and the local competition hypotheses in the cooperatively breeding long-tailed tit Aegithalos caudatus. At the population level, we found that annual brood sex ratio was negatively related to the number of male survivors across years, as predicted by the local competition hypothesis. At an individual level, in contrast to predictions of the repayment hypothesis, there was no evidence for facultative control of brood sex ratio. However, immigrant females produced a greater proportion of sons than resident females, a result consistent with both hypotheses. We conclude that female long-tailed tits make adaptive decisions about brood sex allocation.