Patterns of sex ratio variation and genetic diversity in the dioecious forest perennial <Emphasis Type="Italic">Mercurialis perennis</Emphasis>

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.     

Mortality,fertility, and the OY ratio in a model hunter–gatherer system

An agent‐based model (ABM) is used to explore how the ratio of old to young adults (the OY ratio) in a sample of dead individuals is related to aspects of mortality, fertility, and longevity experienced by the living population from which the sample was drawn. The ABM features representations of rules, behaviors, and constraints that affect person‐ and household‐level decisions about marriage, reproduction, and infant mortality in hunter–gatherer systems. The demographic characteristics of the larger model system emerge through human‐level interactions playing out in the context of “global” parameters that can be adjusted to produce a range of mortality and fertility conditions. Model data show a relationship between the OY ratios of living populations (the living OY ratio) and assemblages of dead individuals drawn from those populations (the dead OY ratio) that is consistent with that from empirically known ethnographic hunter–gatherer cases. The dead OY ratio is clearly related to the mean ages, mean adult mortality rates, and mean total fertility rates experienced by living populations in the model. Sample size exerts a strong effect on the accuracy with which the calculated dead OY ratio reflects the actual dead OY ratio of the complete assemblage. These results demonstrate that the dead OY ratio is a potentially useful metric for paleodemographic analysis of changes in mortality and mean age, and suggest that, in general, hunter–gatherer populations with higher mortality, higher fertility, and lower mean ages are characterized by lower dead OY ratios. Am J Phys Anthropol 154:222–231, 2014. © 2014 Wiley Periodicals, Inc.     

Sex differences in adult mortality rate mediated by early‐life environmental conditions

Variation in sex differences is affected by both genetic and environmental variation, with rapid change in sex differences being more likely due to environmental change. One case of rapid change in sex differences is human lifespan, which has become increasingly female‐biased in recent centuries. Long‐term consequences of variation in the early‐life environment may, in part, explain such variation in sex differences, but whether the early‐life environment mediates sex differences in life‐history traits is poorly understood in animals. Combining longitudinal data on 60 cohorts of pre‐industrial Finns with environmental data, we show that the early‐life environment is associated with sex differences in adult mortality and expected lifespan. Specifically, low infant survival rates and high rye yields (an important food source) in early‐life are associated with female‐bias in adult lifespan. These results support the hypothesis that environmental change has the potential to affect sex differences in life‐history traits in natural populations of long‐lived mammals.     

Demographic effects of temperature‐dependent sex determination: will tuatara survive global warming?

Global climate change is of particular concern for small and isolated populations of reptiles with temperature-dependent sex determination because low genetic variation can limit adaptive response in pivotal temperatures, leading to skewed sex ratios. We explore the demographic consequences of skewed sex ratios on the viability of a tuatara population characterized by low genetic diversity. We studied the rare species of tuatara ( Sphenodon guntheri ) on the 4 ha North Brother Island in New Zealand over two nesting seasons and captured 477 individuals, with a 60% male bias in the adult population. Females first breed at 15 years and have extremely low rates of gravidity, producing clutches of three to eight eggs every 9 years. Simulations of the population using population viability analysis showed that the current population is expected to persist for at least 2000 years at hatchling sex ratios of up to 75% male, but populations with 85% male hatchlings are expected to become extinct within approximately 300 years (some eight generations). Incorporation of inbreeding depression increased the probability of extinction under male biased sex ratios, with no simulated populations surviving at hatchling sex ratios >75% male. Because recent models have predicted that climate change could lead to the production of all male S. guntheri hatchlings by 2085, we examined whether periodic intervention to produce mixed or female biased sex ratios would allow the population to survive if only males were produced in natural nests. We show that intervention every 2–3 years could buffer the effects of climate change on population sex ratios, but translocation to cooler environs might be more cost-effective. Climate change threatens tuatara populations because neither modified nesting behaviour nor adaptive response of the pivotal temperature can modify hatchling sex ratios fast enough in species with long generation intervals.     

Sex‐ and age‐biased mortality in wild Grey Partridge Perdix perdix populations

Differing reproductive effort, individual qualities and local environmental conditions can lead to uneven mortality risk among individuals within populations and may result in survival differences according to age and sex. Identification of factors contributing to unequal operational sex ratios has been important for understanding population dynamics and conservation management. In this study, sex‐ and age‐specific mortality was estimated in three wild Grey Partridge populations from analysis of year‐round radiotracking data from 168 individuals. Survival days were counted in three periods defined individually for each bird: the pairing period (covey break‐up to laying of the first egg); the nesting period (between clutch initiation date and failure of the last nesting attempt, or the date when chicks were 14 days old); and the covey period (the end of the nesting period or joining a group until covey break‐up). Predation was the main cause of mortality. A significant effect of age on survival was found during the pairing period, when older individuals paired off faster and survived better. The highest mortality risk overall was found during the nesting period. Furthermore, significantly higher mortality of females was recorded during the nesting period, suggesting that greater investments in reproduction, behaviour at the nest or the quality of nesting habitats can decrease survival of females and cause a male‐skewed sex ratio. No significant effect of age or sex was found during the covey period, or for the year as a whole, but there was a significant difference in annual mortality rates between the three study populations. Our results confirm age‐ and sex‐specific variation of adult mortality in a ground‐nesting bird with biparental care during the annual cycle, documenting differing sensitivities of various population cohorts to predation.     

Age‐ratio bias among hunter‐based surveys of Eurasian Wigeon Anas penelope based on wing vs. field samples

We compared age and sex ratios among Eurasian Wigeon Anas penelope derived from Danish field observations and hunter‐based shot samples throughout an entire winter. Sex ratios did not differ significantly between the two samples. Overall, first‐year males were more than three times more likely to be represented than adult males in the hunter sample compared with field samples and were 7–20 times overrepresented in the hunting sample at the beginning of the season. These results confirm the need to account for such bias and its temporal variation when using the results of hunting surveys to model population parameters. Hunter‐shot age ratios may provide a long‐term measure of reproductive success of dabbling duck flyway populations given an understanding of such bias.     

Effective size of density‐dependent two‐sex populations: the effect of mating systems

Density dependence in vital rates is a key feature affecting temporal fluctuations of natural populations. This has important implications for the rate of random genetic drift. Mating systems also greatly affect effective population sizes, but knowledge of how mating system and density regulation interact to affect random genetic drift is poor. Using theoretical models and simulations, we compare N_e in short‐lived, density‐dependent animal populations with different mating systems. We study the impact of a fluctuating, density‐dependent sex ratio and consider both a stable and a fluctuating environment. We find a negative relationship between annual N_e/N and adult population size N due to density dependence, suggesting that loss of genetic variation is reduced at small densities. The magnitude of this decrease was affected by mating system and life history. A male‐biased, density‐dependent sex ratio reduces the rate of genetic drift compared to an equal, density‐independent sex ratio, but a stochastic change towards male bias reduces the N_e/N ratio. Environmental stochasticity amplifies temporal fluctuations in population size and is thus vital to consider in estimation of effective population sizes over longer time periods. Our results on the reduced loss of genetic variation at small densities, particularly in polygamous populations, indicate that density regulation may facilitate adaptive evolution at small population sizes.     

Sex ratios at birth in African populations: a review of survey data

This study analyzes the distribution of sex ratio at birth in African populations using data collected in birth histories in sample demographic surveys (Demographic and Health Surveys and World Fertility Surveys). The average sex ratio from 56 surveys, totaling 1.130 million births, was 1.033 (95% CI, 1.029-1.037), significantly different from the world average of 1.055. The distribution of sex ratios across surveys was found to be heterogeneous, and different from what could have been expected from random fluctuations due to sample size. Three subsets were identified: a subset with lower sex ratios, primarily in countries of eastern and southern Africa of Bantu populations (1.010), a subset with average sex ratios (1.035), and a subset of countries with higher sex ratios, in particular Nigeria and Ethiopia (1.070). Further analysis revealed that African populations are as diverse as other populations, with sex ratios ranging from low values (below 1.00) to high values (above 1.08). Results are discussed in light of independent data sources and in comparison with other human populations.     

Patterns of variation in female-biased colony sex ratios in a social spider

Colonies of a social spider Achaearanea wau (Theridiidae) from Papua, New Guinea have adult and juvenile sex ratios that are biased towards females, and this probably represents a primary bias at the egg stage. Adult sex ratios are less female-biased than are juvenile sex ratios, and both vary significantly among colonies. Adult sex ratios covary with colony size: small colonies have a larger proportion of males than large ones. The pattern of variation in adult sex ratio may be due to greater mortality of females than of males during maturation. Juvenile sex ratios do not covary with colony size, nor do they differ among populations. Colony size, however, does have a significant effect on survival and dispersal in colonies. I conclude, therefore, that a conditional sex ratio strategy, in which the primary sex ratio of the colony is adjusted to changing demographic patterns, does not occur in A. wau. I suggest that environmental heterogeneity acting on individual reproductive output may be responsible for the observed variation among colonies in juvenile sex ratios.     

Reproductive biology of captive and free-ranging spider monkeys

Records from 42 zoos and from long-term studies of wild populations were analysed to describe the reproductive biology of spider monkeys (Ateles spp.). Both data sets suggested that spider monkey females typically have their first infant between 7 and 8 years of age with an interbirth interval of approximately 32–36 months. Infant sex ratio for zoo populations was approximately 1 male to 1 female; infant sex ratios from wild populations were variable. Zoo records provided adequate sample size to suggest that interbirth interval was not influenced by the sex of the infant produced, and that the sex ratio and the probability of infant survival did not change with the number of infants the mother had produced. The findings of this study have implications with respect to the conservation of New World primate species. Since spider monkeys take a long time to reach sexual maturity and their interbirth interval is longer than that expected based on their body size, their populations may be slow to recover following disturbances. Thus, particular care should be taken for the protection of these species.     

Sequences of sex allocation and mortality in clutches of Metaphycus parasitoids of soft scale insects and the prevalence of all‐female broods

1. In many gregarious or quasi‐gregarious parasitoids that experience local mate competition, precise sex ratios with low variance are observed. Precise sex ratios can be achieved by laying male and female eggs in non‐random sequences. 2. Developmental mortality can also alter sex ratios of emerging offspring, and subsequently influence sex ratio optima. 3. The present study investigates sex allocation by Metaphycus flavus Howard, M. luteolus Timberlake, and M. angustifrons Compere (Hymenoptera: Encyrtidae), endoparasitoids of soft scale insects, in the laboratory. 4. All three Metaphycus species had precise secondary sex ratios when parasitising brown soft scale, Coccus hesperidum, L. in the laboratory. Moreover, we documented that all three species lay fertilised (= female) eggs first followed by unfertilised (= male) eggs at the end of the oviposition bout. However, there were significant differences in sex allocation sequences among species. 5. Mortality rates of eggs allocated within an oviposition bout also varied considerably, indicating that there is a significant interspecific variation in sequence position‐specific mortality. 6. Using a stochastic Monte Carlo simulation approach, we provide evidence that the incidence of all‐female broods in these parasitoid wasps appears mainly due to developmental mortality and not due to decisions by the ovipositing female. In two species the prevalence of all‐female broods was independent of clutch size, contrary to what is expected from theory. The influence of mortality on optimal sex allocation in these parasitoids is discussed.     

REPLICATED ORIGIN OF FEMALE‐BIASED ADULT SEX RATIO IN INTRODUCED POPULATIONS OF THE TRINIDADIAN GUPPY (POECILIA RETICULATA)

There are many theoretical and empirical studies explaining variation in offspring sex ratio but relatively few that explain variation in adult sex ratio. Adult sex ratios are important because biased sex ratios can be a driver of sexual selection and will reduce effective population size, affecting population persistence and shapes how populations respond to natural selection. Previous work on guppies (Poecilia reticulata) gives mixed results, usually showing a female‐biased adult sex ratio. However, a detailed analysis showed that this bias varied dramatically throughout a year and with no consistent sex bias. We used a mark‐recapture approach to examine the origin and consistency of female‐biased sex ratio in four replicated introductions. We show that female‐biased sex ratio arises predictably and is a consequence of higher male mortality and longer female life spans with little effect of offspring sex ratio. Inconsistencies with previous studies are likely due to sampling methods and sampling design, which should be less of an issue with mark‐recapture techniques. Together with other long‐term mark‐recapture studies, our study suggests that bias in offspring sex ratio rarely contributes to adult sex ratio in vertebrates. Rather, sex differences in adult survival rates and longevity determine vertebrate adult sex ratio.     

Demography and childcare in preindustrial societies

The preliminary comparison of hunter gatherers, horticulturalists, and pastoralists is based on 57 preindustrial populations with demographic and child care data out of a potential of 1264 documented cultures from the Ethnographic Atlas. The purpose of this effort is to demonstrate that the demographic characteristics of a population influence its child care practices and provides clues to understanding child care patterns. Traditional practices and provides clues to understanding child care patterns. Traditional practices including multiple caregiving, multistage play groups, and parents or siblings as cultural transmitters are reviewed in a demographic context. Other emerging practices are also discussed: the role of stepparents and differential parental investment in sons and daughters. Anthropological data published and unpublished included only those using standardized methods on total fertility, infant or child mortality, and/or sex/age distribution. Problems with the data set include limited cultural representation, small study sizes, limited time trends, and reliability. There is a concentration on the ]Kung San, Efe, Aka, Gidjingali, Yanomamo, Dusan, Semai, and Kipsigis. Only 7 of the 57 are outside the tropics. Foragers are farmers are primarily represented, because the pastoralists are primarily East African and smaller samples. Tables provide cultural specific data on total fertility rates (TRF), infant and child mortality, and sex ratios at birth and among the juvenile and adult population. Sections are devoted to methods, general patterns, traditional characteristics of childcare based on 5 hypotheses, and emergent trends with 2 more hypotheses on stepparenting and male preference. 2 patterns prevail: 1) hunter gatherers and horticulturalists/pastoralists show great intercultural variability in fertility and mortality rates, and 2) the ranges and means of both groups are very similiar. In the discussion of specific cultures, the hypothesis is proposed and then examples are drawn from the 57 studies to provide support or rejection of the hypothesis. The 1st postulated that the level of multiple care increases with the number of adult women without children increasing. The 2nd hypothesis is that the greater the density or compactness of the settlement, the greater the level of multiple care. It is reasoned in the 3rd that fertility and mortality patterns influence the nature of indulgent care of infants. The 4th hypothesis is that sex and age distributions and compactness of the camp influence the nature of the play ground and type of supervision. The 5th is that father involvement will be greater in societies with low population densities or isolated. The 6th is that a child rarely stays with natural parents throughout the dependency period. The 7th is that male biased juvenile sex ratios will exist in societies where the cost of raising males is or = that of raising families, or where males contribute more calories to the diet, or where male mortality is high.     

Developmental mortality increases sex‐ratio bias of a size‐dimorphic bark beetle

1. Given sexual size dimorphism, differential mortality owing to body size can lead to sex‐biased mortality, proximately biasing sex ratios. This mechanism may apply to mountain pine beetles, Dendroctonus ponderosae Hopkins, which typically have female‐biased adult populations (2 : 1) with females larger than males. Smaller males could be more susceptible to stresses than larger females as developing beetles overwinter and populations experience high mortality. 2. Survival of naturally‐established mountain pine beetles during the juvenile stage and the resulting adult sex ratios and body sizes (volume) were studied. Three treatments were applied to vary survival in logs cut from trees containing broods of mountain pine beetles. Logs were removed from the forest either in early winter, or in spring after overwintering below snow or after overwintering above snow. Upon removal, logs were placed at room temperature to allow beetles to complete development under similar conditions. 3. Compared with beetles from logs removed in early winter, mortality was higher and the sex ratio was more female‐biased in overwintering logs. The bias increased with overwinter mortality. However, sex ratios were female‐biased even in early winter, so additional mechanisms, other than overwintering mortality, contributed to the sex‐ratio bias. Body volume varied little relative to sex‐biased mortality, suggesting other size‐independent causes of male‐biased mortality. 4. Overwintering mortality is considered a major determinant of mountain pine beetle population dynamics. The disproportionate survival of females, who initiate colonisation of live pine trees, may affect population dynamics in ways that have not been previously considered.     

Demography and Life Histories of Sympatric Patas Monkeys, <Emphasis Type="Italic">Erythrocebus patas</Emphasis>, and Vervets, <Emphasis Type="Italic">Cercopithecus aethiops</Emphasis>, in Laikipia,Kenya

Mortality patterns are thought to be strong selective forces on life history traits, with high adult mortality and low immature mortality favoring early and rapid reproduction. Patas monkeys (Erythrocebus patas) have the highest potential rates of population increase for their body size of any haplorhine primate because they reproduce both earlier and more often. We report here 10 yr of comparative demographic data on a population of patas monkeys and a sympatric population of vervet monkeys (Cercopithecus aethiops), a closely related species differing in aspects of social system, ecology, and life history. The data reveal that 1) adult female patas monkeys have significantly higher mortality than adult female vervets; 2) infant mortality in patas monkeys is relatively low compared to the norm for mammals because it is not significantly different from that of adult female patas monkeys; and 3) infant mortality is significantly higher than adult female mortality in vervets. For both species, much of the mortality could be attributed to predation. An epidemic illness was also a major contributor to the mortality of adult female patas monkeys whereas chronic exposure to pathogens in a cold and damp microenvironment may have contributed to the mortality of infant vervets. Both populations experienced large fluctuations during the study period. Our results support the prediction from demographic models of life history evolution that high adult mortality relative to immature mortality selects for early maturation.     

A methodological note on the relationship between infant mortality and socioeconomic status with evidence from San Antonio, Texas.

A number of previous studies have concluded from social area analyses of medium-size cities that there is no longer a significant correlation between socioeconomic status (SES) and infant mortality in the U.S. To determine if these findings were an artifact of too small samples, the total, neonatal, and postneonatal infant mortality rates were analyzed for 115 census tracts of San Antonio, Texas. The SES of each tract was measured by a score reflecting equally the variables of income, education, and occupation, and allowed assignment of the tracts to 1 of 4 socioeconomic rankings. All 3 infant mortality rates rose as SES decreased, with the most marked relationship being between SES and postneonatal rates. It was also found that of the 3 variables used to measure SES, income bore the strongest relationship to infant mortality. In general it should be noted that social area analysis of infant mortality is limited by the extreme reductions of sample size when additional variables are induced.     

Nuclear sex-determining genes cause large sex-ratio variation in the apple snail Pomacea canaliculata

Evolutionary maintenance of genetic sex-ratio variation is enigmatic since genes for biased sex ratios are disadvantageous in finite populations (the "Verner effect"). However, such variation could be maintained if a small number of nuclear sex-determining genes were responsible, although this has not been fully demonstrated experimentally. Brood sex ratios of the freshwater snail Pomacea canaliculata are highly variable among parents, but population sex ratios are near unity. In this study, the effect of each parent on the brood sex ratio was investigated by exchanging partners among mating pairs. There were positive correlations between sex ratios of half-sib broods of the common mother (r = 0.42) or of the common father (r = 0.47). Moreover, the correlation between full-sib broods was very high (r = 0.92). Thus, both parents contributed equally to the sex-ratio variation, which indicates that nuclear genes are involved and their effects are additive. Since the half-sib correlations were much stronger than the parent-offspring regressions previously obtained, the variation was caused by zygotic sex-determining genes rather than by parental sex-ratio genes. The number of relevant genes appears to be small.     

Child Mortality Estimation: Estimating Sex Differences in Childhood Mortality since the 1970s

Introduction

Producing estimates of infant (under age 1 y), child (age 1–4 y), and under-five (under age 5 y) mortality rates disaggregated by sex is complicated by problems with data quality and availability. Interpretation of sex differences requires nuanced analysis: girls have a biological advantage against many causes of death that may be eroded if they are disadvantaged in access to resources. Earlier studies found that girls in some regions were not experiencing the survival advantage expected at given levels of mortality. In this paper I generate new estimates of sex differences for the 1970s to the 2000s.

Methods and Findings

Simple fitting methods were applied to male-to-female ratios of infant and under-five mortality rates from vital registration, surveys, and censuses. The sex ratio estimates were used to disaggregate published series of both-sexes mortality rates that were based on a larger number of sources. In many developing countries, I found that sex ratios of mortality have changed in the same direction as historically occurred in developed countries, but typically had a lower degree of female advantage for a given level of mortality. Regional average sex ratios weighted by numbers of births were found to be highly influenced by China and India, the only countries where both infant mortality and overall under-five mortality were estimated to be higher for girls than for boys in the 2000s. For the less developed regions (comprising Africa, Asia excluding Japan, Latin America/Caribbean, and Oceania excluding Australia and New Zealand), on average, boys'' under-five mortality in the 2000s was about 2% higher than girls''. A number of countries were found to still experience higher mortality for girls than boys in the 1–4-y age group, with concentrations in southern Asia, northern Africa/western Asia, and western Africa. In the more developed regions (comprising Europe, northern America, Japan, Australia, and New Zealand), I found that the sex ratio of infant mortality peaked in the 1970s or 1980s and declined thereafter.

Conclusions

The methods developed here pinpoint regions and countries where sex differences in mortality merit closer examination to ensure that both sexes are sharing equally in access to health resources. Further study of the distribution of causes of death in different settings will aid the interpretation of differences in survival for boys and girls. Please see later in the article for the Editors'' Summary.     

Sources of stochasticity in models of sex allocation in spatially structured populations

There are many ways to include stochastic effects in models of sex allocation evolution. These include variability in the number of mating partners and fecundity in a rich literature that goes back 20 years. The effects of variance in the fecundity and number of mating partners have typically been considered separately from the stochastic effects of mortality. However, I show that these processes produce mathematically equivalent models with subtly different biological details. These scenarios differ in the way that information becomes available to individuals because the parents often have information on mating partners while they are making sex allocation decisions, but must make these decisions before brood mortality takes place. This makes it possible to test which mechanism, stochastic mortality or variation in mating partners, is responsible for observed sex ratios. Alternatively, asymmetric variance between sexual functions can cause skewed sex allocation, even in the absence of local mate competition. This allows the evolution of either female- or male-biased sex ratios depending on which sexual function is more variable.     

Sex-biased survival predicts adult sex ratio variation in wild birds

Adult sex ratio (ASR) is a central concept in population demography and breeding system evolution, and has implications for population viability and biodiversity conservation. ASR exhibits immense interspecific variation in wild populations, although the causes of this variation have remained elusive. Using phylogenetic analyses of 187 avian species from 59 families, we show that neither hatching sex ratios nor fledging sex ratios correlate with ASR. However, sex-biased adult mortality is a significant predictor of ASR, and this relationship is robust to 100 alternative phylogenetic hypotheses, and potential ecological and life-history confounds. A significant component of adult mortality bias is sexual selection acting on males, whereas increased reproductive output predicts higher mortality in females. These results provide the most comprehensive insights into ASR variation to date, and suggest that ASR is an outcome of selective processes operating differentially on adult males and females. Therefore, revealing the causes of ASR variation in wild populations is essential for understanding breeding systems and population dynamics.