Sex Ratio Patterns Research Articles

The sex-ratio of singleton offspring in assisted-conception pregnancies

To evaluate the effect of treatment and male factor infertility on the sex ratio of singleton births from assisted reproductive technology (ART). Retrospective analysis of singleton live births in the 2005 SART database, including fresh and frozen cycles, autologous and donor oocytes. The study population included 19,048 singleton pregnancies. The data were grouped into four cohorts defined by male factor infertility and either no use of ICSI or ICSI in some or all of the eggs. Factors studied included: maternal age, gravidity, diagnosis, oocyte and semen sources, semen collection method, assisted hatching, and number of oocytes or embryos transferred. Logistic regression was used to model the probability (odds ratio, OR, and 95% confidence interval, CI) of male infant overall, and within three maternal age groups (<35 years, 35–40 years, and >40 years). In the general population in the United States in 2004, 51.2% of live births were male. In the study population, the percentage of males was 49.9%, including 51.7% for births without ICSI and 48.9% with ICSI. The major factors influencing the sex ratio were use of ICSI, presence of male factor infertility and their interaction. The sex ratios for these factors are presented in the table below. Although all factors in the original model differed significantly between the four cohorts defined by ICSI and male factor infertility, none of them affected the sex ratio. The pattern of sex ratio across the four cohorts did not change after stratification by maternal age. TableNo ICSI No Male FactorNo ICSI Male FactorICSI No Male FactorICSI Male FactorN5,9915256,1396,393% Male (all)52.147.247.850.0OR (95% CI)1.00 (Reference)0.82 (0.69–0.98)0.84 (0.78–0.91)0.92 (0.86–0.98)% Male (Mothers <35 yrs)53.147.848.149.5OR (95% CI)1.00 (Reference)0.80 (0.61–1.05)0.82 (0.73–0.92)0.87 (0.78–0.96)% Male (Mothers 35–40 yrs)51.146.148.449.8OR (95% CI)1.00 (Reference)0.83 (0.61–1.14)0.90 (0.79–1.02)0.95 (0.84–1.08)% Male (Mothers >40 yrs)51.645.146.350.8OR (95% CI)1.00 (Reference)0.76 (0.50–1.16)0.82 (0.69–0.96)0.98 (0.81–1.19) Open table in a new tab Male factor infertility and the use of ICSI may reduce the chances of having a male child, although the mechanisms remain unclear.

Population structure, physiology and ecohydrological impacts of dioecious riparian tree species of western North America

The global water cycle is intimately linked to vegetation structure and function. Nowhere is this more apparent than in the arid west where riparian forests serve as ribbons of productivity in otherwise mostly unproductive landscapes. Dioecy is common among tree species that make up western North American riparian forests. There are intrinsic physiological differences between male and female dioecious riparian trees that may influence population structure (i.e., the ratio of male to female trees) and impact ecohydrology at large scales. In this paper, we review the current literature on sex ratio patterns and physiology of dioecious riparian tree species. Then develop a conceptual framework of the mechanisms that underlie population structure of dominant riparian tree species. Finally, we identify linkages between population structure and ecohydrological processes such as evapotranspiration and streamflow. A more thorough understanding of the mechanisms that underlie population structure of dominant riparian tree species will enable us to better predict global change impacts on vegetation and water cycling at multiple scales.

Adult sex ratios in wild bird populations

Offspring sex ratios in wild bird populations, and the extent to which they vary from the equality expected by random genotypic sex determination, have received much recent attention. Adult sex ratios (ASRs) in wild birds, on the other hand, remain very poorly described, and many of the questions about them posed by Ernst Mayr in 1939 remain unanswered. This review assesses population‐level sex ratio patterns in wild bird populations, with an emphasis on the ASR. A quantitative assessment of over 200 published estimates of ASR, covering species from a wide range of taxa, regions and habitats, supported Mayr's assertion that skewed ASRs are common in wild bird populations. On average, males outnumbered females by around 33%, and 65% of published estimates differed significantly from equality. In contrast, population‐level estimates of offspring sex ratio in birds did not generally differ from equality, and mean ASR across a range of wild mammal species was strongly female‐skewed. ASR distortion in birds was significantly more severe in populations of globally threatened species than in non‐threatened species, a previously undescribed pattern that has profound implications for their monitoring and conservation. Higher female mortality, rather than skewed offspring sex ratio, is the main driver of male‐skewed ASRs in birds, and the causes and implications of this are reviewed. While estimates of ASR in wild bird populations may be subject to a number of biases, which are discussed, there is currently no quantitative evidence that an ASR of one male to one female represents the norm in birds. A better understanding and reporting of ASRs in wild bird populations could contribute greatly to our understanding of population processes and could contribute much to theoretical and applied research and conservation.

Life-history and sex allocation in Trypoxylon (syn. Trypargilum) lactitarse (Hymenoptera; Crabronidae)

Hymenopterans have become a model for the study of factors that govern sex allocation. In 1983, Seger proposed a model to study Sphecidae wasps with a strong prediction that sex ratio for univoltine wasps should be 1 : 1 (female : male), and for partially bivoltine species it should be male-biased. The present study investigates if Trypoxylon lactitarse (Saussure, 1867) is a univoltine or a bivoltine species and if Seger's model prediction fits the pattern of sex ratio found in this species. The study was carried out at Parque Municipal das Araucarias, in the municipality of Guarapuava, state of Parana, southern Brazil, from December 2001 to December 2004. Nests of T. lactitarse were obtained using trap-nests drilled longitudinally to a depth to 80 mm with 7.0, 10.0 and 13.0 mm opening diameter. They were placed in a very heterogeneous site filled with araucaria forests, swamps and grasslands. Trypoxylon lactitarse showed two alternative life histories: either they pupated immediately and emerged as adults later in the same season (direct development), or they entered diapause, overwintering and pupating the following spring (delayed development). Its annual sex ratios were not significantly different from 1 : 1, and both sex ratio of overwintering and sex ratio of direct-developing wasps were also not significantly different from 1 : 1, in all years of this study. By examining these results, it was possible to conclude that although T. lactitarse is a multivoltine species, with four generations per year and two alternative life histories, its sex ratio did not support Seger's model.

Test of the First-Cohort Advantage Hypothesis with Feathertail Gliders (Marsupialia)

The first-cohort advantage hypothesis (FCAH) was formulated to explain patterns of biased sex ratios in litters of Virginia opossums (Didelphis virginiana). Females of this species typically produce 2 litters each season, and litters born early in a season are male-biased and those born later in the season are female-biased. Males born earlier are older and heavier in the next breeding season and likely to secure more mates. The hypothesis predicts fitness benefits to mothers that have more males in their 1st litter (the 1st cohort) each season. The feathertail glider (Acrobates pygmaeus), in Victoria, Australia, is another species that produces 2 litters per year and not all males mature before the start of the breeding season after their birth. Hence, feathertail gliders also might be expected to show a pattern of sex bias in litters consistent with the FCAH. They do not. This paper uses data from 2 field studies of feathertail gliders in Victoria over the past 18 years. Sex ratios were recorded for 53 first- and 42 second-cohort litters. There was a trend toward male-biased litters in the 1st cohort, but also in the 2nd, and neither group differed significantly from a binomial distribution of sex ratios. This provides no support for the FCAH in this species. This may be due to the much less sharply defined breeding seasons in feathertail gliders compared to those of Virginia opossums, and to their greater longevity.

Sex ratio and maturity indicate the local dispersal and mortality of adult stoneflies

Summary1. Despite a recent upsurge in interest, there remains remarkably little information about the dispersal and survival of the adults of most stream‐dwelling insects, although this is a basic requirement for understanding their long‐term population dynamics.2. Using Malaise traps for a whole annual flight period, we captured adult stoneflies (Leuctra nigra) along transects perpendicular to three upland Welsh streams. We assessed spatial and temporal patterns in sex ratio to infer local dispersal and, using maturity as an age marker, estimated the mortality of adult females.3. Nearly all adult stoneflies (99%) were taken in the period 26 April–23 July, and the onset of the male and female flight periods was the same. Most males (90%) had been caught by late June. Females were classified as immature (without ripe eggs) or mature, and 90% of immature females had been caught by mid‐late June (depending on catchment). As immature females declined in the catch, mature females increased, 10% having been caught by late‐May to early June and 90% by early to mid‐July. The median catches of immature and mature females were separated by 32 days in all three catchments.4. There was a female bias in the sex ratio overall, which increased as time passed and was attributable partially to the greater longevity of females. Late in the flight period, however, female bias was also greater near the stream channel implying a return of mature females (but not males) from the riparian vegetation, presumably to oviposit.5. The number of mature females was less than the number of immatures in two of the three channels. Over all three catchments, the overall mortality of females over the 32 days taken to mature was estimated at 29%, equivalent to an exponential daily rate of 1.1% day−1. The apparently negative mortality rate in one catchment (i.e. more matures than immatures being caught) could be due to an influx of adult females from elsewhere along the channel to oviposit.6. Natural markers of age and population structure, such as sex ratio and female maturity, thus enabled us to detect a return of females to the stream to oviposit, after prior limited dispersal into the riparian zone, and to infer longitudinal movements in search of suitable sites. We were also able to estimate mortality in the field. Such natural markers seem to have been underexploited in the study of adult aquatic insects.

Sex‐specific sibling interactions and offspring fitness in vertebrates: patterns and implications for maternal sex ratios

Vertebrate sex ratios are notorious for their lack of fit to theoretical models, both with respect to the direction and the magnitude of the sex ratio adjustment. The reasons for this are likely to be linked to simplifying assumptions regarding vertebrate life histories. More specifically, if the sex ratio adjustment itself influences offspring fitness, due to sex-specific interactions among offspring, this could affect optimal sex ratios. A review of the literature suggests that sex-specific sibling interactions in vertebrates result from three major causes: (i) sex asymmetries in competitive ability, for example due to sexual dimorphism, (ii) sex-specific cooperation or helping, and (iii) sex asymmetries in non-competitive interactions, for example steroid leakage between fetuses. Incorporating sex-specific sibling interactions into a sex ratio model shows that they will affect maternal sex ratio strategies and, under some conditions, can repress other selection pressures for sex ratio adjustment. Furthermore, sex-specific interactions could also explain patterns of within-brood sex ratio (e.g. in relation to laying order). Failure to take sex-specific sibling interactions into account could partly explain the lack of sex ratio adjustment in accordance with theoretical expectations in vertebrates, and differences among taxa in sex-specific sibling interactions generate predictions for comparative and experimental studies.

Body size, host choice and sex allocation in a spider-hunting pompilid wasp

Two important relationships in parasitoid evolutionary ecology are those between adult size and fitness and between host quality and sex ratio. Sexually differential size–fitness relationships underlie predicted sex-ratio relationships. Despite each relationship receiving considerable attention, they have seldom been studied simultaneously or using field data. Here we report the biology of Anoplius viaticus paganus Dahlbom, a little known parasitoid of spiders, using field and laboratory data. We found that larger foraging females were able to select larger host spiders from the field, thus identifying a relatively novel component of the size–fitness relationship. Larger offspring developed from larger hosts and, in agreement with the prediction of the host quality model of sex allocation, were generally female. Data on the size–fitness relationship for males are lacking and, in common with many prior studies, we could not evaluate sexually differential size–fitness relationships as an explanation for the observed sex-ratio patterns. Nonetheless, A. v. paganus exhibited one of the strongest relationships between host size and offspring sex ratio yet reported. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 87, 285–296.

Genetics of sex-ratio variation inferred from parent–offspring regressions and sib correlations in the apple snail Pomacea canaliculata

The brood sex ratio in the apple snail Pomacea canaliculata varies almost continuously from all male to all female, but the population sex ratio is nearly 1:1. In this study, regressions of the offspring sex ratio on the sex ratios of the parents' siblings as well as correlations in the brood sex ratios between sisters or brothers were investigated, in order to infer the genetic system that produces the sex-ratio pattern. There were significant positive relationships between the offspring sex ratio and the sex ratio of the mother's siblings (slope=0.28), and between the offspring sex ratios of two sisters (r=0.41). On the other hand, the father-offspring regression (slope=0.10), and the correlations between two brothers (r=-0.13) or between the brother and the sister (r=0.17) were not significant. These patterns differed from predictions using typical cytoplasmic sex factors, sex-ratio genes or sex-determining polygenes. Thus, the results suggest the involvement of either a small number of sex-determining genes or a more complicated system such as sex-ratio or sex-determining polygenes that act nonadditively.

Sex ratio patterns in dioicous bryophytes re-visited

Based on a literature survey and our own investigations of selected species, we present a compilation of expressed sex ratios in 103 taxa of dioicous bryophytes, including 56 mosses and 47 liverworts. We grouped the approaches used to determine sex into two categories: (1) sex assessment per herbarium specimen or per patch in the field; (2) sex assessment by examination of individual shoots or thalli, usually in the field. Eighty-eight per cent of the taxa studied by the former methods, and 68% of those included in the latter, exhibited a female-biased sex ratio. The F : M values varied by a factor of 46 among species, excluding cases with only one sex expressing. There was also within-species variation of sex expression and ratio related to geographical area, altitude, year, substratum, plant maturity, and compared with cultivation conditions, and between the two method categories. Across taxa, proportions of non-expressing and sporophytic samples were negatively related to each other; however, there was no significant relationship between sex ratio and the proportion of sporophytic samples or shoots. The present data do not support the generalization that the most strongly female-biased sex ratios among dioicous bryophytes occur in 'extreme environments'. On the other hand, it is suggested here that phylogenetic history may explain observed species-wide sex ratios better than current habitat conditions in at least some cases.

Sex Ratio and Early Growth Patterns of Roseate Tern Chicks during Five Breeding Seasons at Falkner Island, Connecticut, USA

Abstract Roseate Tern (Sterna dougallii) nests were monitored at Falkner Island, Connecticut, USA between 1998 and 2002. A total of 586 chicks were sexed and their growth measured during the first three days. These data were used to derive growth parameters and predict survival to fledging with high confidence. In contrast to a previous study in Massachusetts, we found no sex-ratio bias at hatching in any single year or in all years combined, nor was there a bias at fledging in all years combined, or in any single year. This difference is interesting because both colonies exhibit a female-biased sex ratio at breeding. After controlling for other factors, there was no difference in early growth or survival between male and female chicks, consistent with our previous studies and with the hypothesis that differences in growth and survival between males and females observed in other species are due to sexual size dimorphism.

Single-locus complementary sex determination absent in Heterospilus prosopidis (Hymenoptera: Braconidae)

In the haplodiploid Hymenoptera, haploid males arise from unfertilized eggs, receiving a single set of maternal chromosomes while diploid females arise from fertilized eggs and receive both maternal and paternal chromosomes. Under single-locus complementary sex determination (sl-CSD), sex is determined by multiple alleles at a single locus. Sex locus heterozygotes develop as females, while hemizygous and homozygous eggs develop as haploid and diploid males, respectively. Diploid males, which are inviable or sterile in almost all cases studied, are therefore produced in high frequency under inbreeding or in populations with low sex allele diversity. CSD is considered to be the ancestral form of sex determination within the Hymenoptera because members of the most basal taxa have CSD while some of the more derived groups have other mechanisms of sex determination that produce the haplo-diploid pattern without penalizing inbreeding. In this study, we investigated sex determination in Heterospilus prosopidis Viereck, a parasitoid from a relatively primitive subfamily of the Braconidae, a hymenopteran family having species with and without CSD. By comparing sex ratio and mortality patterns produced by inbred and outbred females, we were able to rule out sl-CSD as a sex determination mechanism in this species. The absence of sl-CSD in H. prosopidis was unexpected given its basal phylogenetic position in the Braconidae. This and other recent studies suggest that sex determination systems in the Hymenoptera may be evolutionary labile.

Descriptive Ecology of a Turtle Assemblage in an Urban Landscape

We studied turtle populations inhabiting a canal and a lake (both man-made) within a heavily disturbed, urban setting. Six aquatic and semi-aquatic turtle species were collected in both habitats: spiny softshell turtle (Apolone spinifera), painted turtle (Chrysemys picta), common snapping turtle (Chelydra serpentina), common map turtle (Graptemys geographica), common musk turtle (Sternotherus odoratus) and red-eared slider (Trachemys scripta). While G. geographica was the most common species in the canal habitat, T. scripta was most common in the lake habitat. We describe patterns of sexual size dimorphism and sex ratios for the three most abundant species (G. geographica, T. scripta and S. odoratus). We discuss our data in light of problems facing turtle assemblages in urban settings.

The males of Melipona and other stingless bees, and their mothers

Female behaviour in social Hymenoptera and the queen-worker conflict with respect to male production have been the focus of many studies. Although male production is an investment that is in conflict with investment in colony size, males play a vital role in colony reproduction. This paper reviews the production patterns of male stingless bees, their activities once they have reached adulthood and their origin (i.e., are they sons of workers or of queens). The existence of a broad spectrum of species-specific patterns of male production, sex ratios, and male parentage offers ample opportunities to discuss the influence of ecology on the dynamics of stingless bee colony life. The paper also argues that selfishness causes the queen and the workers to compete and each to adopt certain strategies in their effort to produce male progeny. It is this competition, expressed in various forms during the characteristic and socially complex process of cell provisioning and oviposition, that could help explain the variable outcomes of male parentage at the species level as we currently know them. stingless bee male / sex ratio / life history / male aggregation / queen-worker conflict / provisioning and oviposition process

Geographic variation in the pattern of temperature‐dependent sex determination in the American snapping turtle (Chelydra serpentina)

Sex ratios of the American snapping turtle Chelydra serpentina are female-biased at cool temperatures, male-biased at moderate temperatures, and only females are produced at warm temperatures. The laboratory incubation of eggs at several constant temperatures yields patterns of skewed sex ratios. In this study, incubated eggs represented regional samples of C. serpentina from six different latitudes. The sample from the highest latitude yielded male-biased sex ratios across the broadest range of cool temperatures. This male-biased zone and those of the other five samples collectively contributed to a nested arrangement that is approximately symmetrical around a temperature of 24.5°C. Two indices of temperature-dependent sex determination (TSD) patterns (MF pivotal temperature, width of the male-biased zone) were significantly associated with latitude and with published data on several climatic variables (e.g. sunshine and rainfall). At high latitudes, females nest in exposed locations, locally the warmest places. At low latitudes females place their nests in shaded locations. Such nest site selection renders high latitude nests warmer than low latitude nests, even if only intermittently. Given that constant conditions show faster embryogenesis in high vs low latitude eggs, exposed nest sites seem to be an additional adaptation to hasten embryogenesis in high latitude nests. At low latitudes, shading prevents nests from overheating. We propose that a trade-off between evolution, (1) to balance sex ratio and (2) to maintain male-biased sex ratios in a temperature range that favours male fitness, leads to the observed arrangement of concentric TSD patterns centred around male production at 24.5°C.

Sex allocation in Telenomus busseolae, a solitary parasitoid of concealed eggs: the influence of host patch size

Abstract Telenomus busseolae Gahan (Hymenoptera: Scelionidae) is an important egg parasitoid of noctuid stem borers of gramineous crops, attacking egg masses of Sesamia spp. Under natural conditions, and whatever the host species attacked, these egg masses are generally concealed under the leaf sheaths or other narrow spaces, and vary greatly in size. In the work presented here, the influence of host patch size (4, 8, 16, 32, 64, or 128 eggs per mass) on the sex ratio and sex sequence pattern of ovipositing T. busseolae was investigated in the laboratory using Sesamia nonagrioides (Lefebvre) (Lepidoptera: Noctuidae) as host. The results are similar to those described for other parasitoids of aggregated hosts, and are in accordance with the Local Mate Competition model. With increasing egg mass size, the overall sex ratio (proportion of males) decreased, although additional males were laid at the end of the sequence in the larger masses (64 and 128 eggs). Sex sequence pattern always followed a males‐first strategy, i.e., with a higher proportion of males at the beginning, but the whole sex ratio sequence was influenced by the size of the egg mass. Such results in a parasitoid of concealed eggs are compared to those observed in parasitoids of exposed eggs and discussed in terms of parasitoid reproductive strategies and evolutionary adaptations.

우리나라 서해남부 해역에 출현하는 둥근돗대기새우 (Leptochela sydniensis)의 개체군 동태 및 생식생태

Investigations were made on the population structure, sex ratio, growth, and recruitment patterns of Leptochela sydniensis in the southwestern coastal areas of Korea, between May 2000 and December 2001. Spawning period, fecundity, brood loss, reproductive output, and size at sexual maturity also were examined. The results of the sex ratio showed that females were more numerous than males during the overall study period. Parameters of growth were estimated using the modified von Bertalanffy growth function (VBGF) model incorporating seasonal variation into growth. Females grew somewhat faster and reached a larger size than males <TEX>$(L\infty=12.80\;mm\;CL\;and\;K=0.70\;yr^{-1}\;or\;females,\and\;L\infty=12.08\;mm\;CL\;and\;K=0.69\;yr^{-1}$</TEX> for males). The structure of recruitment patterns obtained by the FiSAT program indicated one normally distributed group. Based on the occurrence of ovigerous females and the gonadosomatic index (GSI), the main spawning season was from June to August. Analysis of covariance indicated that brood loss was not observed during the incubation period. The size at which <TEX>$50\%$</TEX> of females are mature is estimated as 5.48 mm CL.

Subtle manipulation of egg sex ratio in birds

Mothers are predicted to overproduce male or female eggs when the relative fitness gains from one sex are higher and outweigh the costs of manipulation. However, in birds such biases are often difficult to distinguish from differential embryo or chick mortality. Using a molecular technique to identify the sex of early embryos, we aim to determine the effect of maternal nutrition on zebra finch (Taeniopygia guttata) egg sex ratios after 2 days of incubation, which is as close to conception as is currently possible. We found no overall bias in the sex ratio of eggs laid and sex did not differ with relative laying order under any diet regime. However, mothers on a low-quality diet did produce a female bias in small clutches and a slight male bias in large clutches. On a high-quality diet, mothers produced a male bias in small clutches and a female bias in large clutches. Those on a standard diet produced a roughly even sex ratio, irrespective of clutch size. These observed biases in egg sex are partly in line with predictions that, in this species, daughters suffer disproportionately from poor rearing conditions. Thus, when relatively malnourished, mothers should only rear daughters in small broods and vice versa. Sex-ratio patterns in this species therefore appear to be subtle.

Protandry in the New Zealand geoduck, Panopea zelandica (Mollusca, Bivalvia)

Summary Unequal sex ratios favouring males, especially in younger or smaller animals, is a feature of many infaunal bivalve species, including geoducks. Size and age at sexual maturity, sex ratios, and growth rates of the New Zealand geoduck, Panopea zelandica (Quoy and Gaimard, 1835), were investigated in specimens collected from Kennedy Bay, Coromandel Peninsula, and Shelly Bay, Wellington, from June 1999 to March 2001. Analyses of internal growth bands of polished shell sections were used to determine age at sexual maturity, and to construct von Bertalanffy growth curves for males and females in both populations. Patterns of sexual maturity were equivalent for both populations with 50% maturity calculated to be at ~55 mm and ~57 mm at Shelly Bay and Kennedy Bay, respectively. Length-frequency analysis of geoducks in Kennedy Bay indicated that females became more prevalent as the size increased, with significantly more females occurring in the largest size classes for both populations. However, sex ratios were equal with respect to age after about age 5 years. Similar patterns were observed in Shelly Bay, although the population was made up of larger, older individuals. Protandric development of P. zelandica for a significant proportion of individuals is considered likely for this species as all immature geoducks matured into males in their third year of life. Differences in growth rates could not be used as an explanation for the observed sex-ratio patterns as they were very similar for both sexes in each population. This paper suggests that functional protandric dioecy exists in P. zelandica. This finding may help to explain apparent sex ratio disparities reported in other geoduck species.

Genetic population structure of the white sifaka (Propithecus verreauxi verreauxi) at Beza Mahafaly Special Reserve, southwest Madagascar (1992–2001)

Gene flow within and between social groups is contingent on behaviourally mediated patterns of mating and dispersal. To understand how these patterns affect the genetic structure of primate populations, long-term data are required. In this study, we analyse 10 years of demographic and genetic data from a wild lemur population (Propithecus verreauxi verreauxi) at Beza Mahafaly Special Reserve, southwest Madagascar. Our goal is to specify how patterns of mating and dispersal determine kinship and genetic diversity among animals in the population. Specifically, we use microsatellite, parentage, and census data to obtain estimates of genetic subdivision (FST), within group homozygosity (FIS), and relatedness (r) within and among social groups in the population. We analyse different classes of individuals (i.e. adults, offspring, males, females) separately in order to discern which classes most strongly influence aspects of population structure. Microsatellite data reveal that, across years, offspring are consistently more heterozygous than expected within social groups (FIS mean = -0.068) while adults show both positive and negative deviations from expected genotypic frequencies within groups (FIS mean = 0.003). Offspring cohorts are more genetically subdivided than adults (FST mean = 0.108 vs. 0.052) and adult females are more genetically subdivided than adult males (FST mean = 0.098 vs. 0.046). As the proportion of females in social groups increases, the proportion of offspring sired by resident males decreases. Offspring are characterized by a heterozygote excess as resident males (vs. nonresident males) sire the majority of offspring within groups. We link these genetic data to patterns of female philopatry, male dispersal, exogamy, and offspring sex-ratio. Overall, these data reveal how mating and dispersal tactics influence the genetic population structure in this species.