Local Sex Ratio Research Articles

Male collared lizards,Crotaphytus collaris(Sauria: Crotaphytidae), signal females by broadcasting visual displays

The signalling function of displays broadcast when animals are distant from conspecifics can be difficult to determine. I tested the extent to which visually transmitted broadcast displays given by free-ranging territorial male collared lizards signalled same-sex rivals or females. One test involved recording the frequency of broadcast displays, aggressive contests with rivals, and courtship encounters with females during ten reproductive seasons when local sex ratios varied markedly. The frequency of broadcast displays decreased as the ratio of male competitors to females increased. The frequency with which males initiated contests with rivals was not related to the ratio of competitors to females, whereas the frequency of courtship interactions decreased with sex ratio because there were fewer females to court. The behaviour of males that defended territories during two successive seasons showed a similar pattern. Broadcast display frequency was positively correlated with courtship frequency, but not with the frequency of contests with rivals. Lastly, individual males gave more broadcast displays during focal observations when they also engaged in courtship encounters with females than other observations when they engaged in aggressive conflicts with rival males. Although these results do not reject the possibility that broadcast displays may also signal male rivals, they support a major role of these displays in advertisement to females.

Local offspring density and sex ratio affect sex allocation in the great tit

The expected fitness gain for offspring of a given sex may depend on local population sex ratio and density. Knowing the influence of such social factors on brood sex ratios may contribute considerably to the understanding of sex allocation in higher vertebrates. For 3 consecutive years, we manipulated sex ratios and densities of juvenile great tits (Parus major) within forest plots and investigated how next year’s brood sex ratios were affected. For yearling female breeders, we considered the treatment of the plot of fledging and settlement and for experienced female breeders the treatment of the breeding plot. Experienced females produced more female offspring at increased plot densities and more males at reduced densities. Yearling female’s brood sex ratio was not affected by the density treatment but was positively related to the natural density in the plot of fledging. The plot sex ratio treatment affected sex allocation differently for yearling and experienced females. Yearling females produced more females when fledged from male-biased plots and more males when fledged from female-biased plots. Experienced females produced more females in control plots and unbiased brood sex ratios in male- and female-biased plots. Our results show that experimental changes in local sex ratio and density generate social or environmental cues that are used in sex allocation. We cannot show that sex allocation was adaptive. However, natural and experimental plot density and experimental plot sex ratio related differently to sex allocation for yearling and experienced females, which suggest that several sex allocation scenarios might act simultaneously. Key words: frequency dependence, local competition, sex-biased dispersal, social environment, Trivers and Willard hypothesis.[Behav Ecol]

High sex ratio as a correlate of depression in Chinese men

There is a large and increasing excess of men in China, especially in poor rural areas, with millions now unable to marry. The study aimed to determine whether these high sex ratios influence the prevalence of depression. The study was a cross-sectional survey, using a self-completion questionnaire, administered to adults of reproductive age in localities with widely varying sex ratios in three provinces: Zhejiang, Yunnan and Guizhou. Depression was measured using the Chinese version of the Beck Depression Inventory (BDI). A total of 7435 men and women aged 19-40 completed questionnaires. Moderate or severe depression was found in 17.6% of men and 17.0% of women, and suicide thoughts and wishes in 18.0% and 20.0% respectively. Independent correlates for depression and suicide ideation in men were high local sex ratio, rural residence, low education and poor economic status. Very high local sex ratio was the strongest independent determinant of depression in men. These include (1) selection bias: with just three provinces and a purposive sampling strategy, (2) reporting bias: the BDI score is arguably a crude measure for a complex psychological condition, and the veracity of responses to sensitive questions cannot be guaranteed, (3) information bias: county-level sex ratio figures may underestimate the actual sex ratio, especially in rural areas. Very high sex ratios in parts of China are contributing to depression and suicidal tendencies in men, and women, but especially unmarried men. Proactive measures need to be taken to identify and treat vulnerable individuals.

Insects perceive local sex ratio in the absence of tactile or visual sex-specific cues

Numerous studies have demonstrated adaptive behavioral responses of males and females to changes in operational sex ratio (the ratio of potentially receptive males to receptive females; OSR), and theory often assumes that animals have perfect instantaneous knowledge about the OSR. However, the role of sensory mechanisms in monitoring the local sex ratio by animals and whether animals can perceive local sex ratio in a manner consistent with model assumptions have not been well addressed. Here, we show that mating water striders Gerris gracilicornis respond to local sex ratio even when visual and physical contact with other individuals were experimentally prohibited. Our study shows that insects are able to estimate local population’s sex ratio and adjust their behavior based on nonvisual cues perceived at a distance or released to the habitat. Hence, the frequent theoretical assumption that individuals have knowledge about their local sex ratio regardless of their direct behavioral interactions may be an acceptable approximation of reality.

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

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

Social environment affects juvenile dispersal in great tits (Parus major)

1. Habitat selection can affect individual fitness, and therefore, individuals are expected to assess habitat quality of potential breeding sites before settlement. 2. We investigated the role of social environment on juvenile dispersal behaviour in the great tit (Parus major). Two main contradictory hypotheses can be formulated regarding social effects on juvenile dispersal as follows: (i) High fledgling density and sex ratio may enhance the intensity of local (kin) competition and, therefore, reduce individual survival chance, enhance emigration and reduce settlement ('repulsion' hypothesis) (ii) Alternatively, high fledgling density and sex ratio may signal high-quality habitat or lead to aggregation and thus increase individual survival chance, reduce emigration and enhance settlement ('attraction' hypothesis). 3. To disentangle positive from negative effects of high density and male-biased sex ratio on dispersal, we manipulated the social composition of the fledgling population in 12 semi-isolated nest-box areas (plots) via a change of fledgling density (low/high) as well as fledgling sex ratio (female-biased/balanced/male-biased) across 3 years. We then tested whether experimental variation in male and female fledgling densities affected variation in local survival, emigration and settlement of juveniles, and whether social effects on survival and dispersal support the 'repulsion' or 'attraction' hypothesis. 4. We found no experimental effects on local survival and emigration probabilities. However, consistent with the 'attraction' hypothesis, settlement was significantly and positively affected by local experimental sex ratio in each of the study years: both male and female juveniles avoided female-biased plots and settled more in plots that were balanced and male-biased the previous year. 5. Our study provides unprecedented experimental evidence that local sex ratio plays a causal role in habitat selection. We suggest that settlers avoid female-biased plots because a high proportion of females may reflect the absence or the low quality of local resources in the habitat. Alternatively, male territory acquisition may be facilitated by a high local density of 'candidate' males, and therefore, juveniles were less successful in settling in female-biased plots.

Mate Availability and Women's Sexual Experiences in China.

Data from the 1999-2000 Chinese Health and Family Life Survey were merged with community-level data from the 1982, 1990, and 2000 Chinese censuses to examine the relationship between the local sex ratio (number of men per 100 women) and sexual outcomes among women (N = 1,369). Consistent with hypotheses derived from demographic-opportunity theory, multilevel logistic regression analyses showed that women are more likely to be sexually active, to have had premarital sexual intercourse, to have been forced to have sex, and to test positive for a sexually transmitted infection when there is a relative abundance of age-matched men in their local community. Education, birth cohort, and geographic location also emerged as significant predictors of women's sexual experiences.

Local sex ratio affects the cost of reproduction

1. Costs and benefits of reproduction are central to life-history theory, and the outcome of reproductive trade-offs may depend greatly on the ecological conditions in which they are estimated. In this study, we propose that costs and benefits of reproduction are modulated by social effects, and consequently that selection on reproductive rates depends on the social environment. 2. We tested this hypothesis in a great tit Parus major population. Over 3 years, we altered parental reproductive effort via brood size manipulations (small, intermediate, large) and manipulated the local social environment via changes in the local fledgling density (decreased, increased) and the local sex ratio (female-biased, control, male-biased). 3. We found that male-biased treatment consistently increased the subsequent local breeding densities over the 3-year study period. We also found that parents rearing small broods in these male-biased plots had increased survival rates compared with the other experimental groups. 4. We conclude that reproductive costs are the product of an interaction between parental phenotypic quality after reproduction and the social environment: raising a small brood had long-lasting effects on some phenotypic traits of the parents and that this increased their survival chances in male-biased environment where habitat quality may have deteriorated (via increased disease/predation risk or intraspecific competition). 5. Our results provide the first experimental evidence that local sex ratio can affect reproductive costs and thus optimal clutch size.

Global invasion by Anthidium manicatum (Linnaeus) (Hymenoptera: Megachilidae): assessing potential distribution in North America and beyond

The wool carder bee, Anthidium manicatum, is the most widely distributed unmanaged bee in the world. It was unintentionally introduced to North America in the late 1960s from Europe, and subsequently, into South America, New Zealand and the Canary Islands. We provide information on the local distribution, seasonal abundance and sex ratio of A. manicatum from samples collected in an intensive two-year survey across Utah, USA. Anthidium manicatum was detected in 10 of the 29 Utah counties, largely in urban and suburban settings. Combining presence-only and MaxEnt background data from literature, museum databases and new records from Utah, we constructed three species distribution models to examine the potential distribution of A. manicatum in its native Eurasian range as well as invaded ranges of North and South America. The A. manicatum model based on locality and background data from the species’ native range predicted 50% of the invasive records associated with high habitat suitability (HS ≥ 0.90). The invasive North American model predicted a much broader distribution of A. manicatum (214% increase); whereas, the South American model predicted a narrower distribution (88% decrease). The poor predictive power of the latter model in estimating suitable habitats in the invasive South American range of A. manicatum suggests that the bee may still be limited by the bioclimatic constraints associated with a novel environment. Estimates of niche similarity (D) between the native and invasive models find that the North America bioclimatic niche is more similar to the bioclimatic niche of the native model (D = 0.78), whereas the bioclimatic niche of the South America invasion is relatively dissimilar (D = 0.69). We discuss the naturalization of A. manicatum in North America, possibly through punctuated dispersal, the probability of suitable habitats across the globe and the synanthropy exhibited by this invasive species.

Sex-specific effects of the local social environment on juvenile post-fledging dispersal in great tits

An individual’s decision to disperse from the natal habitat can affect its future fitness prospects. Especially in species with sex-biased dispersal, we expect the cost–benefit balance for dispersal to vary according to the social environment (e.g., local sex ratio and density). However, little is known about the social factors affecting dispersal decisions and about the temporal and spatial patterns of the dispersal process. In our study, we investigated experimentally the effects of the social environment on post-fledging dispersal of juvenile great tits by simultaneously manipulating the density and sex ratio of fledglings within forest plots. We expected young females in the post-fledging period mainly to compete for resources related to food and, as they are subordinate to males, we predicted higher female dispersal from male-biased plots. Juvenile males compete for vacant territories already in late summer and autumn; thus, we predicted increased male dispersal from high density and male-biased plots. We found that juvenile females had a higher probability to leave male-biased plots and had dispersed further from male-biased plots in the later post-fledging phase when juvenile males start to become territorial and more aggressive. Juvenile males were least likely to leave male-biased plots and had smallest dispersal distances from female-biased plots early after fledging. The results suggest that the social environment differentially affected the costs and benefits of philopatry for male and female juveniles. The local sex ratio of individuals is thus an important social trait to be considered for understanding sex-specific dispersal processes.Electronic supplementary materialThe online version of this article (doi:10.1007/s00265-011-1207-1) contains supplementary material, which is available to authorized users.

The rise and fall of local populations of ortolan buntings Emberiza hortulana: importance of movements of adult males

Changes in population size of local populations of birds have usually been interpreted in relation to adult return rate and recruitment of young individuals after natal dispersal. Little is known about the importance of redistribution of adult individuals through breeding dispersal. The small Norwegian population of ortolan buntings Emberiza hortulana has a patchy distribution with about 30 long-term local populations. During a period of general population decline (29% decrease over 7 years), the population trends of local populations (measured as number of males recorded) were highly variable, with some even increasing four-fold. Comparisons of demographic parameters showed that adult immigration rate (i.e. dispersal of adult males) explained both yearly changes in male population size and population trends over the whole study period better than adult return rate or adult emigration rate, or a measure of recruitment of young males. Adult immigration rates and recruitment rates were correlated, suggesting that both young and adult males find the same places attractive. In the study area, adult sex ratio was strongly male-biased, and immigration rate was higher when local sex ratio was less skewed. In addition, less skewed sex ratio was related to higher adult return rate and lower emigration rate. We found no relationships between measures of breeding success and population change. We suggest that conspecific attraction may explain the observed patterns. Some local populations may act as hot-spots attracting adult males from other populations. Thus, local population changes need not reflect overall population growth rate, but may be a consequence of redistribution of adult birds.

Do sex-specific densities affect local survival of free-ranging great tits?

Competition within sexes is expected when resources are sex specific, whereas competition between sexes can occur when similar resources are exploited. Local population density and sex ratio will determine the amount of sex-specific interactions and thus the potential degree of sex-specific competition. In contrast, high densities and the density of same-sex individuals may also positively influence survival, for example, by facilitating the exploitation of resources. The population density and sex ratio may therefore differently affect survival of males and females and thus also affect the expected fitness gains of producing a certain offspring sex. In this paper, we investigate experimentally whether and how sex-specific local densities affected sex-specific annual local survival of juvenile and adult great tits. We manipulated the density and sex ratio of fledgling great tits in 12 forest plots during 3 consecutive breeding seasons and monitored local survival until the next breeding season. We found no negative effects of the number of same- or opposite-sex competitors on juvenile local survival. Instead, local survival of juveniles of both sexes increased with the density of same-sex fledglings. Adult local survival was negatively affected by an experimental increase in density of nestlings, yet associated positively with the natural breeding pair density in a plot. Juvenile local survival related negatively to breeding pair density. Our results reveal experimental evidence for both negative effects of density on adult local survival and positive sex-specific effects on juvenile local survival, which shape sex-specific fitness prospects and might thus also alter optimal sex allocation decisions.

Imbalanced Sex Ratios, Men’s Sexual Behavior, and Risk of Sexually Transmitted Infection in China

China has been experiencing pronounced changes in its sex ratio, but little research has explored the consequences of these changes for sexual behavior and health. We merge data from the 1999-2000 Chinese Health and Family Life Survey with community-level data from the 1982, 1990, and 2000 Chinese censuses to examine the relationship between the local sex ratio and several dimensions of men's sexual behavior and sexual health. Multilevel logistic regression models show that, when faced with a relative abundance of age-matched women in their community, Chinese men are slightly less likely to have intercourse with commercial sex workers, but are more likely to engage in premarital noncommercial intercourse and to test positive for a sexually transmitted infection. These findings are consistent with hypotheses derived from demographic-opportunity theory, which suggests that an abundance of opposite-sex partners will increase the risk of early, frequent, and multi-partner sex and, through this, sexually transmitted infection risk.

Density‐Dependent Sex Ratio Adjustment and the Allee Effect: A Model and a Test Using a Sex‐Changing Fish

Positive density dependence (i.e., the Allee effect; AE) often has important implications for the dynamics and conservation of populations. Here, we show that density-dependent sex ratio adjustment in response to sexual selection may be a common AE mechanism. Specifically, using an analytical model we show that an AE is expected whenever one sex is more fecund than the other and sex ratio bias toward the less fecund sex increases with density. We illustrate the robustness of this pattern, using Monte Carlo simulations, against a range of body size-fecundity relationships and sex-allocation strategies. Finally, we test the model using the sex-changing polygynous reef fish Parapercis cylindrica; positive density dependence in the strength of sexual selection for male size is evidenced as the causal mechanism driving local sex ratio adjustment, hence the AE. Model application may extend to invertebrates, reptiles, birds, and mammals, in addition to over 70 reef fishes. We suggest that protected areas may often outperform harvest quotas as a conservation tool since the latter promotes population fragmentation, reduced polygyny, a balancing of the sex ratio, and hence up to a 50% decline in per capita fecundity, while the former maximizes polygyny and source-sink potential.

Effects of fine-scale genetic structure on male mating success in gynodioeciousBeta vulgarisssp.maritima

Plant mating systems are known to influence population genetic structure because pollen and seed dispersal are often spatially restricted. However, the reciprocal outcomes of population structure on the dynamics of polymorphic mating systems have received little attention. In gynodioecious sea beet (Beta vulgaris ssp. maritima), three sexual types co-occur: females carrying a cytoplasmic male sterility (CMS) gene, hermaphrodites carrying a non-CMS cytoplasm and restored hermaphrodites that carry CMS genes and nuclear restorer alleles. This study investigated the effects of fine-scale genetic structure on male reproductive success of the two hermaphroditic forms. Our study population was strongly structured and characterized by contrasting local sex-ratios. Pollen flow was constrained over short distances and depended on local plant density. Interestingly, restored hermaphrodites sired significantly more seedlings than non-CMS hermaphrodites, despite the previous observation that the former produce pollen of lower quality than the latter. This result was explained by the higher frequency of females in the local vicinity of restored (CMS) hermaphrodites as compared to non-CMS hermaphrodites. Population structure thus strongly influences individual fitness and may locally counteract the expected effects of selection, suggesting that understanding fine scale population processes is central to predicting the evolution of gender polymorphism in angiosperms.

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

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

Market forces affect patterns of polygyny in Uganda

Polygynous marriage is generally more beneficial for men than it is for women, although women may choose to marry an already-married man if he is the best alternative available. We use the theory of biological markets to predict that the likelihood of a man marrying polygynously will be a function of the level of resources that he has, the local sex ratio, and the resources that other men in the local population have. Using records of more than 1 million men in 56 districts from the 2002 Ugandan census, we show that polygynously married men are more likely to own land than monogamously married men, that polygynous marriages become more common as the district sex ratio becomes more female biased, that owning land is particularly important when men are abundant in the district, and that a man's owning land most increases the odds of polygyny in districts where few other men own land. Results are discussed with reference to models of the evolution of polygyny.

Snow skinks (Niveoscincus ocellatus) do not shift their sex allocation patterns in response to mating history

Theory predicts that where there is variation in the number of males and females available for mating (the breeding/operational sex ratio) frequency-dependent selection should favour overproduction of the less abundant sex. We used mating history as a proxy for male availability to examine female sex allocation responses to variation in the operational sex ratio in a large free-ranging population of Niveoscincus ocellatus, a viviparous skink. While we found evidence for shifts in offspring sex allocation and the local operational sex ratio, both between and within years, we failed to find patterns corresponding to those predicted by theory. These results are in accordance with a growing body of literature on facultative sex allocation, which suggests that, despite plausible benefits, there is often little evidence for adaptive sex allocation shifts in wild vertebrate populations. In this study, we examine the inconsistencies between theoretical predictions and empirical patterns, specifically relating to variation in the operational sex ratio, and the accuracy with which empirical studies adhere to the assumptions of theoretical models. We suggest that alternative hypotheses, which focus on individual level decisions in relation to population parameters influenced by the operational sex ratio, may more closely predict the sex allocation patterns observed.

Multiply Mating Males in Gnamptogenys striatula Mayr (Hymenoptera, Formicidae)

Multiply mating ant males are rare, because in most ant species, multiple mating opportunities for males are scarce. Gnamptogenys striatula is an exception to this rule, as intranidal mating offers protection and a local female-biased sex ratio. In a behavioral experiment, we confirm the males’ ability to inseminate several receptive workers, with four females being the observed maximum.

Local resource competition and local resource enhancement shape primate birth sex ratios.

Sex ratio theory provides a powerful source of testable predictions about sex allocation strategies. Although studies of invertebrates generally support predictions derived from the sex ratio theory, evidence for adaptive sex ratio biasing in vertebrates remains contentious. This may be due to the fact that most studies of vertebrates have focused on facultative adjustment in relation to maternal condition, rather than processes that might produce uniform sex biases across individuals. Here, we examine the effects of local resource enhancement (LRE) and local resource competition (LRC) on birth sex ratios (BSRs). We also examine the effects of sex differences in the costs of rearing male and female offspring on BSRs. We present data from 102 primate species and show that BSRs are skewed in favour of the dispersing sex in species that do not breed cooperatively, as predicted by the LRC model. In accordance with the LRE model, BSRs are generally skewed in favour of the more beneficial sex in cooperatively breeding primate species. There is no evidence that BSRs reflect the extent of sexual size dimorphism, an indirect measure of the costs of rearing male and female offspring. These analyses suggest that adaptive processes may play an important role in the evolution of BSRs in vertebrates.