Sex-specific spatio-temporal variability in reproductive success promotes the evolution of sex-biased dispersal

Inbreeding depression, asymmetries in costs or benefits of dispersal, and the mating system have been identified as potential factors underlying the evolution of sex-biased dispersal. We use individual-based simulations to explore how the mating system and demographic stochasticity influence the evolution of sex-specific dispersal in a metapopulation with females competing over breeding sites, and males over mating opportunities. Comparison of simulation results for random mating with those for a harem system (locally, a single male sires all offspring) reveal that even extreme variance in local male reproductive success (extreme male competition) does not induce male-biased dispersal. The latter evolves if the between-patch variance in reproductive success is larger for males than females. This can emerge due to demographic stochasticity if the habitat patches are small. More generally, members of a group of individuals experiencing higher spatio-temporal variance in fitness expectations may evolve to disperse with greater probability than others.     

Fluctuating sex ratios,but no sex-biased dispersal,in a promiscuous fish

Dispersal in birds and mammals tends to be female-biased in monogamous species and male-biased in polygamous species. However results for other taxa, most notably fish, are equivocal. We employed molecular markers and physical tags to test the hypothesis that Atlantic salmon, a promiscuous species with intense male-male competition for access to females, displays male-biased dispersal. We found significant variation in sex ratios and in asymmetric gene flow between neighbouring salmon populations, but little or no evidence for sex-biased dispersal. We show that conditions favouring male dispersal will often be offset by those favouring female dispersal, and that spatial and temporal variation in sex ratios within a metapopulation may favour the dispersal of different sexes in source and sink habitats. Thus, our results reconcile previous discrepancies on salmonid dispersal and highlight the need to consider metapopulation dynamics and sex ratios in the study of natal dispersal of highly fecund species.     

Conditional dispersal under kin competition: extension of the Hamilton-May model to brood size-dependent dispersal

I consider a site-based model with contest competition among siblings, and assume that dispersal is conditional on the number of offspring in the natal site. Evolutionarily stable populations contain threshold dispersal strategies, which retain a certain number of offspring in the natal site and disperse the rest (if the actual number of offspring is less than the threshold, then all offspring are retained). Due to the discrete nature of the strategy set (the threshold must be integer), the ESS may not be unique or may not exist. In the latter case, two neighboring threshold strategies coexist in the evolutionarily stable population. Dispersal first decreases and then increases as a function of dispersal mortality, such that all but one offspring should be dispersed both when dispersal mortality is very small or very high. Population-level dispersal fractions are often similar to the unconditional ESS, but differ strongly when fecundity is small and dispersal mortality is high.     

Sex allocation and dispersal in a heterogeneous two-patch environment

We investigate the evolution of sex allocation and dispersal in a two-habitat environment using a game theoretic analysis. One habitat is of better quality than the other and increased habitat quality influences the competitive ability of offspring in a sex-specific manner. Unlike previous work, we allow incomplete mixing of the population during mating. We discuss three special cases involving the evolution of sex allocation under fixed levels of dispersal between habitats. In these special cases, stable sex-allocation behaviors can be both biased and unbiased. When sex-allocation behavior and dispersal rates co-evolve we identify two basic outcomes. First-when sex-specific differences in the consequences of spatial heterogeneity are large-we predict the evolution of biased sex-allocation behavior in both habitats, with dispersal by males in one direction and dispersal by females in the other direction. Second-when sex-specific differences are small-unbiased sex-allocation is predicted with no dispersal between habitats.     

Joint evolution of sex ratio and dispersal: conditions for higher dispersal rates from good habitats

We analyze models of evolution of sex ratio conditional on habitat quality and with sex specific dispersal. Previous analysis concluded that the main constraint on sex ratio is habitat choice and leads to overproduction of the most dispersing sex in low quality habitat. Here, we analyze three models with finite local populations and show that constraints on sex ratio can balance constraints on habitat choice. In the first model, dispersal rates are fixed. In the second, sex specific dispersal can evolve independently of the habitat quality. These models suggests that sex ratio evolution can lead to higher global dispersal rates (mean of male and female dispersal rates) from high quality habitats. In the last model dispersal evolves conditionally with both sex and habitat. Our models suggests that conditions for overproduction of the most dispersing sex in high quality habitat are frequent. The predictions of the models with evolving dispersal contrast with patterns generally described in nature. We discuss possible reasons of this difference.     

Natal dispersal in relation to population density and sex ratio in the field vole,Microtus agrestis

Summary In a sample of 240 juvenile field voles 8% of the males and 22% of the females reached sexual maturity within their natal home range. Among individuals retrapped as adults, 58% of males and 23% of females had dispersed, i.e. had moved more than one home range diameter. The mean distance moved for males (58.5 m) exceeded that for females (28.6 m). Male movement distances were negatively associated with total density, and with density of adult females, but not with male density. Female movements were not related to population density. There were no relation between sex ratio and distance moved. The distribution of distances moved for both males and females fit a geometrical distribution, suggesting the importance of competitive processes.     

Advances in our understanding of mammalian sex-biased dispersal

Sex-biased dispersal is an almost ubiquitous feature of mammalian life history, but the evolutionary causes behind these patterns still require much clarification. A quarter of a century since the publication of seminal papers describing general patterns of sex-biased dispersal in both mammals and birds, we review the advances in our theoretical understanding of the evolutionary causes of sex-biased dispersal, and those in statistical genetics that enable us to test hypotheses and measure dispersal in natural populations. We use mammalian examples to illustrate patterns and proximate causes of sex-biased dispersal, because by far the most data are available and because they exhibit an enormous diversity in terms of dispersal strategy, mating and social systems. Recent studies using molecular markers have helped to confirm that sex-biased dispersal is widespread among mammals and varies widely in direction and intensity, but there is a great need to bridge the gap between genetic information, observational data and theory. A review of mammalian data indicates that the relationship between direction of sex-bias and mating system is not a simple one. The role of social systems emerges as a key factor in determining intensity and direction of dispersal bias, but there is still need for a theoretical framework that can account for the complex interactions between inbreeding avoidance, kin competition and cooperation to explain the impressive diversity of patterns.     

Female common lizards (Lacerta vivipara) do not adjust their sex-biased investment in relation to the adult sex ratio

Sex allocation theory predicts that facultative maternal investment in the rare sex should be favoured by natural selection when breeders experience predictable variation in adult sex ratios (ASRs). We found significant spatial and predictable interannual changes in local ASRs within a natural population of the common lizard where the mean ASR is female-biased, thus validating the key assumptions of adaptive sex ratio models. We tested for facultative maternal investment in the rare sex during and after an experimental perturbation of the ASR by creating populations with female-biased or male-biased ASR. Mothers did not adjust their clutch sex ratio during or after the ASR perturbation, but produced sons with a higher body condition in male-biased populations. However, this differential sex allocation did not result in growth or survival differences in offspring. Our results thus contradict the predictions of adaptive models and challenge the idea that facultative investment in the rare sex might be a mechanism regulating the population sex ratio.     

Sex ratio response of the parasitoid wasp Muscidifurax raptor to other females

This study examines the sex ratio response of the parasitoid wasp Muscidifurax raptor to conspecific and confamilial females in relation to two groups of functional sex ratio models, local mate competition and host quality models. In some but not all experiments, M. raptor females produced a greater proportion of sons in the presence of a conspecific female than when alone, and this sex ratio effect carried over for a day after the females were isolated from each other M. raptor females also produced a greater proportion of sons in the presence of a female of the confamilial parasitoid Spalangia cameroni than when alone (although only on the second day of exposure to S. cameroni, not on the first). M. raptor's sex ratio increase in the presence of conspecifics is consistent with local mate competition models but not with host quality models because the presence of a conspecific female did not cause there to be more, and thus potentially smaller, offspring developing per host. In contrast, the presence of a S. cameroni female did cause there to be more offspring developing per host than when a M. raptor female was alone; thus M. raptor's sex ratio increase in the presence of S. cameroni may be explained by host quality models. An alternative explanation for the sex ratio increase in response to confamilials is that only a sex ratio response to conspecifics may be adaptive, due to local mate competition; but M. raptor females may be unable to distinguish between conspecific and S. cameroni females.     

Variable sex ratio strategy of Telenomus heliothidis (Hymenoptera: Scelionidae): adaptation to host and conspecific density

Summary The sex allocation behavior of the solitary egg parasitoid Telenomus heliothidis Ashmead was investigated by examining the response of females reared in isolation and under crowded conditions. Females reared in isolation adjusted their sex ratio with foundress and host number per patch in accordance with the predictions of local mate competition (LMC) theory. However, females did not shift their sex ratio in response to conspecifics foraging on the same host patch or to contact with previously parasitized hosts. Instead, shifts were associated with encounter rate and a sequence of oviposition. Females maintained under crowded conditions responded to host patches much differently. One-day-old females which had lived under crowded conditions for 24 h produced sex ratios similar to those of continuously isolated females. However, females reared under crowded conditions for 7 days consistently produced unbiased sex ratios, and exhibited a different sequence of oviposition. This shift appeared to be due directly to crowding rather than age, oviposition experience or sperm depletion since the effect could be reversed by subsequent isolation.     

Sex allocation by a mite parasitic on insects: local mate competition,host quality and operational sex ratio

The aim of this study was to test the predictions of local mate competition (LMC), host quality (HQ) and operational sex ratio (OSR) models, using a non-arrhenotokous parasitic mite, Hemisarcoptes coccophagus (Astigmata: Hemisarcoptidae). The life-history pattern of this mite meets the assumptions of these sex allocation models. Mating group size (LMC model), HQ and OSR affected the sex allocation of H. coccophagus females. Only young mite females adjusted the sex ratio of their progenies according to the predictions of LMC and HQ models; the sex allocation of old females was contrary to these predictions. We explain these patterns by the dynamic nature of the mite's population structure. When parents are young, their population distribution is patchy and progeny matings are local; hence sex allocation is in accordance with LMC theory. When parents become older, their populations shift towards panmixis; factors which had operated previously no longer exist. Consequently, females adjust the sex ratio of late progenies so that it can compensate for the earlier sex allocation, in order to make their total sex ratio unbiased, as expected in panmictic populations. Our data, expressed as the cumulative sex ratio, support this hypothesis.     

Hypothetical role of men's sexual activity in the regulation of human sex ratio: Analysis of the sex ratio of post-war abandoned children

The regulation of the sex ratio at birth in human species remains poorly understood. After wars, a shift of the sex ratio in favor of men is always observed. Among the different hypothesis to explain this observation, one is to consider that Y-bearing spermatozoids have a weight advantage following insemination and that X-bearing spermatozoids, heavier, are more time-resistant. Following these observations, frequent sex may favor the birth of boys, whether infrequent sex may favor the birth of girls.Sustaining this sperm weight hypothesis, I report here that in France, after the two world wars, there has been an increase of abandoned illegitimate children with a significant shift of the sex ratio in favor of men. These observations may reflect an increase in illegitimate birth and indirectly an increase of men paternity.     

Male-biased brood sex ratio depresses average phenotypic quality of barn swallow nestlings under experimentally harsh conditions

Sex allocation strategies are believed to evolve in response to variation in fitness costs and benefits arising from the production of either sex and can be influenced by the differential susceptibility of sons and daughters to environmental conditions. We tested the effects of manipulating brood size and the sex ratio of the nestmates and the effect of sex on the phenotypic quality of individual barn swallow (Hirundo rustica) nestlings. Brood enlargement, which results in harsh rearing conditions, negatively affected the morphology and immunity of the nestlings. However, the negative consequences of brood enlargement were more marked among male than female offspring. In enlarged but not reduced broods, high proportions of male nestmates resulted in lowered individual body mass, body condition and feather growth. Thus, the consequences of a harsh environment on individual nestlings differed between the sexes and depended on the sex ratio among the other nestlings in the brood. The evolution of sex allocation strategies may therefore depend on the sex of individual nestlings but also on an interaction between environment and progeny sex ratio.     

Daphnia females adjust sex allocation in response to current sex ratio and density

Cyclical parthenogenesis presents an interesting challenge for the study of sex allocation, as individuals’ allocation decisions involve both the choice between sexual and asexual reproduction, and the choice between sons and daughters. Male production is therefore expected to depend on ecological and evolutionary drivers of overall investment in sex, and those influencing male reproductive value during sexual periods. We manipulated experimental populations, and made repeated observations of natural populations over their growing season, to disentangle effects of population density and the timing of sex from effects of adult sex ratio on sex allocation in cyclically parthenogenetic Daphnia magna. Male production increased with population density, the major ecological driver of sexual reproduction; however, this response was dampened when the population sex ratio was more male‐biased. Thus, in line with sex ratio theory, we show that D. magna adjust offspring sex allocation in response to the current population sex ratio.     

Competition between relatives and the evolution of dispersal in a parasitoid wasp

Evolutionary theory predicts that levels of dispersal vary in response to the extent of local competition for resources and the relatedness between potential competitors. Here, we test these predictions by making use of a female dispersal dimorphism in the parasitoid wasp Melittobia australica. We show that there are two distinct female morphs, which differ in morphology, pattern of egg production, and dispersal behaviour. As predicted by theory, we found that greater competition for resources resulted in increased production of dispersing females. In contrast, we did not find support for the prediction that high relatedness between competitors increases the production of dispersing females in Melittobia. Finally, we exploit the close links between the evolutionary processes leading to selection for dispersal and for biased sex ratios to examine whether the pattern of dispersal can help distinguish between competing hypotheses for the lack of sex ratio adjustment in Melittobia.     

The effects of relatedness on progeny sex ratio in spider mites

Theory predicts that optimal sex allocation in subdivided populations is dependent on the genetic relatedness among competing offspring such that when relatedness is high, progeny sex allocation should be more biased than when relatedness is low. In the laboratory we compared the progeny sex ratio of four groups of five ovipositing females of various presumed degrees of relatedness (sisters of inbred laboratory population, sisters of outbred field population, non-sisters of the same field population and non-sisters each from a different, geographically distant field population). We found a greater female bias in the progeny of inbred sisters than for sisters from the field population; the progeny sex ratio was also more strongly female biased for sisters than for non-sisters from the field population. These differences in sex ratio are in line with theoretical predictions. Our results indicate that spider mites are capable of some method of discrimination between kin and non-kin.     

Female ambrosia beetles adjust their offspring sex ratio according to outbreeding opportunities for their sons

Recent studies on the effect of local mate competition (LMC) on sex ratios have focused on the effect of post-dispersal mating success by males. A higher proportion of males is expected to be produced as the potential for outbreeding increases. Here we demonstrate that males of a haplodiploid ambrosia beetle with LMC disperse to seek additional matings, and brood sex ratios increase with outbreeding opportunities in the field. Manipulations in the laboratory confirm that females produce more sons when the post-dispersal mating prospects of their sons are experimentally increased. This is the first study showing that male dispersal options may influence individual female sex allocation decisions in species with strong LMC.     

Partial local mate competition in the wasp Trichogramma euproctidis: the role of emergence sex ratio on female mating behaviour

1. Parasitic wasps with structured populations are generally assumed to follow the local mate competition (LMC) model: females lay only the minimal number of sons necessary to inseminate all daughters in the emergence patch, and increase this number when faced with additional broods from unrelated females. After emergence, daughters mate with local males before dispersing for host location and oviposition. The main predictions from the model have been verified for many species. 2. Conflicting evidence exists on the status of the egg parasitoids Trichogramma regarding their on‐patch versus off‐patch mating. Although the life‐history traits of several species indicate that mating must occur on the emergence patch, recent data suggest that mating could occur outside the natal patch. 3. In this study, we measured the level of off‐patch mating in the egg parasitoid Trichogramma euproctidis using two isofemale lines in a greenhouse experiment. The impact of the sex ratio on the level of off‐patch mating was also tested. 4. The overall off‐patch mating proportion was 40.5% with a range between 0 and 85.7%, and was influenced by the sex ratio on the emergence patch: the more males available at emergence, the less off‐patch mating occurring. 5. The mating structure of this species can be described as partial LMC.