Sex ratios of Sitodiplosis mosellana (Diptera: Cecidomyiidae): implications for pest management in wheat (Poaceae)

Sex ratios of populations of the wheat midge Sitodiplosis mosellana Gehin, developing on wheat Triticum aestivum L., were determined at reproduction, adult emergence, and dispersal. The patterns of sex ratio through the life cycle of S. mosellana result from: (i) a genetic mechanism that causes all or nearly all of the progeny of individual females to be a single sex, with an overall sex ratio that is slightly biased at 54-57% females; (ii) a differential mortality during diapause that increases the sex ratio to 60-65% females; (iii) mating which occurs near the emergence site followed by female dispersal which causes the post-dispersal sex ratio to rise to nearly 100% females; and (iv) oviposition which spreads eggs among different plants and assures that the next generation has a local sex ratio close to the population average. These changes in sex ratio through the life cycle have implications for using crop resistance or pheromones to manage S. mosellana, because mating takes place quickly near emergence sites, and because mated females but not males disperse from emergence sites to oviposition sites. Crop refuges used to protect resistance genes against the evolution of virulence by S. mosellana must be interspersed to prevent assortative mating that would occur in separate blocks of resistant and susceptible plants. Monitoring or mating disruption using a pheromone would be ineffective when wheat is grown in rotation with a non-host crop.     

Insemination Capacity and Dispersal in Relation to Sex Allocation Decisions in Goniozus legneri (Hymenoptera: Bethylidae): Why Are There More Males in Larger Broods?

Models considering sex ratio optima under single foundress strict local mate competition predict that female bias will be reduced by stochasticity in sex allocation, developmental mortality of males and limited insemination capacity of males. In all three cases the number of males per brood is expected to increase with brood size. Sex ratio optima may also be less female biased when several mothers contribute offspring to local mating groups or if non‐local mating occurs between members of different broods; again more males are expected in larger broods. In the parasitoid wasp Goniozus legneri (Hymenoptera: Bethylidae), sex allocation has only a small stochastic component, developmental mortality is low and non‐siblings are unlikely to develop in the same brood. However, the number of males per brood increases with the size of the brood (produced by a single mother). We investigated the further possibilities of limited insemination capacity and non‐local mating using a naturalistic experimental protocol. We found that limited insemination capacity is an unlikely general explanation for the increase in number of males with brood size. All males and females dispersed from both mixed and single sex broods. Although most females in mixed sex broods mated prior to dispersal, these data suggest that non‐local mating is possible, for instance via male immigration to broods containing virgin females. This may influence sex ratio optima and account for the trend in male number.     

Interpersonal Aggression among Aka Hunter-Gatherers of the Central African Republic

Sex differences in physical and indirect aggression have been found in many societies but, to our knowledge, have not been studied in a population of hunter-gatherers. Among Aka foragers of the Central African Republic we tested whether males physically aggressed more than females, and whether females indirectly aggressed more than males, as has been seen in other societies. We also tested predictions of an evolutionary theory of physical strength, anger, and physical aggression. We found a large male bias in physical aggression. Controlling for anger, we found an adult female bias in indirect aggression. Physical strength predicted anger, which predicted hitting, although results were sensitive to the inclusion or exclusion of traditional healers, who were physically and emotionally distinct from other Aka. With some important caveats, our results generally support the predicted sex differences in physical aggression and indirect aggression, and the predicted relationships among anger, strength, and aggression.     

Density-related changes in sexual selection in red deer.

In sexually dimorphic mammals, high population density is commonly associated with increased mortality of males relative to females and with female-biased adult sex ratios. This paper investigates the consequences of these changes on the distribution of male breeding success, the intensity of competition for females and the opportunity for sexual selection. After the red deer (Cervus elaphus L.) population of the North Block of Rum (Inner Hebrides) was released from culling, female numbers rose and male numbers declined, leading to an adult sex ratio of around one male to two females. This change was the result of increased mortality of males relative to females during the first two years of life; of increased emigration rates by young males; and of reduced immigration by males from outside the study area. The increasing bias in the adult sex ratio affected the timing of breeding as well as the distribution of mating success in males. As the adult sex ratio became increasingly biased towards females, the degree of skew in mating success (calculated across all harem-holders) increased, but mature males defended harems for shorter periods and a higher proportion of males held harems. In addition, a higher proportion of calves were fathered by immigrant males and the proportion fathered by males born in the study area declined. These results support the contention that, where high population density is associated with a female-biased adult sex ratio, competition for mates is likely to decline.     

Parameters of Mating Effort and Success in Male European Ground Squirrels,Spermophilus citellus

Behavioural and physiological changes during the reproductive period were documented in male European ground squirrels including: initiated conflict, scent marking, locomotion and range size, as well as testis size, scrotal pigmentation, testosterone levels and changes in body mass. Spring emergence dates in adult males varied but preceded those of females. Gonadal maturation was complete in adult males at emergence. Most yearlings did not mature and emerged with females. After a quiescent premating phase, male range, range overlap, conflict rates and locomotion increased at female emergence. The synchrony of females, their small ranges and short latencies to ovulation produced a highly male-biased operational sex ratio (OSR) and potential for scramble competition polygyny. In contrast to other ground squirrel species, each female apparently mated with only a single male. Scramble competition did appear to occur, in that males tried to contact as many females as often as possible before copulation. Females placed high temporal prerequisites on males. Costs of scrambling were documented by body mass loss during mating, which covaried with the number of ‘acquired’ females. Testicular regression occurred earlier, and fattening, moult and onset of next hibernation were delayed in successful males. The determinant factor in these costs was found to be mass loss intensity during the main mating period. Surprisingly, this factor was best related to feeding changes and not to locomotion or aggression. Time budget constraints perhaps imposed by the female, with compounding influences of exercise and stress effects of conflict, then negatively affected ensuing seasonal activities. The individual changes may adversely affect individual survivability and/or future reproductive success.     

Occurrence of protandry and a female-biased sex-ratio in a sponge-associated water mite (Acari: Unionicolidae)

The sex ratio of aUnionicola crassipes-type mite from a pond in Alberta, Canada, was found to be significantly female-biased at the time of adult emergence. Male mites emerged earlier, but were later surpassed in number by the females. This protandry seems more likely to be due to the smaller size of the males and hence their faster developmental rate than to any mating advatage the male mites gain by early emergence. It is possible that group selection acting on the productivity of mite colonies with different proportions of low-sex-ratio producers has selected for female-bias. Water mites are not known to exhibit any atypical modes of reproduction (e.g.arrhenotoky), nor is it known whether pre-adult maleU. crassipes have a greater mortality rate than females, so the mechanism behind the skewed ratio is not known.     

Aggression in harlequin frogs: male-male competition and a possible conflict of interest between the sexes

Long-term field observations and experimental manipulations were made on a population of harlequin frogs, Atelopus varius, in Costa Rica to document seasonality of intra- and intersexual aggression, determine the function of aggression, and identify the factors that influence success in aggressive encounters. Both males and females defended portions of their home range and exhibited site fidelity. Males were more aggressive towards other males during the pre-breeding and breeding seasons, whereas females were more aggressive towards other females during the post-breeding season. For up to several months prior to oviposition, males attempted to amplex (clasp) females without courtship behaviour, while females chased males from their territories and attempted to dislodge males that amplexed them. In staged encounters the winner was usually the apparent resident, regardless of body size. Frogs from a high-density population were more aggressive than individuals from a low-density population. Subadult males were as aggressive and as likely to win encounters as adult males. The function of aggression seemed to be related to the bizarre reproductives behaviour observed in these frogs. There seemed to be male-male competition for mates, as the operational sex ratio strongly favoured males, oviposition was asynchronous, and amplexus lasted for at least several weeks. Female-female aggression may have been related to defence of foraging or shelter sites, but aggression towards males was likely to be an attempt to thwart unwanted amplexus.     

Sex ratios in field populations of two parasitoids (Hymenoptera: Chalcidoidea) of Coccus hesperidum L. (Homoptera: Coccidae)

We tested several assumptions and predictions of host-quality-dependent sex allocation theory (Charnov et al. 1981) with data obtained for the parasitoid Metaphycus stanleyi Compere on its host, brown soft scale (Coccus hesperidum L.), in a California citrus grove and in the laboratory. Scales ceased growing after parasitization by M.?stanleyi. Thus, M. stanleyi may gauge host quality (=size) at oviposition. Host size positively influenced adult parasitoid size, and parasitoid size in turn influenced adult longevity of M. stanleyi. However, parasitoid fitness gains with host size and adult size were similar in males versus females. Sex allocation to individual hosts by M. stanleyi depended on host size; females consistently emerged from larger hosts than males. Host size was important in a relative sense; the mean host sizes of females versus males, and of solitary versus gregarious parasitoids varied with the available host size distribution. The offspring sex ratio of M. stanleyi reflected the available host size distribution; the sex ratio of emerging parasitoids varied with the available host size distribution. We did not detect a “critical host size” below which males emerged, and above which females emerged; rather, only females emerged from hosts in the upper size range, and a variable ratio of males and females emerged from hosts in the lower size range. We conclude that the sex ratio of field populations of M.?stanleyi is driven largely by the available size distribution of C. hesperidum. In addition, we tested predictions resulting from theoretical analyses of sex allocation in autoparasitoids with data obtained on Coccophagus semicircularis (Förster) parasitizing brown soft scale in the field. The sex ratio of C. semicircularis was consistently and strongly female biased (ca. 90% females). Based on available theoretical analyses, we suggest that this sex ratio pattern may have resulted from a very low encounter rate of secondary hosts coupled with a strong time limitation in C. semicircularis females. This explanation was the most plausible given constraints stemming from the detection of secondary hosts, their variable location within primary hosts, and their handling times. Finally, the size of hosts which yielded single versus multiple parasitoids, and the sizes of these parasitoids, were compared. These comparisons suggested that: (1) M. stanleyi females gauge host sizes precisely, and in terms of female offspring; thus a fitness penalty is not incurred by females which share a host, while males benefit from sharing a host, and; (2) instances where multiple C. semicircularis emerged from a single host were probably the result of parasitism by different females, or during different encounters by a single female.     

Conflict over multiple-partner mating between males and females of the polygynandrous common lizards

The optimal number of mate partners for females rarely coincides with that for males, leading to a potential sexual conflict over multiple-partner mating. This suggests that the population sex ratio may affect multiple-partner mating and thus multiple paternity. We investigate the relationship between multiple paternity and the population sex ratio in the polygynandrous common lizard (Lacerta vivipara). In six populations the adult sex ratio was biased toward males, and in another six populations the adult sex ratio was biased toward females, the latter corresponding to the average adult sex ratio encountered in natural populations. In males the frequency and the degree of polygyny were lower in male-biased populations, as expected if competition among males determines polygyny. In females the frequency of polyandry was not different between treatments, and polyandrous females produced larger clutches, suggesting that polyandry might be adaptive. However, in male-biased populations females suffered from reduced reproductive success compared to female-biased populations, and the number of mate partners increased with female body size in polyandrous females. Polyandrous females of male-biased populations showed disproportionately more mating scars, indicating that polyandrous females of male-biased populations had more interactions with males and suggesting that the degree of multiple paternity is controlled by male sexual harassment. Our results thus imply that polyandry may be hierarchically controlled, with females controlling when to mate with multiple partners and male sexual harassment being a proximate determinant of the degree of multiple paternity. The results are also consistent with a sexual conflict in which male behaviors are harmful to females.     

The reproductive ecology of resident manta rays (Manta alfredi) off Maui, Hawaii, with an emphasis on body size

In resident manta rays (Manta alfredi) off Maui, sexual maturity appears delayed until growth exceeds 90% of maximum size, an indicator that large body size provides a reproductive advantage at the expense of a shorter reproductive time period. In this study, 286 surveys were conducted between 2005 and 2010 using photo-identification and photogrammetry to study the reproductive ecology of a resident population of manta rays off Maui, Hawaii, and investigate the reproductive benefits of large body size in each sex. Although reproductive activities occurred year-round, mating trains and late-term pregnant females were significantly more likely to be observed during the winter months. Some females were pursued by males during both winter and summer of the same year, suggesting multiple ovulations may be possible in a single year. Males likely detect a female’s reproductive state by positioning directly behind her, or passing through her bodily excretions. The mean pregnancy rate was estimated at 0.56 pregnancies/adult female/year with larger females pregnant more often, and more likely in consecutive years. The operational sex ratio was heavily skewed with 2.68 adult males per reproductively available female. Although males appear to compete with one another for females within a mating train, no direct physical competition was ever observed between males. Evidence of highly dynamic mating trains lasting more than one day suggests endurance rivalry may be the primary mating strategy among males, during which larger males may benefit from greater energy reserves. The study area appears to be an important staging area for mating individuals in this population.     

Female mating success and risk of pre-reproductive death in a protandrous grasshopper

Numerous studies have assessed the adaptive value of protandry for males in several insect species, considering that male emergence is determined by female availability. However, the possible advantage of the time of emergence for females on their mating success in protandrous insect species has only been explored theoretically. By studying the grasshopper Sphenarium purpurascens we evaluated the hypothesis that late emergence could be adaptive for females. If female maturation occurs when the population density is higher and the sex ratio (males/females) is biased to males, their probability of mating increases. Thus, in this study we estimated (1) the opportunity for mating in females as a function of their sexual maturation time, population density, and sex ratio at the moment they reached sexual maturity. In addition, (2) an analysis incorporating female body size and the total number of female matings was performed. Both analyses support the occurrence of protandry in the studied population. Under the first approach, females with intermediate maturation time had a higher probability of being mated than earlier and late matured females. Thus, it suggests that stabilising selection is acting on female maturation time and this may affect selection on male maturation time. Furthermore, the proportion of mated females increased when the sex ratio was biased to males, and stabilising selection on maturation time was detected also. However, the number of matings of a female depended on her body size. Females with larger body size had more matings than smaller ones at the beginning of the reproductive season. Because selection acts differently on maturation time in males and females of S. purpurascens this result is consistent with a condition for the maintenance of protandry in the population. The present results are discussed in the light of the models for the evolution of protandry.     

Temporal variation in sex allocation in the mealybug Planococcus citri: adaptation, constraint, or both?

Sex ratio theory has been very successful in predicting under which circumstances parents should bias their investment towards a particular offspring sex. However, most examples of adaptive sex ratio bias come from species with well-defined mating systems and sex determining mechanisms, while in many other groups there is still an on-going debate about the adaptive nature of sex allocation. Here we study the sex allocation in the mealybug Planococcus citri, a species in which it is currently unclear how females adjust their sex ratio, even though experiments have shown support for facultative sex ratio adjustment. Previous work has shown that the sex ratio females produce changes over the oviposition period, with males being overproduced early and late in the laying sequence. Here we investigate this complex pattern further, examining both the robustness of the pattern and possible explanations for it. We first show that this sex allocation behaviour is indeed consistent across lines from three geographical regions. Second, we test whether females produce sons first in order to synchronize reproductive maturation of her offspring, although our data provide little evidence for this adaptive explanation. Finally we test the age at which females are able to mate successfully and show that females are able to mate and store sperm before adult eclosion. Whilst early-male production may still function in promoting protandry in mealybugs, we discuss whether mechanistic constraints limit how female allocate sex across their lifetime.     

An excess of males: skewed sex ratios in bat flies (Diptera: Streblidae)

Ectoparasitic insects often exhibit female-biased sex ratios, a pattern usually explained by greater female longevity and the likelihood that smaller, more active males will disperse or be groomed off the host. Theory predicts that unbalanced sex ratios should favor males when resources are abundant and predictable, and when males are the dispersing sex. Sex ratios of streblid bat flies were evaluated based on a large biodiversity survey in Venezuela–more than 25,000 bats representing 130 species were searched for flies, yielding more than 36,500 bat flies of 116 species. These samples allowed us to analyze sex ratios in 112 bat fly metapopulations. Our results indicate that fully one-third of these metapopulations were significantly male-biased. Traditional explanations for sex-ratio bias, such as sampling effects, unequal longevity between the sexes, and differential dispersal capability are refuted for bat flies in favor of an alternative hypothesis—selective host grooming. Because host grooming is the principal cause of mortality for these slow-reproducing parasites, and because females are larger than males and gravid for a significant portion of their adult life, host grooming activity is more likely to kill or remove females than males. Incomplete understanding of population dynamics, such as mating behavior, dispersal, and reproductive success, cloud applications of male-biased sex ratios in bat flies to support or refute theoretical predictions. Population studies of mating competition and sex-related dispersal dynamics of this male-dominated group should yield important insights into sex ratio theory.     

Growth and maturity of the spider crab Halicarcinus planatus (Brachyura: Hymenosomatidae) females in the southwestern Atlantic Ocean. Can these parameters be influenced by the population sex ratio?

The Halicarcinus planatus populations of the southwestern Atlantic Ocean show a highly variable sex ratio and a large size overlapping between females in the last immature instar (ADO) and mature (MAT) females. We hypothesized that these facts are related and that female impregnation has a central role in this relation. Non-impregnated ADO females delay maturity and would continue to grow, leading to size overlapping. This scenario is most probable in populations with a scarcity of males and could affect the growth, maturity and population structure. The sex ratio and female size distribution of several populations of the southwestern Atlantic Ocean were analysed. Growth and maturity were studied in two populations with different sex ratios (Camarones, a population with males, and Rada Tilly, a population where no males were found). Size overlapping was not related to the population sex ratio. Size overlapping was due to a variable moult increment and, to a lesser extent, to delayed maturity. Females mated before terminal moult in both populations. However, in Camarones, physiological maturity was not related to impregnation, while in Rada Tilly, vitellogenic oocytes were observed only in impregnated females. Also, differences in the number of spermatophores in female spermathecae were related to the sex ratio. Our results showed that the morphological maturity pattern of females was not influenced by the population sex ratio. However, physiological maturity was related to female impregnation and sex ratio. Also, the variation in the number of spermatophores suggests differences in the mating system related to the sex ratio.     

Intranidal mating,emergence, and sex ratio in a communal beeAndrena jacobi Perkins 1921 (Hymenoptera: Andrenidae)

The extent of preemergence intranidal mating, schedules of emergence, and sex ratio at emergence were documented forAndrena jacobi, a communal, univoltine bee, by collecting and dissecting adults as they emerged from their fossorial nests in 1994. Over 70% of females had mated intranidally with nestmate males, thus potentially incestuously, before emerging. Preemergence intranidal mating did not vary during a day or between nests within a day, though it was less frequent at the start and end of the period of emergence. It was independent of a female's size.A. jacobi was protandrous, though some males emerged after all females. The sex ratio at emergence was remarkably female biased, possibly a consequence of local mate competition. Intranidal mating may represent a characteristic trait of communal bees where a high density of receptive females are predictably aggregated within a nest.     

Mating system and sex allocation in the gregarious parasitoid Cotesia glomerata

The gregarious parasitoid Cotesia glomerata (L.) is often presumed to possess the characteristic attributes of a species that manifests local mate competition (LMC), as it commonly produces female-biased broods. However, our field surveys of sex ratio and laboratory observations of adult behaviour showed that this species is subject to partial local mate competition caused by natal dispersal. On average, 30% of males left their natal patch before mating, with the proportion of dispersing males increasing with an increase in the patch's sex ratio (i.e. proportion of males). Over 50% of females left their natal patch before mating, and only 27.5% of females mated with males emerging from the same natal patch. Although females showed no preference between males that were and were not their siblings, broods from females that mated with siblings had a significantly higher mean brood sex ratio (0.56) than broods from females that mated with nonsiblings (0.39). Furthermore, brood sex ratios increased as inbreeding was intensified over four generations. A field population of this wasp had a mean brood sex ratio of 0.35 over 3 years, which conformed well to the evolutionarily stable strategy sex ratio (r=0.34) predicted by Taylor's partial sibmating model for haplodiploid species. These results suggest that the sex allocation strategy of C. glomerata is based on both partial local mate competition in males and inbreeding avoidance in females. In turn, this mating system plays a role in the evolution of natal dispersal behaviour in this species.Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.      

Mating systems and sex ratios in the egg parasitoids, Trichogramma dendrolimi and T. papilionis (Hymenoptera: Trichogrammatidae)

Arrhenotokous parthenogenesis was confirmed in Trichogramma dendrolimi and T. papilionis. Subsequently the possible links between mating systems and biological traits of males, and sex ratios, were investigated in these two species, using Papilio xuthus eggs for their hosts. T. dendrolimi males attained 100% insemination of females in the parasitized host before egress from it. The high insemination rate was guaranteed by male precedence in emergence, lack of courtship in mating behaviour and short copulation time, combined with a long stay of emerging wasps within the host. The males were often the last to leave a host and made no mating attempts outside the host. Most but not all T. papilionis females were also inseminated in the host. The lower insemination rate of this species resulted from almost simultaneous emergence of both sexes, which prevented males from mating with inactive females. Another mating site of T. papilionis was just outside the host, which males left prior to most females. A reduced possibility of outbreeding was inferred in T. dendrolimi on the grounds that males were short-lived, frequently failed to expand their wings and died in the host. The reduced outbreeding was reflected in considerably lower sex ratios in T. dendrolimi.     

Mating Effect on Sex Pheromone Production of the Oriental tobacco budworm,Helicoverpa assulta

This study was undertaken to clarify the suppression phenomenon of sex pheromone production after mating and its relationship to the physiological mechanism in adult females of Helicoverpa assulta, and determine the mating factor from males causing depletion of sex pheromonc production. Sex pheromone production of H. assulta females was mostly terminated in 3 hours after mating. Mated females maintained with a low titer of sex pheromone until 3 days when it started to increase again, which showed a characteristic of species mating more than once. The mated female again produced pheromone upon injection of pheromone biosynthesis activating neuropeptide (PBAN) or extracts of brain-suboesophageal ganglion complexes (Br-Sg) of mated female, which were shown similar pheromonotropic activities as compared with virgin females. These results indicated that the mating did not inhibit the receptivity of pheromone gland itself and PBAN biosynthesis in suboesophageal ganglion of the mated females. And it seems to support that the depletion of sex pheromone production is responsible for blocking of PBAN release from head. To investigate the mating factor from adult males, when extracts of reproductive organs of male were injected into hemocoel of virgin females evoking depletion of sex pheromone production as shown in mated female. The results suggest that a chemical substance(s) from the male reproductive organs could be responsible for the loss of sex pheromone biosynthesis in H. assulta.     

Contest versus scramble competition for mates: The composition and spatial structure of a population of gray mouse lemurs (Microcebus murinus) in North-west Madagascar

The modes of intrasexual competition interacting in many dispersed societies of nocturnal solitary foragers are still poorly understood. In this study we investigate the spatial structure within a free-living population of gray mouse lemurs (Microcebus murinus) in order to test for the first time the predictions from two contrasting models of male intrasexual competition on the population level. The contest competition model predicts an uneven distribution of the sexes in a population nucleus with a female biased sex ratio in the center and a male biased sex ratio in the periphery. In contrast the scramble competition model predicts males and females being distributed evenly throughout their habitat with a constant sex ratio. Nine capture/recapture periods within three consecutive mating seasons revealed a continuous male biased sex ratio in the adult population with even trapping rates for the sexes. The male biased sex ratio could either be explained with postnatal female biased mortality or with a male biased natal sex ratio. This male biased sex ratio was apparent in all parts of the study site, indicating that the population was not subdivided into a female biased core and a male biased periphery. Furthermore, the majority of adult males have been captured at the same site as or in vicinity to females. Consequently, a large proportion of males had spatial access to females during the mating season. No signs of monopolization of females by certain dominant males could be detected. These data support the predictions from the scramble competition model and the concept of a promiscuous mating system for this species.     

Sex ratio variation in dioecious plant species: A comparative ecological study of six species of Lindera (Lauraceae)

Sex ratio variation was investigated in natural populations of six dioecious shrub species of Lindera in Japan. Interspecific differences in sex ratio were examined in relation to patterns of population structure, floral dimorphism, fruit production and intersexual differences in herbivory. Sex ratios tended towards equality or bias in favour of males, except for populations of L. glauca , in which no male plants were found. Sex switching in individual plants was not observed. Although male flowers were generally larger in size than flowers of conspecific females, costs of flowering did not appear to influence sex ratio. However, a relation was found between high allocation to fruit production and increasing male fraction of populations. In addition, girths of flowering plants tended to be larger in females, which may indicate delay in age of the first reproduction compared to males. No intersexual difference was found in degree of leaf damage due to herbivory. Results of this study are discussed in the light of other studies on sex ratio variation in dioecious plant species.