Stochastic effects in LMC models.

LMC (local mate competition) was first introduced by W. D. Hamilton to explain extraordinary female-biased sex ratios observed in a variety of insects and mites. In the original model, the population is subdivided into an infinite number of colonies founded by a fixed number of inseminated females producing the same very large number of offspring. The male offspring compete within the colonies to inseminate the female offspring and then these disperse at random to found new colonies. An unbeatable sex ratio strategy is found to be female-biased. In this paper, the effects of having colonies of random size and foundresses producing a random finite number of offspring are considered. The exact evolutionarily stable strategy (ESS) sex ratio is deduced and comparisons with previous approximate or numerical results are made. As the mean or the variance of brood size increases, the ESS sex ratio becomes more female-biased. An increase in the variance of colony size increases the ESS proportion of males when the mean brood size and colony size are both small, but decreases this proportion when the mean brood size or the mean colony size is large.     

Towards a genetic theory for the evolution of the sex ratio. III. Parental and sibling control of brood investment ratio under partial sib-mating

Models of sex ratio evolution under partial sib-mating are investigated in haplodiploids and diploids. In the cases of parental and sibling control of the brood investment ratio between the sexes in diploids, we find that the “unbeatable” investment ratio obtained by W. D. Hamilton (Science156, 477–488) for his local mate competition model corresponds in our inbreeding models to a weak form ESS (evolutionary stable strategy) fixation state and also to the population investment ratio at certain internal equilibria of our models. For haplodiploids, “strong form ESS” values exist under inbreeding in models involving father and sister control. Under brother and mother control, however, the ESS derived from local mate competition models is unstable in our inbreeding models to the introduction of any other investment ratio. We stress important qualitative differences between models involving local mate competition and inbreeding.     

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.      

Sex-ratio adjustment in response to local mate competition is achieved through an alteration of egg size in a haplodiploid spider mite

Sex-ratio adjustments are commonly observed in haplodiploid species. However, the underlying proximate mechanisms remain elusive. We investigated these mechanisms in Tetranychus urticae, a haplodiploid spider mite known to adjust sex ratio in response to the level of local mate competition (LMC). In this species, egg size determines fertilization probability, with larger eggs being more likely to be fertilized, and thus become female. We explored the hypothesis that sex-ratio adjustment is achieved through adjustment of egg size. By using spider mites from a large population, we found that females produced not only a higher proportion of daughters under high levels of LMC, but also larger eggs. Moreover, in populations experimentally evolving under varying levels of LMC, both the proportion of females and the egg size increased with LMC intensity. These results suggest that sex-ratio adjustment in spider mites is mediated by egg size, although the causal relationship remains to be tested.     

对叶榕传粉小蜂性比率的调节和稳定

传粉榕小蜂呈现偏雌的性比率,单双倍体性别决定系统、局域配偶竞争和近交效应被认为是调节偏雌性比率的3个主要机制。通过研究影响对叶榕传粉小蜂性比率的因素,结果表明传粉榕小蜂的偏雌性比率随局域配偶竞争强度的降低而增加;受母代雌蜂交配次数的影响,随着母代雌蜂交配次数的增加,子代的偏雌性比率逐渐降低,这一结果首次揭示了传粉榕小蜂的交配制次数对性比率的影响,并在个体水平上定量了性比率变异与雌蜂交配频次的关系。传粉小蜂的性比率与共生的非传粉小蜂的关系,非传粉小蜂的介入直接减少了传粉小蜂的数量,甚至对传粉小蜂的种群有显著影响,结果发现非传粉小蜂对传粉小蜂雌雄性的分配比率没有显著影响,传粉榕小蜂仍能正常地进行繁殖。传粉与非传粉者小蜂之间作用关系的确定,可为进一步理解两者的稳定共生的机制提供科学证据。     

Factors influencing brood sex ratios in polyembryonic Hymenoptera

Copidosoma sp. is a polyembryonic encyrtid wasp which parasitizes isolated hosts. Most broods of this wasp are unisexual, but some contain both sexes and the secondary sex ratio of these is usually highly female biased. The overall population secondary sex ratio is female biased. Walter and Clarke (1992) argue that because the majority of individuals must mate outside the natal patch, the bias in the population secondary sex ratio contradicts predictions made by Hamilton's (1967) theory of local mate competition (LMC). We suggest that the primary sex ratio is unbiased and that Walter and Clarke's results do not cast doubt on LMC. Instead these results imply that ovipositing females make a combined clutch size and sex ratio decision influencing whether individuals developing from a particular brood will outbreed or largely inbreed; for each case the predictions of LMC theory are not violated. If this interpretation is correct, what is of interest is the basis on which this decision is made rather than the population secondary sex ratio. We show that host encounter rate influences the proportions of mixed and single sex broods laid by Copidosoma floridanum, a related polyembryonic parasitoid. Among single-sex broods the primary sex ratio is female biased, but our results are in agreement with LMC theory since offspring developing from these broods will probably mate with siblings from adjacent hosts. We consider the egg load of females to be of major influence on oviposition behaviour, and that the mating structure of parasitoid offspring, potentially differential costs of male and female broods and the natural distributions of hosts both at oviposition and eclosion, require further study.     

Observations on sex ratio and behavior of males in Xyleborinus saxesenii Ratzeburg (Scolytinae, Coleoptera)

Strongly female-biased sex ratios are typical for the fungalfeeding haplodiploid Xyleborini (Scolytinae, Coleoptera), and are a result of inbreeding and local mate competition (LMC). These ambrosia beetles are hardly ever found outside of trees, and thus male frequency and behavior have not been addressed in any empirical studies to date. In fact, for most species the males remain undescribed. Data on sex ratios and male behavior could, however, provide important insights into the Xyleborini's mating system and the evolution of inbreeding and LMC in general.In this study, I used in vitro rearing methods to obtain the first observational data on sex ratio, male production, male and female dispersal, and mating behavior in a xyleborine ambrosia beetle. Females of Xyleborinus saxesenii Ratzeburg produced between 0 and 3 sons per brood, and the absence of males was relatively independent of the number of daughters to be fertilized and the maternal brood sex ratio. Both conformed to a strict LMC strategy with a relatively precise and constant number of males. If males were present they eclosed just before the first females dispersed, and stayed in the gallery until all female offspring had matured. They constantly wandered through the gallery system, presumably in search of unfertilized females, and attempted to mate with larvae, other males, and females of all ages. Copulations, however, only occurred with immature females. From galleries with males, nearly all females dispersed fertilized. Only a few left the natal gallery without being fertilized, and subsequently went on to produce large and solely male broods. If broods were male-less, dispersing females always failed to found new galleries.     

Joint evolution of sex ratio and reproductive group size under local mate competition with inbreeding depression

Local mate competition (LMC) may involve some amount of inbreeding between siblings. Because sib-mating is generally accompanied by inbreeding depression, natural selection may favor a reduced rate of sib-mating, possibly affecting the evolution of sex ratio and reproductive group size. The present study theoretically investigated the evolution of these traits under LMC in the presence of inbreeding depression. When the reproductive group size evolves, the determination mechanism of sex ratio is important because the timescale of the sex ratio response to reproductive group size can affect the evolutionary process. We consider a spectrum of sex ratio determination mechanisms from purely unconditional to purely conditional, including intermediate modes with various relative strengths of unconditional and conditional effects. This analysis revealed that both the evolutionarily stable reproductive group size and ratio of males increase with higher inbreeding depression and with a larger relative strength of an unconditional effect in sex ratio determination. Unexpectedly, when the sex ratio is controlled purely conditionally, the reproductive group size cannot exceed three even under the severest level of inbreeding depression (i.e., lethal effect). The present study reveals the conditions for LMC to evolve through the analysis of the joint evolution of reproductive group size and sex ratio.     

Sex allocation of three solitary ectoparasitic wasp species on bean weevil larvae: Sex ratio change with host quality and local mate competition

Sex ratio manipulation by ovipositing females was surveyed in 3 solitary ectoparastic wasp species,Dinarmus basalis (Pteromalidae),Anisopteromalus calanrae (Pteromalidae), andHeterospilus prosopidis (Braconidae), that parasitize azuki bean weevil (Callosobruchus chinensis (L) (Coleoptera: Buruchidae)) larvae within azuki beans (Vigna angularis). Variables were local mate competition (LMC) and host quality (HQ). We used host age as a measure of host quality (from 9-to 16-day-old hosts), changed the number of ovipositing females to control the level of local mate competition (1 female and 10 females), and examined oviposition patterns of the wasps. The offspring sex ratios (proportion of females) of the 3 wasp species respond qualitatively same to HQ and LMC. The common qualitative tendency among the 3 species is an increase of sex ratios increase with host age. In the process of changing the sex ratio (9–13-day-old) 3 wasp species respond only to HQ. In the hosts that end development in size (14–16-day-old) wasps respond to LMC. The response of sex ratio change to LMC in the old host ageclasses are different among the 3 species. In the situation that there exists LMC (10 females) sex ratios are the same among the 3 wasps. However, the sex ratios in no LMC (single female) are heterogeneous among the 3 wasps.     

Brood-size and sex-ratio variation in field populations of three species of solitary aphid parasitoids (Hymenoptera: Braconidae, Aphidiinae)

Solitary parasitoids of colony-forming hosts may produce quasi-gregarious broods, which favours sibmating on the natal patch and local mate competition (LMC). We examined seasonal variations in brood size and sex ratio in three species of solitary parasitoids of aphids associated with trophobiotic ants. Adialytus arvicola, a parasitoid of Sipha agropyrella on grasses, had the smallest broods (mean=4.2, maximum 19), while Lysiphlebus hirticornis, a parasitoid of Metopeurum fuscoviride on tansy, had the largest broods (mean=32.0, maximum 265). In Pauesia pini, a parasitoid of Cinara piceicola on Norway spruce, broods comprised an average of 5.8 (maximum 41) individuals; brood size increased during early summer when hosts became more available but remained the same later in the season. In all three species the sex ratio at eclosion was female-biased, with broods containing approximately two daughters for each son in both A. arvicola and L. hirticornis; the degree of female-bias was least in P. pini. The sex ratio did not vary with brood size. In A. arvicola, the variance of the number of sons declined with an increase in brood size, consistent with "precise" sex allocation. In contrast, in L. hirticornis, the overall sex-ratio variance was greater than its binomial expectation, while it did not differ from binomial in P. pini. A large proportion of broods contained only sons or only daughters, especially in A. arvicola. An excess of male-only broods is expected if constrained females (which can produce only sons) contribute offspring prior to mating. The number of male-only broods, however, did not differ from the number expected if all females are mated and allocate offspring sexes binomially, except in P. pini. In the latter species, broods with two daughters (as opposed to two sons) exceeded binomial expectations. We propose that P. pini is largely outbreeding, while the strongly female-biased sex ratio in A. arvicola and L. hirticornis is consistent with partial sibmating and LMC. Ant-parasitoid interactions could account for a different population mating structure in the three parasitoid species. Both A. arvicola and L. hirticornis mimic the epicuticular hydrocarbon pattern of their aphid hosts. Eclosing wasps are ignored by honeydew-collecting ants and hence can mate and forage on the natal patch. In contrast, P. pini generally depart the mummy area to avoid attacks by trophobiotic ants and mate off patch.     

Differential allocation in a lekking bird: females lay larger eggs and are more likely to have male chicks when they mate with less related males

The differential allocation hypothesis predicts increased investment in offspring when females mate with high-quality males. Few studies have tested whether investment varies with mate relatedness, despite evidence that non-additive gene action influences mate and offspring genetic quality. We tested whether female lekking lance-tailed manakins (Chiroxiphia lanceolata) adjust offspring sex and egg volume in response to mate attractiveness (annual reproductive success, ARS), heterozygosity and relatedness. Across 968 offspring, the probability of being male decreased with increasing parental relatedness but not father ARS or heterozygosity. This correlation tended to diminish with increasing lay-date. Across 162 offspring, egg volume correlated negatively with parental relatedness and varied with lay-date, but was unrelated to father ARS or heterozygosity. Offspring sex and egg size were unrelated to maternal age. Comparisons of maternal half-siblings in broods with no mortality produced similar results, indicating differential allocation rather than covariation between female quality and relatedness or sex-specific inbreeding depression in survival. As males suffer greater inbreeding depression, overproducing females after mating with related males may reduce fitness costs of inbreeding in a system with no inbreeding avoidance, while biasing the sex of outbred offspring towards males may maximize fitness via increased mating success of outbred sons.     

寄生蜂性别分配行为

寄生蜂是性比分配行为领域的研究热点对象,其性别决定方式为单双倍型,一般情况下,未受精的单倍型卵发育成雄蜂,受精的二倍型卵发育为雌蜂。局部配偶竞争和近交等因素使得偏雌性比成为这类生物的进化稳定策略;其性比具有可调节性,产卵个体可以根据对产卵环境的判定来调控后代性比,从而获得最大适合度。在此基础上形成的局部配偶竞争理论阐述了寄生蜂性比的这种可调节性,成为进化论的优秀论据。     

Spatial distribution patterns of three fig wasps on Ficus semicordata: How non-pollinators affect pollinator’s sex ratio

Sex ratio theory is one of the most productive fields in research on evolutionary biology. Pollinating fig wasps, due to their particular natural life history, are considered to be a valuable model for the study of sex ratio evolution. A great deal of research concerning the factors that affect pollinator fig wasp (Agaonidae) progeny sex ratio has been done, and at present three main factors (haplodiploidy, local mate competition and inbreeding) are found to be important at the population level. However, there still exists variation between empirical data and model predictions. Another factor to which little thought has been given before is the effect of non-pollinating fig wasps (NPFWs) which parasitize in the larvae gall of pollinator thus kill pollinators and exploit the fig/fig pollinator mutualistic systems. In this study, we focus on why and how non-pollinating fig wasps distort pollinator fig wasp’s original sex ratio. Through controlling the number of ovipositing foundresses inside a fig, combined with the observation of ovipositing behavior and sequence, we studied three species of wasp in the figs of a dioecious fig Ficus semicordata including the pollinator Ceratosolen gravely and NPFWs Platyneura cunia, Sycoscapter trifemmensis in tropical area of Xishuangbanna from September to December 2009. First, we observed the timing of oviposition of all fig wasps utilizing F. semicordata and found differences when compared to previous studies. Such as P. cunia is the fourth rather then the secondary fig wasps to oviposit on the syconia approximately 10 days after the pollinator. S. trifemmensis oviposits much earlier than previously thought, 14–32 days after the pollinators. We examined the spatial location of male and female progeny of the pollinator. We found foundresses of pollinator prefer to use innermost ovules first. Only at high offspring numbers were the outer ovules used. More male pollinator offspring were developed near the fig cavity, while female pollinator offspring were more evenly distributed among ovule layers. As pollinator offspring numbers increased, this phenomenon became more pronounced. This pattern of segregation of male larvae gall in inner ovules and female larvae gall in outer ovules suggests that female offspring might be more vulnerable to attack by parasitic wasps that oviposit from outside the syconium. Experiments later demonstrated that NPFWs are restricted by their ovipositor length and they prefer to or can only lay their eggs into ovules near the fig wall. Then we examined the spatial location of NPFWs and compared this with the spatial location of male/female progeny of pollinator. NPFWs had a high probability of parasitizing female pollinator larvae. Thus, NPFWs have a substantial effect on the sex ratio of the pollinator, as parasitism risk decreases towards the center of the syconium, where inner ovules provide enemy-free space for most of male pollinator offspring. Partial correlation analyse shows that sex ratio of pollinator progeny has a positive relationship with the number of NPFWs. We suggest that the resulting gradient in offspring viability between male and female contributes to selection on pollinators’ for a less femalebiased sex ratio. When the affect of NPFWs was excluded, the pollinator sex ratio was not in good agreement with local mate competition theory, although it was still female-biased. In addition, the average number of offspring per foundress decreased with increasing foundress number, but pollinator sex ratio was positively related to brood size. Thus, pollinator females do not appear to adjust their sex ratio to foundress density directly, but use brood size and foundress density simultaneously as cues to assess potential LMC.     

More highly female-biased sex ratio in the fig wasp,Blastophaga nipponica Grandi (Agaonidae)

The sex ratio of the pollinator fig wasp,Blastophaga nipponica Grandi (Agaonidae), was examined in an experiment manipulating the number of foundresses. The sex ratio ofB. nipponica was conditional on the number of foundresses and corresponded to the qualitative prediction of the local mate competition (LMC) theory that the proportion of males increases as foundress number increases. However, the sex ratio ofB. nipponica was consistently more female-biased than predicted by extended LMC theories that incorporated effects of inbreeding, and these deviations were statistically significant. Plausible factors that would make predictions more female-biased are discussed.     

Egg load is a cue for offspring sex ratio adjustment in a fig‐pollinating wasp with male‐eggs‐first sex allocation

Fig‐pollinating wasps (Agaonidae) only reproduce within fig tree inflorescences (figs). Agaonid offspring sex ratios are usually female‐biased and often concur with local mate competition theory (LMC). LMC predicts less female‐bias when several foundresses reproduce in a fig due to reduced relatedness among intra‐sexually competing male offspring. Clutch size, the offspring produced by each foundress, is a strong predictor of agaonid sex ratios and correlates negatively with foundress number. However, clutch size variation can result from several processes including egg load (eggs within a foundress), competition among foundresses and oviposition site limitation, each of which can be used as a sex allocation cue. We introduced into individual Ficus racemosa figs single Ceratosolen fusciceps foundresses and allowed each to oviposit from zero to five hours thus variably reducing their eggs‐loads and then introduced each wasp individually into a second fig. Offspring sex ratio (proportion males) in second figs correlated negatively with clutch size, with males produced even in very small clutches. Ceratosolen fusciceps lay mainly male eggs first and then female eggs. Our results demonstrate that foundresses do not generally lay or attempt to lay a ‘fixed’ number of males, but do ‘reset to zero’ their sex allocation strategy on entering a second fig. With decreasing clutch size, gall failure increased, probably due to reduced pollen. We conclude that C. fusciceps foundresses can use their own egg loads as a cue to facultatively adjust their offspring sex ratios and that foundresses may also produce more ‘insurance’ males when they can predict increasing rates of offspring mortality.     

Factors affecting female-biased sex ratio in a trap-nesting wasp, Trypoxylon malaisei

We examined the female-biased sex ratio of a trap-nesting wasp Trypoxylon malaisei considering the following factors: (1) local mate competition (LMC), (2) resource quality, (3) partial bivoltinism, and (4) presence of constrained females. The sex ratio (expressed as male ratio) at emergence was strongly female biased, i.e., 0.30 and 0.19, in terms of the number and investment, respectively. To evaluate the primary sex ratio, we analyzed the data from nests where all the offspring successfully emerged, excluding nests composed of single-sex offspring. The primary sex ratio was also female biased, at 0.33 and 0.21, in terms of the number and investment, respectively. LMC was highly responsible for the female-biased sex ratio because both the nonrandom oviposition sequence [females at inner cells and male(s) at outer cells] and earlier emergence of males allowed sib-matings to occur. In contrast, the other three factors little affected the female-biased sex ratio: the sex ratio was fairly constant when resource quality (nest size) varied, partial bivoltinism was extremely rare or absent, and constrained females were absent or did not reproduce at all. Received: June 19, 1998 / Accepted: January 18, 1999     

Family planning in a stemborer parasitoid: sex ratio, brood size and size-fitness relationships in Parallorhogas pyralophagus (Hymenoptera: Braconidae), and implications for biological control

Various aspects were studied of the brood size and sex allocation strategies, and of size-fitness relationships in Parallorhogas pyralophagus (Marsh), a gregarious ectoparasitoid of Eoreuma loftini Dyar. Brood size was significantly correlated with host size; larger hosts were allocated larger broods. Brood sex ratios were fixed precisely at 1 male per 4 females, and eggs were likely to be deposited in that order; differential mortality did not contribute to this precise sex ratio. The sex allocation strategy of P. pyralophagus is likely to conform to strict, i.e. single foundress, local mate competition. Adoption of this strategy is probably influenced by a limited insemination capacity of males; a smaller proportion of females (0.09 vs. 0.21) remained virgin in broods with precise or higher sex ratios (> or = 0.20 males) relative to broods with lower than precise sex ratios (< 0.20 males). Moreover, all females were inseminated in most broods (60%) with precise or higher sex ratios, whereas this did not occur in broods with lower than precise sex ratios. The hypothesized occurrence of strict local mate competition in P. pyralophagus was supported also by observations that: (i) offspring brood sex ratios were independent of maternal brood sex ratios and number of parental females concurrently allocating offspring to a group of hosts, and; (ii) the rate of superparasitism under no-choice conditions was low (approximately 20%), suggesting that rates of outbreeding in the field are low. Other results suggested that fitness in P. pyralophagus was correlated with adult size; longevity and reproductive capacity both increased with adult size in males and females. However, adult size may be more important for females than for males because the differences in reproductive capacity between the largest and smallest individuals was up to 7.3 times greater in females versus < 2 times in males.     

A Comparative Study of Sex Ratio and Clutch Size in Gregarious Ichneumonoid Wasps

Some convincing support for sex ratio theory comes from the cross-species relationship between sex ratio and brood size in gregarious bethylid wasps (Hymenoptera: Bethylidae), in which the proportion males declines as brood size increases as predicted under local mate competition. It is unknown how widely such relationships hold within parasitoid wasps as a whole. We assemble a dataset on sex ratio and brood size for gregarious Braconidae and Ichneumonidae. Their sex ratios deviate substantially from those of bethylids; sex ratios differ widely across species; and they are not significantly related to brood size across species. Several factors explain the heterogeneity in sex ratios including across-species differences in mating system, sex determining mechanism, and sexual asymmetries in larval competition and polyembryony leading to single-sexed broods.     

Female-biased sex allocation in wild populations of the eriosomatine aphid Prociphilus oriens: local mate competition or transgenerational effects of maternal investment?

Several aphid species exhibit female-biased sex allocation. Local mate competition (LMC) has been postulated to be the evolutionary factor of the female-biased sex allocation. We estimated individual sex allocation in the eriosomatine aphid Prociphilus oriens and explained the observed pattern of sex allocation based on a hypothesis other than LMC. On the basis of the relationship between maternal body size and brood size, we estimated the cost of producing a female to be 1.85 times the cost of producing a male. The population-wide allocation to males was 22–24 %. Winged mothers exhibited a large variation in the number of male and female embryos they had, including 23–30 % of winged mothers producing only female embryos. There was polymorphism in the sex-ratio expression. Thus, the constant male hypothesis assuming LMC was not supported. Winged mothers that produced an all-female brood contained larger female embryos than did mothers that produced a bisexual brood. Previous studies have indicated that a large sexual female produces a single large egg, which hatches into a first-instar larva containing a larger amount of gonads. Thus, in eriosomatine aphids, maternal investment in daughters directly affects the potential fecundity of granddaughters, whereas investment in sons does not. We propose a hypothesis that higher fitness returns from maternal investment in daughters than in sons may have primarily led to the evolution of highly female-biased sex allocation in P. oriens.     

Effects of time-dependent competition for oviposition sites on clutch sizes and offspring sex ratios in a fig wasp

Fig wasps have been known as one of the best-documented examples of female-biased sex ratio predicted from the local mate competition (LMC) theory. However, observed sex ratios appear more female-biased than predicted. Before a close match between theory and observation can be claimed, the number and sex ratio of offspring left by each foundress in a multi-foundress syconium need to be determined. We examined the clutch size and sex ratio of individual females of the pollinator fig wasp Blastophaga nipponica (Agaonidae) in experiments using a pair of fertile and sterile females in which sequence and time interval of entering syconia were manipulated. To determine the number and sex ratio of offspring left by each foundress in a multi-foundress syconium, we prepared sterilized females that could oviposit ordinarily but whose offspring could not develop at all, by irradiating the females with ⁶⁰Co gamma rays. Female fig wasps contributed different numbers and sex ratios of offspring to the total brood within a syconium, due to different entry times among them. The variation in clutch sizes with different entry times appeared to be caused by competition for oviposition sites, and sex ratios to be adjusted according to the clutch size.