Roots, shoots and reproduction: sexual dimorphism in size and costs of reproductive allocation in an annual herb

Females tend to be smaller than males in woody dioecious plant species, but they tend to be larger in herbs. The smaller size of females in woody species has been attributed to higher reproductive costs, yet no satisfactory explanation has been provided for their larger size in herbs. Because herbs have higher nitrogen concentrations in their tissues than woody plants, and because pollen is particularly rich in nitrogen, we predicted that male growth would be more compromised by reproduction than female growth. To test this hypothesis, we conducted three experiments on the annual dioecious herb Mercurialis annua. First, we compared the timing of reproduction between males and females and found that males started flowering earlier than females; early flowering is expected to compromise growth more than later flowering. Second, we compared plants allowed to flower with those prevented from flowering by experimental debudding and found that males incurred a higher reproductive cost than females in terms of both biomass and, particularly, nitrogen. Third, we grew plants under varying levels of nitrogen availability and found that although sexual size dimorphism was unaffected by nitrogen, females, but not males, decreased their relative allocation to both roots and reproduction under high nitrogen availability. We propose that males deal with the high cost of pollen production in terms of nitrogen by allocating biomass to nitrogen-harvesting roots, whereas females pay for carbon-rich seeds and fruits by investing in photosynthetic organs. Sexual dimorphism would thus seem to be the outcome of allocation to above- versus below-ground sinks that supply resources (carbon versus nitrogen) limiting the female and male reproduction differentially.     

Reproduction of an inbred strain of Culex pipiens prevented by loss of Wolbachia pipientis

The development time for an autogenous strain of Culex pipiens was prolonged after the mosquitoes were “cured” of their symbiotic Wolbachia pipients with tetracycline. Eggs produced by aposymbiotic females caged with either aposymbiotic or normal males failed to hatch. Neither growth rate nor reproduction was improved by feeding the larvae a commensal flagellate, bovine blood cells, or adenylic acid.     

Effect of sex ratio,habitat factors and neighborhood competition on stem growth in the dioecious tree Fraxinus mandshurica

Growth rates of male and female trees are often different in a dioecious species. In this study, we analyzed sex ratios and the effect of gender, neighborhood competition and habitat factors on the stem growth of dioecious Fraxinus mandshurica trees in a secondary conifer and broad-leaved mixed forest in the Changbai Mountains of northeastern China. The sex ratio in the 5.2-ha study area does not deviate significantly from the expected 1:1 ratio, except for trees in the large diameter classes. For dbh >40 cm, the sex ratio is male-biased. This result suggests that males have a faster rate of stem growth than females, which is usually explained by the higher cost of reproduction in the fruit-bearing females. An analysis of the dbh distributions of two successive measurements showed that the rate of stem growth of the (27) females drops off with increasing dbh and remains below that of the (35) males. A causal model was used to analyze factors affecting the rate of stem growth, showing that these rates are affected significantly and positively by soil moisture and tree size in both genders and that within-gender competition is mainly for nitrogen. Our study suggests that neighborhood competition does not affect stem growth significantly, which is a rather surprising result.     

Growth,plant quality and leaf damage patterns in a dioecious tree species: is gender important?

Frequently, female plants allocate more resources to reproductive structures and defense-related secondary compounds in comparison with male plants that invest more resources to growth, reflecting trade-offs between reproduction, growth and defense. Therefore, differences in herbivory can be expected between genders. In this study, over two years, we analyzed the differences in plant chemical defense, nutritional quality, plant size and herbivory between genders in the dioecious tree, Spondias purpurea in a Mexican tropical dry forest. We estimated the total leaf area and the area consumed by folivory using a digital image of each leaf. The nutritional quality was estimated as water content, and the concentration of chlorophyll and total nonstructural carbohydrates. The secondary metabolites analyzed were total content of soluble phenolics, flavonoids, protein precipitation capacity of tannins, gallotannins, soluble proanthocyanidins, hydrolyzable tannins and ellagitannins. Our results differ from most of studies that analyze the differential herbivory patterns in dioecious plants. We found that female trees had higher levels of herbivory than male trees of S. purpurea. In the same way, female trees showed higher size and nutritional quality than males, while chemical defense was higher in male trees. The higher percentage of folivory in female trees of S. purpurea is associated with greater nutritional quality and lower chemical defenses. Our results show that male-biased herbivory might not be universal in dioecious species. Therefore, studies of fitness components affected by herbivory are necessary to understand the evolution of dioecy and the importance of herbivores as selective agents on breeding system features.     

Lonely hearts advertisements reflect sexually dimorphic mating strategies

Lonely hearts personal advertisements (LHPA) became popular during the 1980s and now appear in nearly every major newspaper. They appear to reflect common male and female reproductive themes. Our analyses of 49 advertisements written by males and 49 advertisements written by females indicate that males offer resources to females and ask for youth and attractiveness, and that females offer youth and attractiveness and ask for resources. When subjects judge these advertisements on a 5-point scale, advertisements are easily grouped into three levels of attractiveness. The attributes of preferred advertisements are defined by those things offered not those things sought. Words or phrases extracted from these advertisements are readily categorized by subjects along a 5-point dimension of desirability. Males and females generally agree on the degree of preference for these 105 words or phrases (r = 0.94), yet differ in degree of preference on 39. Words or phrases preferred by females focus on commitment (e.g., “loving,” “monogamous,” “unattached”). Those preferred by males focus on sexual qualities (e.g., “good figure,” “sexy,” “young”). Individuals of both sexes who indicate a high level of self-confidence prefer words or phrases indicating adventuresome and outgoing qualities. Lack of self-confidence is related to preference for inward-directed qualities. When advertisements are artificially constructed from these words or short phrases, the rating of the advertisements corresponds to the desirability of the individual words. A factor analysis of the words reveals three major factors: (1) words that males prefer; (2) words that females prefer; (3) words that neither males nor females prefer. More highly rated words appear in Factors 1 and 2 than in Factor 3. A survey of 91 lonely hearts advertisement writers demonstrate the same sex differences in what individuals seek and what they offer. Males seek attractivity and offer resources; females seek resources and offer attractivity. After the numerous responses are categorized, only about eight categories for solicitations and eight categories of offers are evident. Interests in resources and attractivity prevail and show sexual dimorphism. Interests in the six remaining categories are nearly identical for the two sexes. Males receive fewer responses to their advertisements than do females. Lengthy advertisements do better for males and shorter ones do better for females. LHPA appear to reflect sexual differences in reproductive concerns. They offer an obvious entry into the motivational systems underlying sexual interactions.     

Sex and tree rings: Females neither grow less nor are less water-use efficient than males in four dioecious tree species

Females of woody dioecious species usually expend more resources on reproduction than males. Therefore, it is expected that females incur greater costs of reproduction than males, and, as a result, trade-offs between reproduction and growth should emerge. The aim of this study is to test those hypotheses by analyzing the differences between genders regarding radial growth (basal area increment) and wood carbon isotope composition (δ¹³C), a proxy of water-use efficiency. We compared these two variables in males and females of four dioecious tree species inhabiting drought-prone Mediterranean sites in Spain (Pistacia terebinthus, Ilex aquifolium, Juniperus thurifera and Ailanthus altissima). We analyzed the influence of sex on the radial growth patterns throughout the tree life considering the growth stage of individuals, the differences in the response of genders to climate variables (air temperature, precipitation and drought severity), and the δ¹³C during a severe drought period. One site was studied for each species and 21–33 trees per species were sampled in each site. No differences in growth were found between genders for any of the four species throughout their life span. No significant interactions between gender and precipitation were found, although A. altissima males were more responsive to summer (June-July) temperature. No differences in δ¹³C were found between genders excepting for P. terebinthus, indicating that the males of this species show a less efficient water use during drought events than the females. These results do not support the broad assumption that females of woody dioecious plants show lower growth and are less water-use efficient than males or that they respond differently to precipitation variability, except for P. terebinthus during drought events. Further analyses could be performed in other dioecious species inhabiting seasonally dry regions to confirm or reject our conclusions.     

Sex ratios and clonal growth in dioecious Populus euphratica Oliv., Xinjiang Prov., Western China

Using a microsatellite assay, we investigated sex ratios at three levels (apparent, intrinsic, genet) for Populus euphratica stands in Xinjiang, China and possible consequences of sex-specific costs of reproduction in terms of clonal growth and individual growth or mortality. Sex ratios at all levels tended to be male biased (60 % of 3,295 flowering trees were male), although male excess was least pronounced at the genet level (52 % of 850 genets were male). Male clones comprised significantly more (708 vs. 572) trees than female clones. Reproductive investment was measured in terms of carbon (C) and nitrogen (N) contents of male and female reproductive organs: single flowers or fruit capsules, whole inflorescences or infructescences, and whole branches of ca. 2 cm diameter. Male flowers and catkins require less N than female fruits and catkins, but on average only 16 % of female catkins develop into fruits. This changes the measured investment for reproduction at branch level: now male branches spent 3.3 times more N than their female counterparts. This coincides with the annual increment of branches, measured as a possible trade-off for costs of reproduction: female branches needed 2 years less to reach a diameter of 2 cm. We conclude that full fruit set of females would give males a heavy comparative advantage, but frequent abortion of whole infructescences by females seems to be a powerful mechanism to compensate a higher reproductive effort, thus avoiding a pronounced runaway effect by more vigorous clonal growth of male trees over a long time.     

Demographic and spatial analysis of a population of Juniperus oxycedrus L. in an abandoned grassland

A study of the structural, dynamic, and space occupancy characteristics of a prickly juniper (Juniperus oxycedrus) population developed on an abandoned secondary grassland of the Central Apennines (Italy), is presented here. The data were collected from a permanent plot, limited by mixed deciduous woods dominated by Ostrya carpinifolia Scop. by a plantation of Pinus nigra Arnold, and by a forest road, and consisting of 1744 prickly juniper individuals, as well as other pre-forest and forest species. The age and sex distributions of this population were assessed. With these data, the age pyramid was constructed, within which three developmental phases of the population became apparent, as confirmed by a study of the logistic curve of development of the population. Based on an estimate of the intensity function of the spatial distribution of the prickly juniper individuals, the hypothesis of complete randomness has to be rejected. On the other hand, the correlation between sex and population density was significant. The mode of space occupancy of the prickly juniper population was investigated also with respect to the other populations of shrubs and forest species. It was shown to spread out in all directions, following lines of diffusion that could be randomly determined. Finally, the presence, within the plot, of some pasture areas not occupied by prickly juniper may be derived from lack of germination of the fruits, or development of the seedlings owing to unfavourable micro-environmental conditions.     

Electrophysiological and behavioral response of the oriental fruit moth <Emphasis Type="Italic">Grapholita</Emphasis><Emphasis Type="Italic">molesta</Emphasis> (Busck, 1916) (Lepidoptera: Tortricidae) to the volatiles of apple fruits at different stages of development

The oriental fruit moth, Grapholita molesta, occurs in Southern Brazil throughout the year, and migrates from peach to apple orchards. Because moths rely on volatile organic compounds (VOCs) during the host-location process, variations in the emission of these compounds during fruit maturation can influence the time of infestation and preference of the moths for a particular genotype. The aim of this work was to identify VOCs emitted by the apples “Eva” and “Gala” at different stages of development and to determine the behavioral and electrophysiological responses of G. molesta to these compounds. For this purpose, VOCs from immature, maturing, and mature fruits of both cultivars were collected and analyzed by gas chromatography and gas chromatography-mass spectroscopy. The response of the antennae of virgin males and females and mated females to volatiles released by the three fruit stages was registered by gas chromatography coupled to an electroantennography. A dual-choice behavioral test for the different combinations of insect groups and fruit stage was also performed. Amongst the volatiles released by mature fruits, twelve compounds elicited a response. The antennae of the oriental fruit moth did respond to isoamyl hexanoate and α-farnesene emitted by “Eva” maturing fruits. In general, virgin females did not respond to volatiles in olfactometer bioassays and mated females were attracted to volatiles released by mature fruits. Our results show that the variation in the emission during the maturation of fruits can influence the orientation of G. molesta.     

SEXUAL DIMORPHISM IN BIOMASS ALLOCATION AND CLONAL GROWTH OF XANTHOXYLUM AMERICANUM

Reproduction can have a high resource cost. It has been suggested that greater investments in sexual reproduction by female dioecious plants leads to a lower rate of vegetative growth in females than in males. In this study, we investigated sexual dimorphism in biomass allocation and genet growth of the dioecious clonal shrub, northern prickly ash (Xanthoxylum americanum). The allocation of biomass over the course of one growing season to reproductive tissue, leaves, and growth of aboveground first-year wood, was compared in 18 clones growing in fields and six clones in woods in southeastern Wisconsin during 1985 and 1986. In addition, the number of shoots per clone, and weight of nonfirst-year wood (accumulated biomass) above- and below-ground were estimated. In open field sites, male clones allocated more biomass to new wood and less to reproduction than females, although males allocated more to flowers alone. Accordingly, male clones had significantly more shoots and more accumulated biomass both above- and below-ground than female clones. In the woods, where fruit set was near zero, there were few significant differences between male and female clones in either biomass allocation or accumulated biomass. These results support the hypothesis that the high resource investment in fruit production by females reduces their vegetative growth relative to males.     

Body condition but not dietary restriction prolongs lifespan in a semelparous capital breeder

Effects of diet on longevity are complex because acquired resources are shared among growth, reproduction and somatic maintenance. We simplify these axes by examining how dietary restriction and competitive contexts affect longevity using semelparous males of the Australian redback spider (Latrodectus hasselti). Plastic development of L. hasselti males results in trade-offs of body condition against faster development if females are present, facilitating scramble competition. In the absence of females, males develop slowly as high body condition adults, and are better equipped for mate searching. Here we focus on effects of diet and competitive context on body condition and longevity. Although male survival depended on body condition and exercise, contrary to studies in a wide range of taxa, dietary restriction did not increase longevity. However, there was an interactive effect of diet and competitive context on lifespan, because high-diet males reared in the absence of females lived longer than males reared in the presence of females. Thus males near females pay a survival cost of developing rapidly. This shows that life-history trade-offs affected by competitive context can impose longevity costs independent of the direct energy expenditure of searching, courtship, competition or reproduction.     

Sexual and vegetative reproduction in the aboveground part of a dioecious clonal plant, <Emphasis Type="Italic">Dioscorea japonica</Emphasis> (Dioscoreaceae)

In dioecious clonal plants, the reproductive effort required to set seeds will be responsible for the larger investment in sexual reproduction by females. If there will be a trade-off in resource allocation between sexual and clonal reproduction, this differential sexual reproduction will lead to sexual differentiation in the relative amount of clonal reproduction. To test this prediction, we studied differences between the sexes in their phenologies and investments in sexual and vegetative reproduction (clonal reproduction by means of bulbils) with respect to ramet size in a dioecious clonal plant, Dioscorea japonica Thunb. The period of bulbil production overlapped the period during which infructescences developed. Females flowered later, produced heavier inflorescences, and fewer flowers per inflorescence than did males. Regression analysis using the size of the individual plants demonstrated that large females made smaller investments in inflorescences and larger investments in sexual reproduction than did large males. In contrast, females invested fewer resources in vegetative reproduction than did males. However, the total investments in sexual and vegetative reproduction did not differ between the sexes. These results supported our hypothesis on the sexual differentiation in sexual and clonal reproduction.     

Masting uncoupling: mast seeding does not follow all mast flowering episodes in a dioecious juniper tree

Evolutionary selective forces, like predator satiation and pollination efficiency, are acknowledged to be major causes of masting (the variable, periodic and synchronic production of seeds in a population). However, a number of recent studies indicate that resources might also play an important role on shaping masting patterns. Dioecious masting species offer a privileged framework to study the role of resources on masting variation, since male and female plants often experience different reproductive costs and selective pressures. We followed masting and reproductive investment (RI) of the dioecious tree Juniperus thurifera in two populations along 10 years and studied the different response of males and females to experimentally increased water and nutrient availability in a third population. Juniperus thurifera females invested in reproduction three times more resources than males. Such disparity generated different resource‐use strategies in male and female trees. Tree‐ring growth and water use efficiency (WUE) confirmed that sexes differed in their resource investment temporal pattern, with males using current resources for reproduction and females using resources accumulated during longer periods. Watered and fertilized female trees presented significantly higher flowering reproductive investments than males and experienced an extraordinary mast‐flowering event. However, seeding RI and mast seeding were not affected by the treatment. This suggests that although resource availability affects the reproductive output of this species, there are other major forces regulating masting on J. thurifera. During 10 years, J. thurifera male and female trees presented high and low flowering years more or less synchronously. However, not all mast flowering episodes resulted in mast seeding, leading to masting uncoupling between flowering and seeding. Since flowering costs represent only 1% of females’ total reproductive investments, masting uncoupling could be a beneficial bet‐hedging strategy to maximize reproductive output in spite of unpredictable catastrophic events.     

Sexual dimorphism in resource acquisition and deployment: both size and timing matter

Background and Aims

The males and females of many dioecious plant species differ from one another in important life-history traits, such as their size. If male and female reproductive functions draw on different resources, for example, one should expect males and females to display different allocation strategies as they grow. Importantly, these strategies may differ not only between the two sexes, but also between plants of different age and therefore size. Results are presented from an experiment that asks whether males and females of Mercurialis annua, an annual plant with indeterminate growth, differ over time in their allocation of two potentially limiting resources (carbon and nitrogen) to vegetative (below- and above-ground) and reproductive tissues.

Methods

Comparisons were made of the temporal patterns of biomass allocation to shoots, roots and reproduction and the nitrogen content in the leaves between the sexes of M. annua by harvesting plants of each sex after growth over different periods of time.

Key Results and Conclusions

Males and females differed in their temporal patterns of allocation. Males allocated more to reproduction than females at early stages, but this trend was reversed at later stages. Importantly, males allocated proportionally more of their biomass towards roots at later stages, but the roots of females were larger in absolute terms. The study points to the important role played by both the timing of resource deployment and the relative versus absolute sizes of the sinks and sources in sexual dimorphism of an annual plant.     

Nutrient deficiency promotes male-biased apparent sex ratios at the ramet level in the dioecious plant <Emphasis Type="Italic">Myrica gale</Emphasis> var. <Emphasis Type="Italic">tomentosa</Emphasis> in oligotrophic environments in bogs

In populations of dioecious plants, the differences in the cost of reproduction between male and female plants can promote a male-biased sex ratio. In this study, we examine the macronutrient levels in tissues of the dioecious wetland shrub Myrica gale to identify the cost of reproduction for male and female plants and to examine the effect of nutrients on the apparent sex ratio at the ramet level. We examined plants across 12 populations of M. gale inhabiting bogs and fens in Japan. For each population, we used line transects to estimate the apparent sex ratio and measured the concentrations of nitrogen (N), phosphorus (P), and potassium (K) in the leaves sampled from male and female plants and in the fruits from female plants. For five of the populations, we calculated the flowering frequency, mortality, and the recruitment rate (as the rate of clonal propagation). We found that the proportion of females was positively affected, and the male bias of sex ratios reduced, by increases in P concentration in leaves sampled from female plants. Neither mortality nor recruitment was affected by sex or by the nutrient concentration (P, K). The flowering frequency was not affected by sex or by K concentration, but decreased with decreases in the P concentration measured in leaves. This study confirmed that reproduction in M. gale is P-limited. We found no distinct differences in the flowering frequency, mortality, or recruitment rate between the male and female plants.     

The potential for disruptive selection on growth rates across genetically influenced alternative reproductive tactics

A trade-off between survival to sexual maturity and mating success is common across alternative reproductive tactics (ARTs), and can lead to tactical disruptive selection on shared traits (i.e. positive selection gradient in one tactic, and negative selection gradient in another). We were interested in examining the theoretical possibility of tactical disruptive selection on intrinsic growth rate. The male ARTs in Xiphophorus multilineatus express two distinct life histories: “courters” optimize mating success by maturing later at larger size and coaxing females to mate, while “sneakers” optimize survival to sexual maturity by maturing earlier at a smaller size, using both coaxing and coercive mating behaviors. In addition to differences in mating behaviors, body length, body depth, and the pigment pattern vertical bars, courter males grow faster than sneaker males. We present a new hypothesis for differences in growth rates between genetically influenced ARTs. The “growth-maturity optimization” hypothesis suggests that ARTs with differences in the probability of surviving to sexual maturity may have different optimal growth rates, leading to tactical disruptive selection. We also present a simple model to suggest that when considering both a cost and benefit to faster growth, tactical disruptive selection on growth rates is theoretically possible. In our model, the value that determines when tactical disruptive selection on growth rate will occur is the increase in probability of survival to sexual maturity gained through faster growth multiplied by the cost of faster growth (reduced longevity). Finally, we present empirical evidence to support the prediction that faster growth has a cost in X. multilineatus: in a controlled laboratory setting, courter males that did not survive 1.2 years past sexual maturity grew faster as juveniles (14–70 days) than those that survived.     

Sex ratio and sexual dimorphism in mountain dioecious thuriferous juniper (Juniperus thurifera L., Cupressaceae)

Thuriferous juniper ( Juniperus thurifera L), a dioecious bush or tree is only found in isolated parts of the western Mediterranean: France, Spain, Algeria and Morocco. These mountain juniper stands are seriously endangered in Morocco as a result of intensive wood removal, and in Europe as a result of recolonization of stands by pines or oaks. Field studies were conducted to investigate sex ratio and sexual dimorphism, never previously examined, in eight different populations in the Atlas mountains and, for comparison, in one of two populations in the French Pyrenees. The sex ratio was female-biased for six of the eight Moroccan stands and especially for the oldest populations. The Pyrenean population showed a similar female-biased ratio. This particular sex ratio is possibly linked to cost of reproduction, paid by both males and females. Sex ratios can also be linked to population dynamics. Males begin to flower slightly younger than females, which explains their apparent dominance in young populations in Morocco or in a recolonization zone with young trees in the Pyrenees. Studies concerning sexual dimorphism in the western High Atlas sites showed no significant difference in phytomass between males and females. Females appear to be significantly taller but with a lower radial growth. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 138 , 237–244.     

Sex ratio and sexual dimorphism in the dioecious Borderea pyrenaica (Dioscoreaceae)

Sex ratio and sexual dimorphism of Borderea pyrenaica, a long-lived dioecious geophyte endemic to the Pyrenees (north-east Iberian Peninsula), were examined in three alpine populations. In this species, age can be estimated and the sex of nonreproductive adult plants identified. Male plants attain sexual maturity earlier, flower more frequently and grow faster than female plants, whereas females allocate a higher biomass to reproduction than males. These results support the hypothesis that female plants incur a higher cost of sexual reproduction and that this higher cost is measurable as reduced vegetative growth and lower flowering frequency. Variation of sex ratio among young, intermediate and old adults within populations suggests, however, that this higher female reproductive investment does not result in sexual differences in mortality. The overall male-biased sex ratio in B. pyrenaica is mainly a consequence of the tendency of males to reproduce at an earlier age and more frequently than females.     

GENDER VARIATION IN A RED MAPLE POPULATION (ACER RUBRUM; ACERACEAE): A SEVEN-YEAR STUDY OF A “POLYGAMODIOECIOUS” SPECIES

Red maple trees have been described as having a complex breeding system, categorized as “polygamodioecious.” However, the seven-year, quantitative investigation of 79 trees presented here shows that the breeding system is relatively simple, using the gender equations of Lloyd (1980). Out of the entire maple population, 55 individuals are constant males while 4 individuals are inconstant males that on occasion produce some female inflorescences. Twelve individuals are constant females and 6 individuals are inconstant females that on occasion produce some male inflorescences. Only 2 of the plants have a functional gender which fluctuates dramatically in successive years between maleness and femaleness. It might be predicted that maple plants may specialize in one gender at a small size and switch over to produce both pollen and ovules at a larger size. However, these data show that plants of inconstant gender are not larger than plants of constant gender. Further, the inconstant plants actually appear to have a lower mean fitness than plants of constant gender. These results suggest that inconstancy in red maple gender does not increase fitness and may be an example of an imperfectly developed canalization process.     

Sexual dimorphism and intraspecific aggression,and their relationship to sex ratios in Caprella gorgonia Laubitz & Lewbel (Crustacea: Amphipoda: Caprellidae)

The amphipod Caprella gorgonia Laubitz & Lewbel is an obligate commensal on gorgonian octocorals. Its primary host is Lophogorgia chilensis (Verrill), found below 20 m.C. gorgonia breeds throughout the year, with wide fluctuations in abundance. Mating and oviposition follow molting. Sex reversal does not occur; two distinct sexes are present from the first instar after emergence from the brood pouch.Young males and females grow at approximately the same rate, but males are larger by a relatively constant increment. Males continue to grow at their original rate to a maximum size (about twice that of females). The growth rate of females is not limited by the onset of reproduction and brooding, but rather by an approach to maximum size when the rate is greatly reduced. Fecundity of females is not affected by size.The population sex ratio is about 1:3 (males:females), and about 1:4 among adults. The secondary sex ratio is 1:1. The post-emergence sex ratio bias is a result of heavier mortality among males. Sex ratios drop from 50% at emergence to 25% as females approach maximum size, then rise to 100% in larger size classes.Differential predation on males did not appear to be a source of any sex ratio bias. Adult males possess a “poison spine”, a puncturing weapon on the large second gnathopod, which functions in mating-related intraspecific combat with other males. Intraspecific male aggression during mating is a major cause of sex ratio bias. In the laboratory, increased density in breeding groups may affect mortality due to male aggression. In nature, adult sex ratios are negatively correlated with population density. The reproductive capacity of the population is not limited by a shortage of adult males, despite the low adult sex ratio.