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We assess the causes of adult sex ratio skew in marine pelagic copepods by examining changes in these ratios between the juveniles and adults, sexual differences in juvenile stage durations, and mortality rates of adults in the field and laboratory (when free from predators). In the field, late copepodite stages (CIV and CV) commonly have sex ratios that are either not significantly different from equity (1 : 1), or slightly male biased. By contrast, in adults, these ratios are commonly significantly biased toward female dominance. Sex ratio skews are therefore primarily attributable to processes in adults. Members of the non-Diaptomoidea have especially skewed adult ratios; in the members Oithonidae and Clausocalanidae this is not generated from differences between male and female adult physiological longevity (i.e., laboratory longevity when free of predators). In the genera Acartia, Oithona, and Pseudocalanus, we estimate that predation mortality contributed [greater than or equal to] 69% of the field mortality rate in adult males, whereas in Acartia, Oithona, and Calanus adult females, this is [greater than or equal to] 36%. We conclude that (1) adult sex ratio skew in pelagic copepods is primarily due to differential mortality of the sexes in the adult stage and not in juveniles, (2) mortality rates of adult Acartia, Pseudocalanus, and Oithona are dominated by predation mortality rather than physiological longevity (except under extreme food limitation), and (3) in Pseudocalanus and Oithona, elevated mortality rates in adult males to females is predominantly due to higher predation on males. Our work demonstrates that we now need to develop a more comprehensive understanding of the importance of feeding preferences in predators. doi: 10.4319/lo.2010.55.5.2193

The proximity of mates can influence mating opportunities and the quantity and quality of offspring, especially in dioecious plant species. Progeny sex ratios modulated by environmental conditions is one of the most radical ways in which offspring quality may be influenced, yet it has rarely been reported in plants. A mechanism proposed to influence progeny sex ratios in dioecious plants involves competition between female- and male-determining microgametophytes (certation) as a result of variation in pollination intensity. However, the role of selective fertilization in dioecious plants is controversial and has not been demonstrated under field conditions. Here we investigate whether natural variation in the spatial arrangement of females and males influences pollination intensity and progeny sex ratios in the wind-pollinated herb Rumex nivalis. Based on previous experimental manipulation of pollination intensity in this species, we predicted that maternal parents in close proximity to males would produce more strongly female-biased progeny sex ratios. We tested this prediction in six alpine populations in Switzerland by measuring the distance between focal females and neighboring males and assessing pollen loads and seed sex ratios of maternal parents. In four of the six populations, females positioned in close proximity to males captured more pollen and exhibited more female-biased sex ratios. Our results demonstrate that demographic aspects of the maternal mating environment can influence progeny sex ratios. The most probable explanation for biased primary sex ratios in Rumex is selective fertilization resulting from pollen tube competition. female-biased sex ratios | pollination intensity | selective fertilization

Alpine ibex (Capra ibex) are polygynous ungulates that exhibit extreme sexual dimorphism and segregation. To test the hypothesis that risk of predation plays a key role in the development of sexual segregation (habitat segregation) in this species, compositions and locations of groups of ibex were recorded from February 2003 to October 2004. Individual fixes of marked adult males and females in the Gran Paradiso National Park in Italy were collected monthly over a 4-year period (January 2000--December 2003). Distances were measured between each location and the nearest refuge area (rocky slopes), and between each location and the nearest source of disturbance (roads and hiking trails). Group size was not related to distances from refuge areas or from sources of disturbance, but sex, age, and weaning played a key role in spatial segregation. During the period of rut, females kept nearer to rocky slopes and further from hiking trails than males. The presence of young also influenced the spatial behavior of females: mothers made use of refuge areas more frequently and of areas near hiking trails less frequently than other females. In contrast, no difference between the spatial behaviors of pregnant and nonpregnant females was recorded in spring. Age played an important role in space use by males, but not by females. Young males (2-5 years) stayed closer to rocks than did middle-aged males (6-11 years), and both stayed closer to rocks than did adult males (> 11 years). Adult males stayed closer to hiking trails than middle-aged males; likewise, the latter stayed closer to hiking trails than young males. Males stayed near areas with human presence, likely because of good foraging opportunities. Although our findings do not exclude other explanations, they support the reproductive strategy-predation risk hypothesis. Moreover, predation risk was shown as an important factor leading to both sexual and age segregation in males. Key words: alpine ibex, antipredator behavior, Capra ibex, predation risk, sexual segregation

The sex ratio of post-emergence offspring of the European badger Meles meles in Wytham Woods, Oxfordshire, UK did not differ significantly from 50% between 1987 and 2001. Insofar as male offspring are more costly to rear successfully, mothers optimized cub productivity with regard to the level of resources that they could invest. When females had a relatively high index of body condition, they implanted early; when implantation was early, the cub sex ratio was male biased. Additionally, years with a male-biased cub sex ratio coincided with a significantly larger cub cohort. These results do not support the local resource competition hypothesis, which proposes that in years when female body condition is poor, females should lower competition for local resources by producing the dispersing sex, i.e., males. The potential mechanisms by which offspring sex ratios may be skewed are discussed. Overall, it appears that bias in the cub sex ratio may result from a combination of adaptive parental strategies, differential infanticide by individuals other than the mother, and differential fetal mortality between the sexes. Key words: body condition; delayed implantation; European badger; local resource competition; Meles meles; offspring sex ratio; parental investment; sex differences in fitness; sex differences in mortatity; Wytham Woods.

Using molecular-genetic techniques, we determined the gender of 202 Southwestern Willow Flycatcher (Empidonax traillii extimus) nestlings from 95 nests sampled over a five-year period. Overall nestling sex ratio did not vary significantly from 50:50 among years, by clutch order, or by mating strategy (monogamous vs. polygamous pairings). However, we did observe significant differences among the four sites sampled, with sex ratios biased either toward males or females at the different sites. Given the small population sizes and geographic isolation of many of the endangered subspecies' breeding populations, sex-ratio differences may have localized negative impacts. Key words: Empidonax traillii, endangered species, nestling sex ratio, Willow Flycatcher.

Using museum records, we examined sex ratios for 32 collections of eastern red bats (Lasiurus borealis) in the eastern United States. Results from logistic regression analyses indicate that female-dominated sex ratios are associated with warmer, mean high temperatures in June. Male-dominated collections, and those with approximately even sex ratios, occurred disproportionately in the Appalachian Highlands where monthly mean high temperatures were lower than in either the Central Lowlands-Interior Low Plateaus or Coastal Plain-Piedmont regions. Efforts to maximize conservation of summer roosting and foraging habitat that favors the female population segment of eastern red bats should be directed at areas where June mean high temperatures exceed 28.5 C.

Research points out that female nest-site philopatry is sex-specific, culturally inherited in reptiles, influenced by environmental sex determination, and exhibits highly female-biased sex rations. Data indicate that these attributes facilitate the maternal manipulation of offspring sex ratios in favor of females.

Research into male-killing, bacteriophage infection, nematode infections, and virulence of Wolbachia cytoplasmic bacteria is reviewed. The dynamics of embryonic male-killing are reviewed. Arthropod host-Wolbachia phylogenies are not congruent, which suggests horizontal transmission. The opposite has been shown in nematode-Wolbachia phylogenies, which indicates long-term association and vertical transmission. Horizontal transmission of the phage, which is extensive, may explain the plasticity of Wolbachia's effect on arthropod hosts. Wolbachia variation in reproductive effects would likely indicate a high degree of evolutionary plasticity.

Sex chromosome meiotic drive, which is the unequal transmission of X and Y chromosomes from individuals of the heterogametic sex, is discussed. Mean fitness in a population may be reduced when the population has driving sex chromosomes. Other possible results include intragenomic conflict between the X chromosome, the Y, and the autosomes. The intensity or mode of sexual selection may be changed. Sex chromosome drive could be an agent of species-level selection. Sex chromosome drive, or its genetic equivalent, occurs in mammals, plants, and flies.

Research is presented on plant gynodioecy, or the reproduction structure in which hermaphrodites and females co-occur, and the influence of plant population dynamics on plant sex differentiation. It is posited that the evolution of sex ratio reflects a species' metapopulation context; in the case of test species Silene vulgaris, sex ratio results from its existence as a weedy species that has modified itself to a disturbed habitat.

Those who claim that the male/female ratio has been decreasing because of endocrine disruptors from environmental pollution are mistaken. Changes in the male/female ratios are not related to environmental pollution. An examination of the data shows that air pollution was increasing during the 1940s, 1950s and early 1960s when male/female ratio was high. Air pollution declined since 1970 which was the time when the sex ratio started to decrease.

Adaptive sex ratios among the eggs of Seychelles warblers, Acrocephalus sechellensis, were determined via randomly amplified polymorphic DNA (RAPD)analysis of warbler broods. RAPD analysis of Seychelles warbler offspring from bird populations located in high-quality and low-quality environments indicated the presence of adaptive sex ratios that are dependent on the availability of food. Warblers in low-quality environments produce more male offspring than females to reduce competition on limited resources.

Human-environment interactions can affect the sex ratios of resource-dependent societies in a variety of ways. Historical and contemporary data on Alaska Native populations illustrate such effects. Some eighteenth and early nineteenth century observers noted an excess of females, which they attributed to high mortality among hunters. Population counts in the later nineteenth century and well into the twentieth found instead an excess of men in many communities. Female infanticide was credited as the explanation: since family survival depended upon hunting success, males were more valued. Although infanticide explanations for the excess of males have been widely believed, available demographic data point to something else: higher adult female mortality. Finally, in the postwar years, the importance of mortality differentials seems to have faded - and also changed direction. Female outmigration from villages accounts for much of the gender imbalance among Native populations today. Natural-resource development, particularly North Slope oil, indirectly drives this migration. In Alaska's transcultural communities, the present gender imbalances raise issues of individual and cultural survival.

Females of the southern green stinkbug, Nezara viridula (Hemiptera: Pentatomidae), initiating copulation in a female-biased environment produced relatively more sons than females initiating copulation in a male-biased environment. Although families suffering greater mortality yielded more sons, the difference in offspring sex ratio between treatments was not due to differential mortality by sex since the distribution of family size did not vary between treatments. Female condition, indexed by female size and size of the first egg mass laid, did not vary between treatments and, therefore, apparently did not contribute to the results. Thus, it appears that the stinkbug is capable of facultative sex ratio adjustment in response to the operational sex ratio, increasing the production of sons when males are rare. In nature, overlapping generations and female-biased operational sex ratios may occur; conditions under which selection for sex ratio adjustment is most intense.

Bias in the adult sex ratio of a population, if not due to a biased primary sex ratio, may be due to differential mortality of males and females, which can occur during any life history stage. Yet, although adult sex-ratio bias is common in dioecious plant species, little is known about the demography of prereproductive individuals. Using a RAPD-PCR (randomly amplified polymorphic DNA) marker that co-segregates with female phenotype, I studied juvenile demography in the saltmarsh grass Distichlis spicata, which exhibits within-population sex-ratio variation (spatial segregation of the sexes) and has genetic sex determination. First, I surveyed populations of D. spicata to ascertain whether any environmental factor co-varies with local sex ratio, and I found that habitats with female majorities are located at significantly lower elevations in the marsh than are habitats with male majorities. In randomly collected seeds, the sex ratio was significantly biased toward females, but male and female seeds did not differ in mass. Using a subsample of these seeds, I conducted greenhouse and field experiments to examine whether differences in seed germination or seedling survival between the sexes explain sex-ratio variation along the micro-elevation gradient in the marsh. In greenhouse treatments designed to mimic field habitats with female majorities, more male seeds germinated than did female seeds. In habitats with female majorities in the field, significantly more female seedlings survived an extreme high tide than did male seedlings (25.17% of females vs. 13.49% of males). These results suggest that, in habitats with female majorities, gender-specific bias in seedling survivorship, rather than seed germination, is a causal factor in the underlying sex-ratio bias. In habitats with male majorities, seed germination and seedling survival did not differ between the sexes. This study is the first to document environment-dependent differential seed germination and seedling mortality between males and females in a dioecious plant species. Key words: dioecy; Distichlis spicata; germination; life histories; molecular ecology; RAPD (randomly amplified polymorphic DNA) markers; saltmarsh grass; seedling survival; seed mass; sex ratio; spatial segregation of the sexes.

To test alternative hypotheses concerning life history flexibility in variable environments I focused on the genotypic and environmental components of variation in Sex Ratio and Reproductive Allocation in clones of a benthic modular animal, the bryozoan Celleporella hyalina, grown in different flow regimes through all seasons. Clones of this bryozoan growing in autumn and winter varied significant in growth rate, modular sex ratio, and relative investment in sexual reproduction. Experimental reduction of ambient water flow significantly reduced growth rate but did not affect Reproductive Allocation or Sex Ratio. Clonal differences in Reproductive Allocation were associated with a trade-off between somatic and sexual functions: clones investing more heavily in sex reached a smaller somatic size. In spring and summer, Celleporella clones showed an inconsistent and less pronounced growth response to flow regime. Significant genetically based variation was found for all measured characters except Reproductive Allocation. In both seasonal groups, clonal Sex Ratio was genetically canalized, and insensitive to the experimental treatments. The presence of clonal variability in Reproductive Allocation in autumn/winter but not in spring/summer may arise from seasonal differences in lifespan on natural substrata. These may select for maximal sexual investment in short-lived generations, but allow scope for alternative strategies if potential longevity is greater. The genetic constraint on clonal Sex Ratio may not conflict with models predicting flexible sex expression in variable environments, since the season fluctuations experienced probably do not affect sex-specific reproductive success. The small Genotype x Treatment interactions and the positive correlations between clonal performance in different treatments and seasons, suggest that the relative fitness of clones does not vary, a result at odds with assumptions underlying current hypotheses for the adaptive basis of sexual reproduction.

The Seychelles warbler, Acrocephalus sechellensis, exhibits an adaptive sex-ratio variation that correlates with Fischer's equilibrium sex-ratio theory. Adaptive sex-ratio variation in Acrocephalus sechellensis is affected by the availability of resources or food. Adult warblers in high-quality territories produce more females compared to warblers in low-quality territories. The low numbers of female offspring in low-quality territories will reduce competition for food among warblers.