An evolutionary explanation for seasonal trends in avian sex ratios

We present an extensive set of data for five species of raptorialbirds to demonstrate that some raptor species produce an excessof daughters early in the season and an excess of sons in latenests, while others show the reverse. By means of a simulationmodel we investigate an evolutionary explanation for this phenomenonin terms of sex-specific differences in the relation betweenage at first breeding and date of birth. The model predictsthat that gender should be produced first in the season whoseage of first breeding is more strongly accelerated by an earlybirth date. We argue that this tends to be the male gender inraptor species, such as the common kestrel (Falco tinnunculus),which tend to breed early in life, while it is the female genderin larger species with later onset of breeding, such as themarsh harrier (Circus aeruginosus). The empirical evidence isqualitatively consistent with this hypothesis. Our model isquite general in that it makes no assumptions about the mechanism(primary sex-ratio bias at egg laying or secondary sex-differentialmortality before fledging) by which the bias is generated. Yetit is able to create quantitative predictions for species wheresufficient demographic and life-history data are available.From the available data set in the common kestrel we derivea quantitative prediction for the seasonal trend in brood sexratio. The observed trend is in good agreement with this prediction.[BehavEcol 7: 426–430 (1996)]     

Secular trends in twinning rates

Based on national data from civil birth registration systems, this paper provides an overview of trends in the twinning birth rates in fifteen developed countries. Patterns and differentials in such rates across populations are described, and trends over time are given. The evolution of the twinning rate in industrialized countries over the last century follows broadly the same pattern. One exception is the period around World War I, with a peak in the twinning rate being observed during the war, or just after it, although this was not registered in all countries. Since the mid-1970s twinning rates have increased in many developed countries in response to a growing use of fertility-stimulating treatments such as in vitro fertilization.     

Secular trends in some Russian populations

Secular changes of body measurements in children have been the subject of studies in many different countries. In recent years, there has been an increase in BMI associated with a significant trend towards obesity in both Europe and the US. The aim of the present study was to analyze trends in body measurements and BMI in Russia from the 1960's to the beginning of the 21st century. This was done at three locations of the Russian Federation: the city of Moscow, the cities of Saratov and Naberezhnye Chelny in the Volga-river area. In addition, data on secular changes of Abkhazian children were analyzed. A large number of anthropometric measurements were taken on each individual including height, weight, arm, leg and trunk lengths (estimated), body diameters and circumferences, skinfold thickness, head and face dimensions. Stages of secondary sex characteristics also were evaluated; data on menarcheal age were collected by status-quo and retrospective methods. Changes in hand grip strength have been evaluated in some of the samples. While stature was increasing during these years, weight, chest circumference and BMI were characterized by negative changes, which became more obvious in elder girls. Changes in handgrip strength also showed negative trends. There were noticeable changes in head and face measurements, which were expressed in more elongated head and face forms, i.e. the head became longer and narrower with narrower and higher faces. Secular changes in head and facial morphology may be considered part of the general trend.     

Secular trends of demographic parameters

The aim of this paper is to describe the secular changes of selected demographic parameters and to investigate the possible causes for such changes as well as the biological relevance. We took into account the following parameters: population density, mean live expectancy, average age at marriage of until then unmarried persons, rate of live births, and number of children per woman. The results show that the population density (global and in Germany) especially in the twentieth century increased dramatically. We found a striking increase of life expectancy in Germany. Essential causes are rapid decreases in infant and maternal mortality. Since the 80s of the twentieth century the average age at marriage of until then unmarried persons as well as the number of single mothers show a permanent increase. Generally, the average age of mothers increased (for live and legitimate births). In the past 150 years we found a decrease in fertility rates in Germany. The registered demographic parameters show temporal and regional variations. These differences, especially between East Germany and West Germany, are emphasized.     

Secular trends of twinning rate in Sweden.

The change in parity and maternal age in Sweden accounts for the decline in the twinning rate from the 19th century to the middle of the 20th century. The later decline in twinning rate cannot be explained by changes in maternal age and parity. Fecundity appears to be higher in mothers of twins than in other mothers.     

Declining sex ratios in Canada

OBJECTIVE: To examine the trends in the proportion of annual live births that were male in Canada and to compare the trends with those in the United States. DESIGN: Analysis of census data. SETTING: Canada as a whole and 4 main regions (West, Ontario, Quebec and Atlantic). SUBJECTS: All live births from 1930 to 1990. OUTCOME MEASURES: Sex ratio (expressed as the proportion of total live births that were male [male proportion]) overall and by region. RESULTS: The male proportion in Canada decreased significantly after 1970 (p < 0.001); this represented a cumulative loss of 2.2 male births per 1000 live births from 1970 to 1990. Although a decrease was observed in all four regions studied, only that in the Atlantic region was significant (p < 0.001), representing a cumulative loss of 5.6 male births per 1000 live births from 1970 to 1990. A significant decrease in the male proportion was also observed in the United States from 1970 to 1990 (p < 0.001), although to a lesser degree than that observed in Canada, and represented a cumulative loss of 1.0 male births per 1000 live births. CONCLUSIONS: The decreased sex ratio in Canada adds to the growing debate over changes in biological markers and their potential causes. In addition, the study illustrates the potential use of the sex ratio as a widely available, unambiguous measure of the reproductive health of large populations.     

Evolutionarily stable sex ratios and sex allocations

The paternal fitness of a sexual individual is equated with the fitness of those eggs of its potential mates which it is able to fertilize. This property enables the total sexual fitness of individuals to be expressed in terms of female gamete contributions in separate equations for a cosex (an individual in a population composed of a single sexual class which combines male and female functions) and for parents in a dioecious population. The general equations are used in phenotypic models of selection which examine conditions maximizing the fitness advantage of one phenotype over another with a different sex ratio or allocation. As an example, it is shown that finite population size confers full stability on the sexual allocations in a cosexual population and on the sex ratio in a dioecious population.The use of fitness advantages provides the outcome of selection for all frequencies of contrasted phenotypes. It is therefore possible to redefine an ESS to allow for persistent variability in a population. A phenotype is an ESS in a population if, from any initial frequency, it is protected from loss by its fitness advantage. The conditions for a rare mutant to spread invariably coincide with those for its fixation only if an individual of any phenotype affects the fitness of other individuals of all phenotypes in identical ways.     

A snail with unbiased population sex ratios but highly biased brood sex ratios

Extraordinary sex ratio patterns and the underlying sex-determining mechanisms in various organisms are worth investigating, particularly because they shed light on adaptive sex-ratio adjustment. Here, we report an extremely large variation in the brood sex ratio in the freshwater snail, Pomacea canaliculata. In eight rearing series originating from three wild populations, sex ratios were highly variable among broods, ranging continuously from almost exclusively males to almost exclusively females. However, sex ratios were similar between broods from the same mating pair, indicating that sex ratio is a family trait. Irrespective of the large variations, the average sex ratios in all rearing series were not significantly different from 0.5. We argue that Fisher's adaptive sex-ratio theory can explain the equal average sex ratios, and the results, in turn, directly support Fisher's theory. Polyfactorial sex determination (in which sex is determined by three or more genetic factors) is suggested as the most likely mechanism producing the variable brood sex ratio.     

Optimal sex ratios in structured populations

In this paper, we develop a general method to determine evolutionary equilibrium sex ratios and to check evolutionary stability, continuous stability and invadability in exact genetic models with or without dominance. This method is then applied to three kinds of models for structured populations: the first one concerns Hamilton's LMC model, except that only a fraction beta of female offspring mate with male offspring born in the same colonies, while a fraction 1-beta mate with male offspring chosen at random within the whole population; in the second model, it is assumed that partial dispersal of inseminated females occurs after mating; in the third model, partial dispersal of male and female offspring occurs before mating. In the first model, the effect of population regulation is studied while, in the other models, two kinds of dispersal are considered: proportional and uniform.     

Problems with primate sex ratios

Birth sex ratios of baboons in Gombe National Park, Tanzania, show an overall male bias of ca. 20%, but there is no obvious explanation for this trend. Individual females did not alter their sex ratios according to persistent levels of local resource competition. Sex ratios showed an unexpected relationship between age and rank: subordinate females had more sons when they were young; dominant females had more sons when they were old. The sex ratio of low-ranking females also varied with the severity of environmental conditions during pregnancy. Our findings suggest that mammalian sex ratios might be the product of more complex processes than is generally recognized or that sex-determining mechanisms impose sufficient constraints to prevent adaptive variation in all contexts.     

Inbreeding and parasite sex ratios

The breeding system of parasitic protozoa affects the evolution of drug resistance and virulence, and is relevant to disease diagnosis and the development of chemo- and immunotherapy. A major group of protozoan parasites, the phylum Apicomplexa, that includes the aetiological agents of malaria, toxoplasmosis and coccidiosis, all have dimorphic sexual stages. The sex ratio (proportion of males produced by parasites) is predicted to depend upon the inbreeding rate, and it has been suggested that sex-ratio data offer a relatively cheap and easy method for indirectly estimating inbreeding rates. Here, we exploit a new theoretical machinery to show that there are generally valid relationships between f, Wright's coefficient of inbreeding, and sex ratio, z(*), the generality being with respect to population structure. To focus the discussion, we concentrate on malaria and show that the previously derived result, f = 1 - 2z(*), does not depend on the artificial assumptions about population structure that were previously made. Not only does this justify the use of sex ratio as an indirect measure of f, but also we argue that it may actually be preferable to measure f by measuring sex ratios, rather than by measuring departures from Hardy-Weinberg genotypic proportions both in malaria and parasites more generally.     

Adult sex ratios in wild bird populations

Offspring sex ratios in wild bird populations, and the extent to which they vary from the equality expected by random genotypic sex determination, have received much recent attention. Adult sex ratios (ASRs) in wild birds, on the other hand, remain very poorly described, and many of the questions about them posed by Ernst Mayr in 1939 remain unanswered. This review assesses population-level sex ratio patterns in wild bird populations, with an emphasis on the ASR. A quantitative assessment of over 200 published estimates of ASR, covering species from a wide range of taxa, regions and habitats, supported Mayr's assertion that skewed ASRs are common in wild bird populations. On average, males outnumbered females by around 33%, and 65% of published estimates differed significantly from equality. In contrast, population-level estimates of offspring sex ratio in birds did not generally differ from equality, and mean ASR across a range of wild mammal species was strongly female-skewed. ASR distortion in birds was significantly more severe in populations of globally threatened species than in non-threatened species, a previously undescribed pattern that has profound implications for their monitoring and conservation. Higher female mortality, rather than skewed offspring sex ratio, is the main driver of male-skewed ASRs in birds, and the causes and implications of this are reviewed. While estimates of ASR in wild bird populations may be subject to a number of biases, which are discussed, there is currently no quantitative evidence that an ASR of one male to one female represents the norm in birds. A better understanding and reporting of ASRs in wild bird populations could contribute greatly to our understanding of population processes and could contribute much to theoretical and applied research and conservation.     

Uniform female-biased sex ratios in alpine willows

? Premise of the study: The development of biased sex ratios in dioecious plant species has been ascribed to either (1) factors influencing differential adult mortality of male and female plants or (2) factors acting at an early life stage that determine seed sex ratio or seedling survival. ? Methods: To discriminate between these two competing hypotheses, we surveyed sex and age of 379 individuals from five species of the genus Salix across 11 alpine valleys in the southwest Yukon. ? Key results: We observed uniformly female-biased sex ratios of approximately 2:1 across all adult age cohorts and patch sizes of the five willow species. No spatial variation in sex ratio occurred that could be associated with site-specific characteristics such as elevation or aspect. ? Conclusions: Our results indicate that the female-biased sex ratios in the alpine willow species investigated in this study are not a consequence of ecological processes acting on established adult plants. The sex ratio is instead determined at an early life stage by a mechanism that remains unknown.