Extreme sex ratio variation in relation to change in condition around conception

Adaptive theory predicts that mothers would be advantaged by adjusting the sex ratio of their offspring in relation to their offspring's future reproductive success. Studies investigating sex ratio variation in mammals have produced notoriously inconsistent results, although recent studies suggest more consistency if sex ratio variation is related to maternal condition at conception, potentially mediated by changes in circulating glucose level. Consequently, we hypothesized that change in condition might better predict sex ratio variation than condition per se. Here, we investigate sex ratio variation in feral horses (Equus caballus), where sex ratio variation was previously shown to be related to maternal condition at conception. We used condition measures before and after conception to measure the change in condition around conception in individual mothers. The relationship with sex ratio was substantially more extreme than previously reported: 3% of females losing condition gave birth to a son, whereas 80% of those females that were gaining condition gave birth to a son. Change in condition is more predictive of sex ratio than actual condition, supporting previous studies, and shows the most extreme variation in mammals ever reported.     

Maternal control of offspring sex and male morphology in the Otitesella fig wasps

Models concerning the evolution of alternative mating tactics commonly assume that individuals determine their own strategies. Here we develop a computer-based ESS model that allows mothers, ovipositing in discrete patches, to choose both the sex and the male mating tactics (natal-patch mating or dispersing) of their offspring based only on how many other mothers have used the specific patch before them. Data for three species of nonpollinating fig wasps from the Otitesella genus agree quantitatively with the model's assumptions and predictions. This suggests that females respond to population densities at the level of individual figs. The alternative male tactics in the species we studied are probably a result of a conditional strategy exercised by the mother that laid them. In addition, as females were only allowed to lay one egg per patch, our results suggest a new mechanism that can skew population sex ratios towards a female bias.     

The variations of human sex ratio at birth during and after wars, and their potential explanations

Data on wartime sex ratios (proportions male at birth) are reviewed. Two sorts of variation are empirically well supported viz. (a) rises during and just after both World Wars and (b) a fall in Iran during the Iran-Iraq War. Potential explanations are offered here for these rises and fall. The fall seems plausibly explained by psychological stress causing pregnant women disproportionately to abort male fetuses. The rises may be explained by either or both of two different forms of hypothesis viz. (i) Kanazawa's “returning soldier” hypothesis and (ii) variation in coital rates. The coital rate hypothesis potentially accounts, in slightly different ways, for the rises both during, and just after, some wars. The argument that coital rate affects sex ratio just after wars seems to be supported by evidence that in some combatant countries, dizygotic (DZ) twinning rates (which also reportedly vary with coital rate) peaked after the World Wars. The suggestion that war is associated with rises in sex ratio at birth was first made more than two centuries ago. However, I have been unable to locate direct supporting sex ratio data relating to any conflict before World War One. So it would be useful if historical demographers were to search for such data relating to these earlier wars.     

Offspring allocation in externally ovipositing fig wasps with varying clutch size and sex ratio

The simultaneous optimization of clutch size and sex ratio isa tricky problem. Unless parameters such as host size or fecundityexist to pin down the optimal clutch size, this problem remainselusive to analytical analysis. This is because the fitnesslandscape with respect to clutch size and sex ratio does nothave one single evolutionarily stable peak toward which thepopulation can evolve. To solve this problem, I used a computeremulation to optimize both clutch size and sex ratio using externallyovipositing fig wasps as a model taxon. The simulation approachallows the use of integer numbers of eggs rather than assumingthat females can produce any sex ratio between 0 and 1. Whenfemales have no information about the patches on which theyoviposit, they produce either large clutches with a strong femalebias or clutches of a single male egg. When females have completeknowledge of their oviposition site, a set of conditional substrategiesis evolutionarily stable. Again, these substrategies are eitherlarge clutches with a female bias or dutches consisting of asingle male egg. This dichotomous oviposition pattern resultsin unrelated males sharing a fig, a condition conducive to theevolution of fatal fighting. Selection on female ovipositionstrategies may therefore be an important driving force behindhigh levels of fighting observed between male fig wasps.     

Egg sex ratio and paternal traits: using within-individual comparisons

Empirical studies of sex ratios in birds have been limited dueto difficulties in determining offspring sex. Since molecularsexing techniques removed this constraint, the last 5 yearshas seen a great increase in studies of clutch sex ratio manipulationby female birds. Typically these studies investigate variationin clutch sex ratios across individuals in relation to environmentalcharacteristics or parental traits, and often they find no relationships. In this study we also found that clutch sex ratiosdid not vary in relation to a number of biological and environmentalfactors for 238 great tit Parus major nests. However, interestingsex ratio biases were revealed when variation in clutch sexratios was analyzed within individual females breeding in successiveyears. There was a significant positive relationship betweenthe change in sex ratio of a female's clutch from one yearto the next and the relative body condition of her partner.Females mating with males of higher body condition in yearx + 1 produced relatively male-biased sex ratios, and the oppositewas true for females mated with lower condition males. Within-individualanalysis also allowed investigations of sex ratio in relationto partner change. There was no change in sex ratios of femalespairing with the same male; however, females pairing with anew male produced clutches significantly more female biased. Comparisons of clutch sex ratios within individuals may be apowerful method for detecting sex ratio variation, and perhapsfemale birds may indeed manipulate egg sex but require personalcontextual experience for such decisions.     

Skewed sex ratios and female homosexual activity in Japanese macaques: An experimental analysis

In this paper we present the results of a behavioral experiment conducted to test whether homosexual consortships and sexual solicitations among female Japanese macaques (Macaca fuscata) increase in the context of operational sex ratios that are heavily skewed towards females. The study involved a baseline period of observation on an intact social group which had a female-biased sex ratio typical of this species. During the experimental period which followed, we created a sub-group with an operational sex ratio that was heavily skewed towards females. Compared to the baseline period, females solicited significantly more same-sex individuals for sex and formed significantly more homosexual consortships during the experimental period of the study. Females did not appear to engage in homosexual activity during the study's experimental period simply because they lacked heterosexual alternatives. Instead, we suggest that an abundance of certain types of preferred, same-sex sexual partners and/or a scarcity of opposite-sex sexual competitors best account for the increased levels of female homosexual behavior observed at this time.     

Hatching sex ratio and sex specific chick mortality in common terns Sterna hirundo

Bias in sex ratios at hatching and sex specific post hatching mortality in size dimorphic species has been frequently detected, and is usually skewed towards the production and survival of the smaller sex. Since common terns Sterna hirundo show a limited sexual size dimorphism, with males being only about 1–6% larger than females in a few measurements, we would expect to find small or no differences in production and survival of sons and daughters. To test this prediction, we carried out a 2-year observational study on sex ratio variation in common terns at hatching and on sex specific post hatching mortality. Sons and daughters hatched from eggs of similar volume. Post hatching mortality was heavily influenced by hatching sequence. In addition, we detected a sex specific mortality bias towards sons. Overall, hatching sex ratio and sex specific mortality resulted in fledging sex ratios 8% biased towards females. Thus, other reasons than body size may be influencing the costs of rearing sons. Son mortality was not homogeneous between brood sizes, but greater for two-chick broods. Since adults rearing two-chick broods were younger, lighter and bred consistently later than those rearing three-chick broods, it is suggested that lower capacity of two-chick brood parents adversely affected offspring survival of sons. Though not significantly, two-chick broods tended to be female biased at hatching, perhaps to counteract the greater male-biased nestling mortality. Thus, population bias in secondary sex ratio is not limited to strongly size dimorphic species, but species with a slight sexual size dimorphism can also show sex ratio bias through a combination of differential production and mortality of sons and daughters.     

Hypothetical role of men's sexual activity in the regulation of human sex ratio: Analysis of the sex ratio of post-war abandoned children

The regulation of the sex ratio at birth in human species remains poorly understood. After wars, a shift of the sex ratio in favor of men is always observed. Among the different hypothesis to explain this observation, one is to consider that Y-bearing spermatozoids have a weight advantage following insemination and that X-bearing spermatozoids, heavier, are more time-resistant. Following these observations, frequent sex may favor the birth of boys, whether infrequent sex may favor the birth of girls.Sustaining this sperm weight hypothesis, I report here that in France, after the two world wars, there has been an increase of abandoned illegitimate children with a significant shift of the sex ratio in favor of men. These observations may reflect an increase in illegitimate birth and indirectly an increase of men paternity.     

Sex expression and sex dimorphism in sporophytic populations of the desert moss Syntrichia caninervis

Ten sporophytic populations of Syntrichiacaninervis contained an average of 22 individualscm^–2, with a sex ratio of 7.9 Female:1 Male: 3.1 Nonexpressing(N = 300). In each of the populations, female individuals outnumberedmale individuals. A representative population size of 50 × 25cm was estimated to contain 27,250 organically distinct individualplants. Given the wide disparity in reproductive investment in this dioeciousspecies, sex-specific traits were investigated at the level of the individualinbiomass, total stem length, number of ramets, number of branches, length ofannual growth interval, length of longest ramet, age, number of inflorescences,number of ramets expressing sex, and consecutive seasons of sex expression. Theonly significant sex dimorphism recorded was in consecutive seasons of sexexpression, with nonsporophytic female individuals exhibiting a higherfrequencythan males. However, nonexpressing individuals had lower biomass, shorter totalstem length, fewer branches, and shorter ramets than males and females, andfewer ramets than female individuals. When the biomass of female and maleindividuals was controlled for inflorescence number, no significant differencesbetween the sexes in biomass were found. There appears to be a threshold sizefor sex expression, with all individuals above 2.0 mg dry weightexpressing sex (N = 108). Biomass and total stem length are strongpositive correlates (r = 0.88), and individual biomass is a betterpredictor of the number of inflorescences produced per individual than is stemlength (r = 0.85 vs. r = 0.69, respectively). Independent of stemlength, individual biomass had a positive and significant relationship withinflorescence number; however, independent of individual biomass, stem lengthwas not associated with inflorescence number. Overall sex expression was 0.74(individual level) and 0.65 (ramet level). The functional sex ratio wasassessedat the inflorescence level, and ranged from 9.7:1, to14.9:1 over the most recent four years. Fertilization frequencywas 0.69 (individual level) and 0.34 (perichaetial level). Over the last threeyears, 63% of all fertilized perichaetia resulted in an abortivesporophyte.     

Evolutionarily stable strategies in differential and difference equation models

Summary A definition for an evolutionarily stable strategy (ESS) is given which is applicable to a general differential equation population model and two difference equation analogs. With the introduction of a fitnessgenerating function, it is possible to develop necessary conditions for the determination of an ESS for each of these systems. In most situations, an ESS for one system will also be an ESS for the other. Necessary conditions for an ESS are obtained. Under certain restrictions, they are shown to be valid, even under an unstable equilibrium in population density. the results are illustrated with an example which has the same ESS solution whether a continuous or discrete model is used. The behavior of the ESS for the discrete model is then examined under unstable equilibrium conditions in population density.     

Plant sex and hare feeding preferences

Summary To evaluate the general extent to which sex-related differences in palatability occur in boreal dioecious woody plants, males and females of five dioecious woody plant species were presented to free-ranging mountain hares (Lepus timidus) during winter. Hares strongly preferred branches from male plants when feeding on Populus tremula and Salix caprea and weakly preferred male S. pentandra. However, they did not show any sex-related preference when feeding on the other two species studied (Myrica gale and Juniperus communis). Nitrogen concentration and, to some degree, digestibility showed strong relationships with hare food preferences. By contrast, the concentration of phenolics was only weakly related to feeding preference. Phenolics could, nevertheless, still be important if only one or a few specific compounds deter hare feeding. These results indicate that sex-related differences in plant palatability in the boreal forest might be more widespread than previously believed, particularly for species of the family Salicaceae. Thus, herbivores might be responsible for the female-biased sex ratios found in willow populations in northern Scandinavia (e.g. Elmqvist et al. 1988).     

Factors affecting secondary sex ratio in Iranian Holsteins

The objective of this study was to evaluate the factors affecting secondary sex ratio (SSR) in Iranian Holsteins. Data of 942,941 Holstein calving events from the Animal Breeding Center of Iran, recorded between January 1996 and December 2007, were used in the analysis. A multivariable logistic regression model was used to model the logit of the probability of a male calf being born. Male births accounted for 49.6% of the total observations. The ratio of males to females varied from 52.5:47.5 in calving year 1996-1999 (odds ratio (OR) = 1.18; P < 0.0001), to 48.5:51.5 in calving year 2004-2007. The greatest occurrence of male births was observed in spring (OR = 1.02; P < 0.0001), and the lowest incidence of male births was for summer or fall calvings. Also, the frequency of male births decreased from parity 1 to parity 4 and beyond (P < 0.0001; OR = 1.11). The greatest number of sires had the SSR equal to 0.5 with a minimum SSR of 32% while the maximum was 97%. Among cows that had a male birth, the chance of delivering a male calf again was 25.5% when cows had delivered a male once (OR = 1.14; P < 0.0001), and 12.7% if a male calf was delivered twice by a cow. This indicated that characteristics peculiar to the dam influence the sex of her offspring and suggests some degree of repeatability of calf sex within cows.     

Male-biased sex ratio in litters of Alpine marmots supports the helper repayment hypothesis

In a French population of Alpine marmots (Marmota marmota),the sex ratio at weaning was biased in favor of males. Thisbias also seemed to exist at birth. Under Fisher's equal allocationprinciple, this means that daughters should be more costlyto produce than sons. Because the Alpine marmot can be considereda cooperative breeding species, we investigated whether thedifferential cost between sons and daughters may be explainedby the helper repayment hypothesis. The Alpine marmot usessocial thermoregulation during hibernation, allowing juvenilesto better survive over winter. In the study population, juvenilesurvival during winter increased with group size. More precisely,juvenile survival during winter increased with the number andwith the proportion of subordinate males in the hibernatinggroup, but juvenile survival did not depend on the number of subordinate females. As our results did not support alternativehypotheses to explain the observed bias in sex ratio amongoffspring at emergence, we conclude that the helper repaymenthypothesis is the best candidate to explain the observed offspringsex ratio bias in Alpine marmots. By participating in socialthermoregulation, subordinate males may repay part of the investment they received from their parents and thus become less costlyto produce. We suggest that only subordinate males helped becausethey may gain direct fitness benefits, whereas subordinatefemales may only expect indirect fitness benefits from helping.Finally, the offspring sex ratio per individual parent wasmale biased, but mothers adjusted the size and the sex compositionof their litters according to their phenotypic condition asexpected from the Trivers-Willard hypothesis.     

Hatching order and size-dependent mortality in relation to brood sex ratio composition in chinstrap penguins

The differential environmental sensitivity of the sexes hasstrong implications in the evolutionary history of species asit can alter sexual size dimorphism, population sex ratios,and the faculty of parents to manipulate offspring sex in relationto environmental conditions. We studied sexual differences inhatching patterns and evaluated sex- and size-related mortalityin relation to hatching order and brood sex ratios in the chinstrappenguin Pygoscelis antarctica, a moderately size-dimorphic species,with a modal clutch size of 2 eggs. We found that male, second-hatched,and large eggs showed shorter hatching periods than female,first-hatched, and small eggs. We also found a male-biased mortalityof nestlings in the colony. However, male mortality patternsdiffered depending on the brood sex ratio composition. Mortalityof male chicks in all-male broods was higher than in mixed broodsand higher than female mortality in all-female broods. Contrary,females from mixed brood showed higher mortality than theirmale nest mates and higher too than females in all-female broods.Second-hatched chicks also suffered from higher mortality thanfirst-hatched chicks. Our results indicate that both the superiorcompetitive capacity and the higher energy demand of the largersex constitute 2 causal factors explaining patterns of sex-biasedmortality. Both factors occur in the same species and in differentsituations of sibling competition shaped by brood sex ratiocomposition. This study constitutes a good example of how patternsof sex-related mortality can vary depending on nest environmentalcircumstances. Furthermore, our study suggests that hatchingperiod can be a mechanism underlying sexual differences in theembryonic period of birds.     

Blocks and bodies: sex differences in a novel version of the Mental Rotations Test

A novel version of the Mental Rotations Test (MRT) that alternated the standard block figures with three-dimensional human figures was administered to 99 men and 129 women. Women and men differed predictably in their retrospective reports of childhood play and digit ratios, a putative measure of prenatal androgen action. Compared to the block figure items, human figure items on the modified MRT were associated with an improvement in performance in both sexes. However, consistent with the study hypothesis, the enhancing effect of the human figure condition on performance as measured by conventional scores was smaller in men compared to women and not at all evident in men when performance was measured by ratio scores. A closer inspection of the human figures effects on test scores showed performance in women improved for both male and female figure items. In contrast, relative to scores on block figure items, performance in men improved when stimuli were male figures but did not improve when stimuli were female figures. These results add to the evidence that the magnitude of sex differences in scores on the MRT may vary according to the test content and item properties. The findings suggest that online measures of cognitive processing in response to different classes of test stimuli (e.g., animate vs. inanimate objects, self-relevant vs. neutral stimuli) may prove useful in research aimed at understanding the hormonal and social factors contributing to the sex difference in performance on the MRT.     

Size distributions and sex ratios of colonizing lizards

This paper reports the body size distributions and sex ratios of four species of phyrnosomatid lizard that colonized experimentally created density sinks. The experiments were conducted in western Colorado in 1992, and lizards colonizing the habitats in 1993 were compared to those removed in 1992 and those present in 1991. Lizards colonizing the density sinks were able to disperse from adjacent habitat. For two of the species (Urosaurus ornatus and Sceloporus undulatus), colonizing lizards were significantly smaller than either those removed in 1992 or those present in 1991. Two other species (S. graciosus and Uta stansburiana) showed no difference in the size distribution of colonizing and removed lizards. In addition, sex ratios of colonizing lizards did not differ from those removed in 1992 or present in 1991. The results of the experiments have implications for the dynamics of the target populations, rescue of local populations from extinction, the regional persistence of populations subject to turnover in patchy environments, and priority effects in colonization. Received: 3 February 1997 / Accepted: 20 May 1998     

Evidence of genetic and maternal effects on secondary sex ratio in cattle

There is a paucity of estimates of genetic variation for secondary sex ratio (i.e., sex ratio at birth) in dairy cattle. The objective of this study was to estimate the direct and maternal genetic variance as well as maternal permanent environmental variance for offspring sex in dairy herds. The data consisted of 77,508 births from 61,963 dams and 2,859 sires in 1,369 Irish dairy herds across the years 2003 to 2008, inclusive. Mixed models were used to estimate all parameters. Significant genetic variation in sex ratio existed, with a heritability for secondary sex ratio estimated at 0.02; the genetic standard deviation was 0.07 percentage units. No maternal genetic effects on secondary sex ratio were identified but the proportion of phenotypic variance in secondary sex ratio attributable to maternal permanent environmental effects was similar to that attributable to the additive genetic variance (i.e., 0.02). These results, therefore, suggest that the paternal (genetic) influence on secondary sex ratio is just as large as the maternal (non-genetic) influence, both of which are biologically substantial. The results from this study will be useful in generating a sample population of divergent animals for inclusion in a controlled experiment to elucidate the physiological mechanism underpinning differences in secondary sex ratio.     

Do females adjust the sex of their offspring in relation to the breeding sex ratio?

When the adult sex ratio differs between years in local populations, but still is predictable between adjacent years, it has been proposed that the best strategy would be to bias the offspring sex ratio in favour of the rare sex. We tested this hypothesis using a data set of great reed warbler offspring, sexed by molecular techniques, that were collected over 11 breeding seasons at two adjacent reed marshes. Three important assumptions for this hypothesis are fulfilled in the studied great reed warbler population. First, a substantial proportion of great reed warblers are living in small local populations where sex ratio distortions would be sufficiently large and common. Second, breeding adults and their offspring return to breed in the local population to a high degree. Third, females have a possibility to assess the breeding sex ratio before laying their eggs. At our study site, the breeding sex ratio was positively correlated between successive years. However, contrary to our prediction, female great reed warblers seemed not to adjust their offspring sex ratio in relation to the local breeding sex ratio.     

Strong but variable associations between social dominance and clutch sex ratio in a colonial corvid

We studied primary sex ratio of clutches in relation to socialdominance for 6 years in a colony of free-living jackdaws, asmall corvid. Social dominance was strongly associated withclutch sex ratio, with the difference in clutch sex ratio betweenthe most and least dominant pairs being 30–40%. To ourknowledge, this is the first demonstration of an associationbetween social dominance and sex allocation in birds. However,the direction of this effect varied between years. Dominantjackdaws produced more sons during the first years of the studybut fewer sons during the last years. Offspring sex was notrelated to laying order within a clutch, and the effect of socialdominance on sex ratio was similar on eggs laid first, middle,or last. We investigated the effect of 2 factors (laying dateand parental condition) that could have mediated the shift inthe effect of social dominance on sex allocation in the courseof the study. Laying date was positively associated with theproportion of males, but this effect was independent of socialdominance. Maternal condition (residual mass over tarsus andegg volume) was related to social dominance but not to clutchsex ratio. Paternal condition (residual mass over tarsus) wasnot related to clutch sex ratio. We discuss how spatial or temporalvariation in effects of variables such as social dominance onsex allocation can contribute to our understanding of the evolutionof sex allocation in species with complex life histories.