Violent men have more sons: further evidence for the generalized Trivers-Willard hypothesis (gTWH)

The generalized Trivers-Willard hypothesis (gTWH) [Kanazawa, S., 2005a. Big and tall parents have more sons; further generalizations of the Trivers-Willard hypothesis. J. Theor. Biol. 235, 583-590] proposes that parents who possess any heritable trait which increases the male reproductive success at a greater rate than female reproductive success in a given environment have a higher-than-expected offspring sex ratio, and parents who possess any heritable trait which increases the female reproductive success at a greater rate than male reproductive success in a given environment have a lower-than-expected offspring sex ratio. One heritable trait which increases the reproductive success of sons significantly more than that of daughters in the ancestral environment is the tendency toward violence and aggression. I therefore predict that violent parents have a higher-than-expected offspring sex ratio (more sons). The analysis of both American samples and a British sample demonstrates that battered women, who are mated to violent men, have significantly more sons than daughters.     

Revised evidence for facultative sex ratio adjustment in birds: a correction

We provide a revision to the calculation of effect sizes and heterogeneity statistics in our original article, 'Facultative primary sex ratio variation: a lack of evidence in birds' (Ewen et al. 2004). Our revision shows that significant heterogeneity in sex ratio study effect sizes does indeed exist and that for a series of key traits the average effect sizes (while still weak) are in fact significantly different from zero.     

A simple model for adaptive variation in the sex ratios of mammalian offspring

We present a simple mathematical model that describes how primary and secondary sex ratios of offspring may vary adaptively in order to maintain equal numbers of the sexes at the age of reproductive maturity. The model postulates that the sex of an offspring depends probabilistically on a weighted linear combination of maternal testosterone and male vulnerability. The model operates at population level, and is based on three physiological phenomena: first that maternal testosterone in follicular fluid is normally distributed, with levels above the mean more likely to be associated with the conception of males; secondly, that males are more vulnerable than females from conception onwards; and thirdly that under conditions of chronic stress, increased secretion of female testosterone coincides with increased male vulnerability. Thus during times of chronic stress, more males are conceived, but their number of live births is moderated by increased male loss. Variations in secondary sex ratios should therefore be related not only to the stressfulness of environmental conditions, but also to the timing of changes in stressfulness.     

Facultative adjustment of the offspring sex ratio and male attractiveness: a systematic review and meta‐analysis

Females can benefit from mate choice for male traits (e.g. sexual ornaments or body condition) that reliably signal the effect that mating will have on mean offspring fitness. These male‐derived benefits can be due to material and/or genetic effects. The latter include an increase in the attractiveness, hence likely mating success, of sons. Females can potentially enhance any sex‐biased benefits of mating with certain males by adjusting the offspring sex ratio depending on their mate's phenotype. One hypothesis is that females should produce mainly sons when mating with more attractive or higher quality males. Here we perform a meta‐analysis of the empirical literature that has accumulated to test this hypothesis. The mean effect size was small (r = 0.064–0.095; i.e. explaining <1% of variation in offspring sex ratios) but statistically significant in the predicted direction. It was, however, not robust to correction for an apparent publication bias towards significantly positive results. We also examined the strength of the relationship using different indices of male attractiveness/quality that have been invoked by researchers (ornaments, behavioural displays, female preference scores, body condition, male age, body size, and whether a male is a within‐pair or extra‐pair mate). Only ornamentation and body size significantly predicted the proportion of sons produced. We obtained similar results regardless of whether we ran a standard random‐effects meta‐analysis, or a multi‐level, Bayesian model that included a correction for phylogenetic non‐independence. A moderate proportion of the variance in effect sizes (51.6–56.2%) was due to variation that was not attributable to sampling error (i.e. sample size). Much of this non‐sampling error variance was not attributable to phylogenetic effects or high repeatability of effect sizes among species. It was approximately equally attributable to differences (occurring for unknown reasons) in effect sizes among and within studies (25.3, 22.9% of the total variance). There were no significant effects of year of publication or two aspects of study design (experimental/observational or field/laboratory) on reported effect sizes. We discuss various practical reasons and theoretical arguments as to why small effect sizes should be expected, and why there might be relatively high variation among studies. Currently, there are no species where replicated, experimental studies show that mothers adjust the offspring sex ratio in response to a generally preferred male phenotype. Ultimately, we need more experimental studies that test directly whether females produce more sons when mated to relatively more attractive males, and that provide the requisite evidence that their sons have higher mean fitness than their daughters.     

Complementation hypothesis: the necessity of a monoallelic gene expression mechanism in mammalian development

Gene expression from both parental alleles (biallelic expression) is beneficial in minimizing the occurrence of recessive genetic disorders in diploid organisms. However, imprinted genes in mammals display parent of origin-specific monoallelic expression. As some imprinted genes play essential roles in mammalian development, the reason why mammals adopted the genomic imprinting mechanism has been a mystery since its discovery. In this review, based on the recent studies on imprinted gene regulation we discuss several advantageous features of a monoallelic expression mechanism and the necessity of genomic imprinting in the current mammalian developmental system. We further speculate how the present genomic imprinting system has been established during mammalian evolution by the mechanism of complementation between paternal and maternal genomes under evolutionary pressure predicted by the genetic conflict hypothesis.     

Facultative sex ratio adjustment in response to male tarsus length in the Varied Tit Parus varius

During the last decade, evidence from a number of studies has suggested systematic deviations from a 1 : 1 primary sex ratio in birds, in spite of the fact that birds have chromosomal sex determination systems; the mechanism of sex allocation is not fully understood. However, it still remains uncertain whether adaptive manipulations of primary sex ratio occur, especially in Parus species. We studied sex ratio variation in the Varied Tit Parus varius , which is a socially monogamous species similar to the Great Tit P. major and the Blue Tit P. caeruleus . In total, 362 chicks that hatched from 72 broods over 3 years were sexed. Of all nestlings, 51.9% (188/362) were male. The nestling sex ratio did not differ significantly from unity. However, the proportion of sons in each brood was significantly and positively related to the father's tarsus length. This corresponds with our predictions, given that larger males have higher resource holding potential if tarsus length is a heritable character between fathers and sons.     

Further support for the hypothesis that parental hormone levels around the time of conception are associated with human sex ratios at birth

During the past year, data have been published on the offspring sex ratios of people diagnosed with toxoplasmosis, hepatitis B, and pre- and post-menopausal breast cancer. It is shown here how these offspring sex ratios constitute further support for the hypothesis that mammalian (including human) parental hormone concentrations around the time of conception partially control the sexes of the resulting infants. If this interpretation were correct, then hormonal treatments might be considered for some or all of these conditions. It is intended that anyone who has read the present note and my two previous papers (James, 1996, 2004) should be aware of all the data relating to the hypothesis.     

Sex-specific growth rates in zebra finch nestlings: a possible mechanism for sex ratio adjustment

Wild and captive zebra finches (Taenopygia guttata), like severalother species, produce a male-biased sex ratio at fledging whenfood is scarce. This is due to primary sex-ratio adjustmentand female-biased nestling mortality. Given that young femalesfledging at low body masses have been shown to have low fecundityas adults, lower returns to parents from producing female offspringin conditions of restricted food has been raised as a functionalexplanation (Trivers and Willard's hypothesis of adaptive sexualinvestment; 1973). However, an alternative, mechanistic hypothesisis that under restricted conditions female chicks are more costlyto produce. In consequence, lower returns to parents under theseconditions would happen earlier in the life of female offspringrather than later. To test this hypothesis, I hand-reared chickson a food gradient. In the absence of parent-offspring and sib-sibinteractions, final body mass and growth rates for females werelower in conditions of restricted food. For males, final bodymass and growth rates did not differ with food condition. Lowfemale growth rates in food-restricted conditions might be onepotential mechanism causing female-biased mortality in birds.More importantly, this result is the strongest evidence yetof female offspring experiencing higher marginal fitness benefitsfrom additional food than males and it has implications forprimary and secondary sex-ratio adjustment. Also, as this mechanismhas been shown in the absence of parent-offspring interactions,significant questions can now be raised as to how parental andoffspring behavior interact in their effects on secondary sex-ratioadjustment.     

Marsupials and Eutherians reunited: genetic evidence for the Theria hypothesis of mammalian evolution

The three living monophyletic divisions of Class Mammalia are the Prototheria (monotremes), Metatheria (marsupials), and Eutheria (`placental' mammals). Determining the sister relationships among these three groups is the most fundamental question in mammalian evolution. Phylogenetic comparison of these mammals by either anatomy or mitochondrial DNA has resulted in two conflicting hypotheses, Theria and Marsupionta, and has fueled a ``genes versus morphology' controversy. We have cloned and analyzed a large nuclear gene, the mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R), from representatives of all three mammalian groups, including platypus, echidna, opossum, wallaby, hedgehog, mouse, rat, rabbit, cow, pig, bat, tree shrew, colugo, ringtail lemur, and human. Statistical analysis of this nuclear gene unambiguously supports the morphology-based Theria hypothesis that excludes monotremes from a clade of marsupials and eutherians. The M6P/IGF2R was also able to resolve the finer structure of the eutherian mammalian family tree. In particular, our analyses support sister group relationships between lagomorphs and rodents, and between the primates and Dermoptera. Statistical support for the grouping of the hedgehog with Feruungulata and Chiroptera was also strong. Received: 8 December 2000 / Accepted: 01 February 2001     

The categories of evidence relating to the hypothesis that mammalian sex ratios at birth are causally related to the hormone concentrations of both parents around the time of conception

This note categorizes the evidence for the hypothesis that mammalian offspring sex ratios (proportions male) are causally related to the hormone levels of both parents around the time of conception. Most of the evidence may be acknowledged to be correlational and observational. As such it might be suspected of having been selected; or of having been subject to other forms of bias or confounding; or, at any rate, of being inadequate as a firm basis for causal inference. However, there are other types of evidence that are not vulnerable to these types of criticism. These are from the following sources: (1) previously neglected data from Nazi Germany and Soviet Russia; (2) fulfilled predictions; (3) genetics; and (4) a network of logically (mathematically) related propositions, for some of which there is overwhelming empirical evidence. It is suggested that this variety of evidence confers greater overall credibility on the hypothesis than would be the case if all the evidence were of the same observational/correlational status. This observational/correlational evidence is tabulated to illustrate its consistency.     

The mechanism of sex ratio adjustment in a pollinating fig wasp

Sex ratio strategies in species subject to local mate competition (LMC), and in particular their fit to quantitative theoretical predictions, provide insight into constraints upon adaptation. Pollinating fig wasps are widely used in such studies because their ecology resembles theory assumptions, but the cues used by foundresses to assess potential LMC have not previously been determined. We show that Liporrhopalum tentacularis females (foundresses) use their clutch size as a cue. First, we make use of species ecology (foundresses lay multiple clutches, with second clutches smaller than first) to show that increases in sex ratio in multi-foundress figs occur only when foundresses are oviposition site limited, i.e. that there is no direct response to foundress density. Second, we introduce a novel technique to quantify foundress oviposition sequences and show, consistent with the theoretical predictions concerning clutch size-only strategies, that they produce mainly male offspring at the start of bouts, followed by mostly females interspersed by a few males. We then discuss the implications of our findings for our understanding of the limits of the ability of natural selection to produce 'perfect' organisms, and for our understanding of when different cue use patterns evolve.     

Big and tall parents have more sons: further generalizations of the Trivers-Willard hypothesis

This paper proposes the generalized Trivers-Willard hypothesis (gTWH), which suggests that parents who possess any heritable trait which increases male reproductive success at a greater rate than female reproductive success in a given environment will have a higher-than-expected offspring sex ratio, and parents who possess any heritable trait which increases female reproductive success at a greater rate than male reproductive success in a given environment will have a lower-than-expected offspring sex ratio. Since body size (height and weight) is a highly heritable trait which increases male (but not female) reproductive success, the paper hypothesizes that bigger and taller parents have more sons. The analysis of both surviving children and recent pregnancies among respondents of the National Child Development Survey and the British Cohort Survey largely supports the hypothesis.     

Beautiful parents have more daughters: a further implication of the generalized Trivers-Willard hypothesis (gTWH)

The generalized Trivers-Willard hypothesis (gTWH) [Kanazawa, S., 2005. Big and tall parents have more sons: further generalizations of the Trivers-Willard hypothesis. J. Theor. Biol. 235, 583-590) proposes that parents who possess any heritable trait which increases the male reproductive success at a greater rate than female reproductive success in a given environment will have a higher-than-expected offspring sex ratio, and parents who possess any heritable trait which increases the female reproductive success at a greater rate than male reproductive success in a given environment will have a lower-than-expected offspring sex ratio. One heritable trait which increases the reproductive success of daughters much more than that of sons is physical attractiveness. I therefore predict that physically attractive parents have a lower-than-expected offspring sex ratio (more daughters). Further, if beautiful parents have more daughters and physical attractiveness is heritable, then, over evolutionary history, women should gradually become more attractive than men. The analysis of the National Longitudinal Study of Adolescent Health (Add Health) confirm both of these hypotheses. Very attractive individuals are 26% less likely to have a son, and women are significantly more physically attractive than men in the representative American sample.     

No sex please, we're mitochondria: a hypothesis on the somatic unit of inheritance of mammalian mtDNA

In this article we develop a model for the organization and maintenance of mitochondrial DNA (mtDNA) in mammalian somatic cells, based on the idea that the unit of genetic function comprises a group of mtDNA molecules that are semi-permanently associated as a mitochondrial nucleoid. Different mtDNA molecules within a nucleoid need not be genetically identical. We propose that nucleoids replicate faithfully via a kind of mitochondrial mitosis, generating daughter nucleoids that are identical copies of each other, but which can themselves segregate freely. This model can account for the very slow rates of mitotic segregation observed in cultured, heteroplasmic cell-lines, and also for the apparently poor complementation observed between different mutant mtDNAs co-introduced into rho(0) cells (cells that lack endogenous mtDNA). It also provides a potential system for maintaining the mitochondrial genetic fitness of stem cells in the face of a presumed high somatic mutation rate of mtDNA and many rounds of cell division in the absence of phenotypic selection. BioEssays 22:564-572, 2000.     

Experimental evidence that corticosterone affects offspring sex ratios in quail

Recent studies have shown that some species of birds have a remarkable degree of control over the sex ratio of offspring they produce. However, the mechanism by which they achieve this feat is unknown. Hormones circulating in the breeding female are particularly sensitive to environmental perturbations, and so could provide a mechanism for her to bias the sex ratio of her offspring in favour of the sex that would derive greatest benefit from the prevailing environmental conditions. Here, we present details of an experiment in which we manipulated levels of testosterone, 17beta-oestradiol and corticosterone in breeding female Japanese quail (Coturnix coturnix japonica) using Silastic implants and looked for effects on the sex ratio of offspring produced. Offspring sex ratio in this species was significantly correlated with faecal concentrations of the principal avian stress hormone, corticosterone, and artificially elevated levels of corticosterone resulted in significantly female-biased sex ratios at laying. Varying testosterone and 17beta-oestradiol had no effect on sex ratio alone, and faecal levels of these hormones did not vary in response to corticosterone. Our results suggest that corticosterone may be part of the sex-biasing process in birds.     

Split sex ratios in the social Hymenoptera: a meta-analysis

The study of sex allocation in social Hymenoptera (ants, bees,and wasps) provides an excellent opportunity for testing kin-selectiontheory and studying conflict resolution. A queen–workerconflict over sex allocation is expected because workers aremore related to sisters than to brothers, whereas queens areequally related to daughters and sons. If workers fully controlsex allocation, split sex ratio theory predicts that colonieswith relatively high or low relatedness asymmetry (the relatednessof workers to females divided by the relatedness of workersto males) should specialize in females or males, respectively.We performed a meta-analysis to assess the magnitude of adaptivesex allocation biasing by workers and degree of support forsplit sex ratio theory in the social Hymenoptera. Overall, variationin relatedness asymmetry (due to mate number or queen replacement)and variation in queen number (which also affects relatednessasymmetry in some conditions) explained 20.9% and 5% of thevariance in sex allocation among colonies, respectively. Theseresults show that workers often bias colony sex allocation intheir favor as predicted by split sex ratio theory, even iftheir control is incomplete and a large part of the variationamong colonies has other causes. The explanatory power of splitsex ratio theory was close to that of local mate competitionand local resource competition in the few species of socialHymenoptera where these factors apply. Hence, three of the mostsuccessful theories explaining quantitative variation in sexallocation are based on kin selection.     

Low parasitism rates in parthenogenetic bagworm moths do not support the parasitoid hypothesis for sex

The parasite hypothesis for sex is one of the many theories that have been suggested to solve the mystery of the widespread occurrence of sex despite its high short‐term costs. It suggests that sexual lineages have an evolutionary advantage over parthenogens because they can frequently generate new genotypes that are temporarily less prone to coevolving parasites. In this study, we looked for further supporting evidence for the parasite hypothesis of sex in an attempt to understand the coexistence of sexual and parthenogenetic bagworm moths (Naryciinae). The bagworm moths and their parasitoids form one of the few natural host–parasite systems where sexual and parthenogenetic hosts are apparently not separated by ecological or geographical barriers. Furthermore, in support of the parasite hypothesis for sex, parthenogenetic presence is negatively correlated with parasitism rate. We specifically tested, by identifying the reproductive mode of the parasitized individuals, whether parasitoids preferentially attack the parthenogens in sites with both sexual and parthenogenetic forms, as predicted by the parasite hypothesis. We collected hosts from sites with different frequencies of parthenogenetic and sexual moths. A DNA barcoding approach was used to determine the reproductive mode of the parasitized hosts. Furthermore, we investigated whether differences in host and parasitoid phenology could provide an alternative explanation for the variation in parasitism rates between parthenogens and sexuals. Our results contradict the prediction of the parasite hypothesis because parthenogenetic bagworm moths were less parasitized than sexuals in sympatric sites. Our findings can be explained by differences in phenology between the parthenogenetic and sexual moths rather than genetic incompatibility between parthenogenetic hosts and parasitoids. The stable coexistence of sexual and parthenogenetic Naryciinae despite the many apparent costs of sex in this system remains a mystery. Our work adds to the list of studies were the assumptions of the parasite hypothesis for sex are not all met.