Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men

Meta-analysis of aggregate data from 14 samples representing 10,143 male subjects shows that homosexuality in human males is predicted by higher numbers of older brothers, but not by higher numbers of older sisters, younger brothers, or younger sisters. The relation between number of older brothers and sexual orientation holds only for males. This phenomenon has therefore been called the fraternal birth order effect. Research on birth order, birth weight, and sexual orientation suggests that the developmental pathway to homosexuality initiated by older brothers operates during prenatal life. Calculations assuming a causal relation between older brothers and sexual orientation have estimated the proportion of homosexual men who owe their sexual orientation to fraternal birth order at 15% in one study and 29% in another. The maternal immune hypothesis proposes that the fraternal birth order effect reflects the progressive immunization of some mothers to male-specific antigens by each succeeding male fetus and the increasing effects of such immunization on sexual differentiation of the brain in each succeeding male fetus. There are at least three possible mechanisms by which the mother's immune response could influence the fetus: the transfer of anti-male antibodies across the placenta from the maternal into the fetal compartment, the transfer of maternal cytokines across the placenta, and maternal immune reactions affecting the placenta itself. This hypothesis is consistent with recent studies showing that the quantity of fetal cells that enter the maternal circulation is greater than previously thought, and that the number of male-specific proteins encoded by Y-chromosome genes is greater than previously thought.     

The cause(s) of the fraternal birth order effect in male homosexuality

It has been established that the probability that a man is homosexual is positively related to his number of older brothers, but not older sisters when the brothers are accounted for. This is known as the 'fraternal birth order' effect. In the past, efforts have been made to explain this phenomenon in terms of several alternative biological hypotheses and a psychosocial hypothesis. This note examines how well these hypotheses accommodate the fraternal birth order effect. It is concluded that: (1) the evidence for the hypothesis of maternal immunoreactivity to the male fetus is weak; (2) the evidence for the intrauterine hormone exposure hypothesis is also weak; (3) the evidence for the hypothesis of postnatal learning is stronger. Lastly, there seem likely to be causes common to male homosexuality and paedophilia. They may include sexual (or quasi-sexual) experience in childhood or adolescence.     

Ectoparasitism in mothers causes higher positional fluctuating asymmetry in their sons: implications for sexual selection

Random deviations from perfect bilateral symmetry, fluctuating asymmetry, arise from developmental instability. I tested experimentally whether parasitism in female Drosophila nigrospiracula increases fluctuating asymmetry in male offspring. I also developed a novel measure for estimating developmental instability in a meristic trait called positional fluctuating asymmetry, which is based on the difference in the position of thoracic bristles between the right and left sternopleuron. I expected this measure to be a more sensitive indicator of developmental instability than the traditional numerical fluctuating asymmetry, because the latter is based on the difference in the actual presence or absence of bristles. Female flies burdened with hematophagous mites, Macrocheles subbadius (Macrochelidae), produced sons with significantly higher positional fluctuating asymmetry than did females carrying no mites. This effect, which may have resulted from impaired provisioning of oocytes by infested females, was dose dependent and magnified in the progeny of younger (18-20 d) versus older (30-32 d) females. This apparent magnification resulted from a slight but not significant increase in asymmetry of offspring of the older and unparasitized females. In contrast, the same mite loads had no effect on offspring numerical fluctuating asymmetry. If low-positional fluctuating asymmetry males enjoy a mating advantage, then with appropriate genetic variability, sexual selection could drive the evolution of host resistance in host populations. However, variability in neither kind of asymmetry influenced male mating success in nature. Thus, although male positional fluctuating asymmetry is causally associated with parasitism via maternal effects, asymmetry-based sexual selection is unlikely to influence the evolution of mite resistance in D. nigrospiracula. The value of the sensitivity afforded by positional fluctuating asymmetry is discussed in the context of sexual selection and conservation biology.     

Fraternal birth order and the maternal immune hypothesis of male homosexuality

In men, sexual orientation correlates with an individual's number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%. It has been hypothesized that this fraternal birth order effect reflects the progressive immunization of some mothers to Y-linked minor histocompatibility antigens (H-Y antigens) by each succeeding male fetus and the concomitantly increasing effects of such maternal immunization on the future sexual orientation of each succeeding male fetus. According to this hypothesis, anti-H-Y antibodies produced by the mother pass through the placental barrier to the fetus and affect aspects of sexual differentiation in the fetal brain. This explanation is consistent with a variety of evidence, including the apparent irrelevance of older sisters to the sexual orientation of later born males, the probable involvement of H-Y antigen in the development of sex-typical traits, and the detrimental effects of immunization of female mice to H-Y antigen on the reproductive performance of subsequent male offspring. The maternal immune hypothesis might also explain the recent finding that heterosexual males with older brothers weigh less at birth than heterosexual males with older sisters and homosexual males with older brothers weigh even less than heterosexual males with older brothers.     

Fluctuating asymmetry and fitness in Drosophila melanogaster

Models predict that developmental stability measured by fluctuating asymmetry should be positively correlated with fitness. Although such a correlation has often been suggested by indirect studies, there is still a lack of direct experimental evidence. In this note, I have measured the fluctuating asymmetry of sternopleural bristle counts in 32 lines of Drosophila melanogaster sharing the same genetic background but displaying all combinations of five visible mutations. Fluctuating asymmetry was heterogeneous among lines, suggesting a direct impact of the mutations on developmental stability. Two measures of fitness were made for each line: productivity (a combined measure of fecundity and egg‐to‐adult survivorship) and competitive male mating success. Fluctuating asymmetry was correlated with neither of these two components of fitness. This suggests that generalizations about fluctuating asymmetry must be taken with care.     

Birth order and male androphilia in Samoan fa'afafine

The manner in which male androphilia is publicly expressed varies cross-culturally. As such, it is unclear whether distinct or common underlying causal processes characterize male androphilia in different cultures. Establishing the existence of cross-cultural universals in male androphilia is one means of ascertaining whether common biological bases underlie this phenomenon despite its culturally distinct forms. The evidence that the number of older biological brothers increases the odds of androphilia in later-born males has been well documented for Western samples (i.e. the fraternal birth order effect); but there is little evidence for this effect in non-Western samples. Here, we compare the birth order of androphilic males (i.e. fa'afafine) and gynephilic males from the politically autonomous Polynesian nation of Independent Samoa. Results indicate that relative to gynephilic males, fa'afafine tend to have more siblings and are generally later born when birth order is quantified using Slater, fraternal and sororal indices. More specifically, fa'afafine tend to have a greater number of older brothers, older sisters and younger brothers. We discuss the observed effects in relation to the differing reproductive patterns exhibited by the mothers of fa'afafine and gynephilic males, and to existing social and biological theories for sexual orientation.     

Facial sexual dimorphism,developmental stability,and susceptibility to disease in men and women

We investigated aspects of self-reported health history–the number and duration of respiratory and stomach or intestinal infections and the number of uses of antibiotics over the last 3 years–in relation to measured facial masculinity, developmental instability [facial asymmetry and body fluctuating asymmetry (FA)] and facial attractiveness in a sample of 203 men and 203 women. As predicted from the hypothesis that the degree of facial masculinity is an honest signal of individual quality, men's facial masculinity correlated negatively and women's positively with respiratory disease number and duration. Stomach illness, however, was not associated significantly with facial masculinity and antibiotic use correlated significantly (negatively) only with men's facial masculinity. For both facial asymmetry and body FA, significant, positive associations were seen with the number of respiratory infections. In addition, facial asymmetry was associated positively with the number of days infected and marginally, in the same direction, with antibiotic use. Facial attractiveness showed no significant relationships with any of our health-history measures. This study provides some evidence that facial masculinity in both sexes may signal disease resistance and that developmental stability covaries positively with disease resistance. The validity of our health measures is discussed.     

Testis asymmetry and sperm length in Rhacophorus omeimontis

Theory predicts that the degree of testes asymmetry should be positively correlated with male body condition in species with directional testis asymmetry. We tested this prediction in Rhacophorus omeimontis, a species in which females mate with more than one male. Our results showed that the treefrogs did not exhibit the absence of directional asymmetry in testis size, but rather the occurrence of fluctuating asymmetry. Moreover, we also tested differences in body size, body mass, testis mass, testis asymmetry, and sperm size among initially paired, jointly paired, and unpaired males. We found that body size and mass, testis mass, testis asymmetry and sperm length did not differ among the three male types. Testis mass showed a positive relationship with soma mass, but the correlations between the extent of fluctuating testis asymmetry and sperm length, and between testis mass and sperm length were not significant. Our data suggest that testes size and sperm length do not play an important role in determining male mating success in the presence of sperm competition.     

Mounting frequency and number of cloacal contacts increase with age in common terns Sterna hirundo

We investigated courtship feeding and copulatory behaviour of common terns Sterna hirundo in relation to age. We found a significant positive correlation between the number of courtship feedings delivered to females and the number of mountings. Moreover, both courtship feeding and mounting rates were positively correlated with female and male age. Also, the number of cloacal contacts per mounting increased significantly with male or female age, which may indicate an increase in copulatory ability with age. The number of mountings for older birds was up to three times that for younger birds. Presuming that transfer of sperm is proportional to the number of cloacal contacts, older birds may have transferred up to six times more sperm than younger birds during the pre-laying and laying period. These results may account for the preference of young females for older mates as well as for older extra-pair mates.     

Fluctuating asymmetry in antlers of fallow deer (Dama dama): the relative roles of environmental stress and sexual selection

Antler lengths were recorded of a total of 250 male fallow deer (Dama dama (L)). Animals sampled were from marked populations where the majority of individuals were of known age. Asymmetry in antler length was normally distributed with a mean not significantly different from zero, confirming that differences in length between the antler pair constitute a true fluctuating asymmetry (FA). We found no clear relationship between the degree of asymmetry in antler length of an individual male and either population density or actual body mass. We did, however, detect a significant relationship between asymmetry and deviation from maximum cohort bodyweight. If deviation from maximum weight within a cohort may be considered some index of competitive success or increasing environmental stress, this may suggest that asymmetry in antler length relates in some way to developmental stress suffered by the individual concerned. The degree of asymmetry recorded in antler length also showed a significant decline with animal age, with antlers of animals of 2 years or older showing significantly greater symmetry. This is consistent with a hypothesis that despite continued competition for resources, there is a changing balance of selection pressure as animals reach maturity, with increased pressure from sexual selection requiring males to produce significantly more symmetrical ornaments.     

Sibling composition during childhood and adult blood pressure among native Amazonians in Bolivia

Sibling configuration, including birth order, or the number, age, and sex of siblings is associated with parental resource allocation between children and is thus associated with a person's well-being. Little is known about the association between specific types of siblings and adult health outcomes. Here we test several hypotheses about sibling composition (number of older brothers, older sisters, younger sisters, younger brothers) and adult blood pressure in a foraging-farming society of native Amazonians in Bolivia (Tsimane’). We collected data in 2007 from 374 adults (16–60 years of age) from 196 households in 13 villages. Household random-effects multiple regressions were run using systolic (SBP) or diastolic blood pressure (DBP) as outcomes; covariates included the four sibling categories and control variables (e.g., sex, age, education, body mass index [BMI]). Mean SBP and DBP were 114 (SD = 14) and 66 (SD = 11) mmHg. The prevalence of hypertension was 5.08%. Having an additional younger brother bore a small (3.3–5.9%) positive association with both SBP and DBP, with the effect weakening as people aged. Having an additional younger sister was associated with a small (3.8%) increase in SBP among women, with the magnitude shrinking as people aged. In a large family, the number of younger brothers may exert an impact on an individual's blood pressure.     

Effect of male and female age on the mating success of the obliquebanded leafrollerChoristoneura rosaceana (Lepidoptera: Tortricidae) under different ecological conditions

The present study was undertaken to test the hypothesis that male and female age, as well as temperature, can affect the mating success ofChoristoneura rosaceana, given the role that these two factors play in the pheromone biology of this species. In the laboratory, the mating success of females generally declined linearly with age, whereas in males, it increased during the first 3 days and then decreased. The decline in female mating success was more pronounced under warm than cool thermocycles, while the changes observed in males were greater under fluctuating than constant temperature regimes. The onset time of mating was unaffected by male age, however, older females always mated earlier than younger ones, with the advance being more pronounced at cool than warm temperatures. Similar results were also obtained under field conditions. When a single 3-day-old male was provided with 0-, 3-, and 5-day-old females simultaneously, older females obtained mates significantly more often than younger individuals at all temperatures, indicating that calling earlier may afford a reproductive advantage to older females. At both constant temperatures, the time spent mating was longer in older than in younger females, but not under fluctuating thermocycles. Very young and very old males generally spent more time in copula than middle-aged individuals both at constant temperatures and under the warm thermocycle. Under the cool thermocycle, the duration of mating was considerably prolonged at all ages, which could increase the risk of predation.     

Mating asymmetry and the direction of evolution in the Hawaiian cricket genus Laupala

Based on studies from native Hawaiian Drosophila, a model was proposed to explain sexual isolation and mating asymmetry, from which one could potentially infer the 'direction of evolution'. We examined sexual isolation between allopatric cricket species of the genus Laupala, another endemic Hawaiian insect with an elaborate mating system, to begin to explore the nature of sexual isolation and mating asymmetry in closely related Hawaiian organisms. We studied sexual isolation and mating asymmetry in two contrasts. First, an inter-island comparison, including L. makaio from the older island of Maui and L. paranigra from the younger island of Hawaii, and second, an intra-island (Hawaii) comparison, including L. nigra from the older volcano of Mauna Kea and L. paranigra with a primary distribution on the younger volcanoes of Mauna Loa and Kilauea. We used a 'no-choice' experimental design, pairing individual males and females in homospecific or heterospecific combinations. Several behavioural aspects of courtship (proportion of male singing, latency to male singing, production of spermatophores and courtship initiation speed) were quantified as well as the success or failure of matings. We demonstrate asymmetry in sexual isolation between reciprocal combinations of L. makaio and L. paranigra. This result is examined in light of the differences in courtship behaviour manifest in the experiments with these two species. We did not find evidence of asymmetry in sexual isolation between L. nigra and L. paranigra, although differences in courtship initiation speed were evident between reciprocal combinations of these two species. In addition to the geological argument that species on older islands and older volcanoes give rise to species on younger islands and younger volcanoes, we discuss phylogenetic evidence consistent with these biogeographic hypotheses of relationships among the focal taxa. The patterns of asymmetrical sexual isolation and mating asymmetry are consistent with those found in the native Hawaiian Drosophila.     

Fruit abortion,developmental selection and developmental stability in<Emphasis Type="Italic"> Quercus ilex</Emphasis>

Fruit abortion has been hypothesized to be a parental means of selective removal of propagules with low viability. In particular, aborted zygotes have been suggested to have developmentally deviant phenotypes, and surviving offspring may therefore give rise to adults with a developmentally stable phenotype. We tested predictions from this hypothesis using acorns of holm oaks Quercus ilex as a model system. Fecundity of oak trees was negatively related to mean fluctuating asymmetry of leaves, and abortion rates were positively related to leaf fluctuating asymmetry in at least one population. Aborted acorns were asymmetric in 83-99% of cases in three samples, while mature acorns were only asymmetric in 57-78% of cases. Acorn asymmetry was unrelated to germination probability and germination date, and had no significant effect on number of leaves, leaf mass, stem mass, seedling height or leaf area of seedlings. However, acorn asymmetry affected the trade-off between number and size of leaves in seedlings. Seedlings from asymmetric acorns showed a positive relationship between acorn size and number of leaves, but no relationships between acorn size and leaf area, while symmetric acorns showed the opposite. A positive relationship between acorn size and number of leaves in spring was found for naturally emerged seedlings that died during their first summer, whereas the number of leaves produced by surviving seedlings did not depend on acorn size. These findings are consistent with the hypothesis that stressed trees selectively abort propagules of low viability, and that developmental selection acts on a measure of developmental instability of fruits.     

Parasites increase fluctuating asymmetry of maleDrosophila nigrospiracula: Implications for sexual selection

Fluctuating asymmetry (minor deviations from perfect bilateral symmetry) is manifested by individuals less able to buffer environmental stress during development. I utilized a system of two naturally-occurring parasites ofDrosophila nigrospiracula to test whether parasitic infection during host development yields elevated degrees of fluctuating asymmetry in two morphological traits of males. This hypothesis has important implications for sexual selection, as it may explain why asymmetric males are often found to be sexually disadvantaged. In my system, nematodes infect larvae and therefore are more likely to disrupt development than mites which only parasitize adult flies. As predicted, nematode-infected maleD. nigrospiracula had a higher degree of bristle asymmetry than did mite-infested and control (carrying neither parasite) males. There was also a significant relation between nematode number and degree of asymmetry. There was a significant negative relation between nematode load and size of adult males, implicating a causal link between nutritional stress during host development and fluctuating asymmetry. Patterns of wing length asymmetry were inconsistent with those of bristle asymmetry. Nematode-infected males did not differ in wing length asymmetry relative to mite-infested and control males, nor was there a significant relation between nematode number and wing asymmetry. This inconsistency in expression of asymmetry may reflect different intensities of selection operating on each morphological trait.     

The effect of age on encounters between male crab spiders

In males that compete aggressively for females, size and agemay determine which males obtain access to these females. Inthe present study, we use the crab spider, Misumena vatia, aspecies with males that do not grow after becoming sexuallymature adults, to test the hypothesis that age affects the successof males competing for access to females. M. vatia is an excellentspecies to test this hypothesis because it is possible to disentangleage from size, characters that typically vary together in thespecies usually tested. We staged encounters between similar-sizedolder and younger adult male M. vatia in the presence of a femaleto determine the role of age in male access to females. Encountersbetween the males occurred during 63.3% of these pairings. Youngermales won significantly more (70.2%) of the encounters thandid older ones, but did not initiate significantly more encountersthan did older ones (62.5%). Although older males won only 29.8%of these encounters, they initiated significantly more (76.5%)of them than predicted by chance. This design also allowed usto test Parker's hypothesis that older individuals should exhibita higher level of aggression than younger ones. However, attacksby younger males were most likely to include extensive bodilycontact, whereas attacks by older males involved significantlyless contact. These results counter the frequent assertion thatolder individuals usually prevail over younger ones in contestsfor access to females, and that older males are more likelyto engage in highly overt aggression than are younger ones.Aging may decrease reproductive opportunities and success ratesof male M. vatia, affecting as many as nearly one-fourth oftheir encounters.     

Birth weight, sexual orientation and the sex of preceding siblings.

This study's first objective was to compare the mean birth weights of homosexual and heterosexual men and women. Its second objective was to investigate whether prior male and female fetuses have different effects on the birth weight of subsequent fetuses. The subjects were 3229 adult men and women (the probands), who weighed at least 2500 g at birth, and whose mothers knew the sex of the child (or fetus) for each pregnancy prior to the proband. Information on birth weight, maternal gravidity and other demographic variables was reported on questionnaires completed by the probands' mothers. The results confirmed earlier reports that boys with older brothers weigh less at birth than boys with older sisters, but they did not confirm reports that girls with older brothers weigh less than girls with older sisters. The results did not show across-the-board differences in the mean birth weights of homosexual versus heterosexual women or homosexual versus heterosexual men. However, the homosexual males with older brothers weighed about 170 g less at birth than the heterosexual males with older brothers. It is suggested that this pattern of results may reflect a maternal immune response to Y-linked minor histocompatibility antigens (H-Y antigens). According to this hypothesis, when the maternal immune response is mild, it produces only a slightly reduced birth weight, but when it is stronger, it produces a markedly reduced birth weight as well as an increased probability of homosexuality.     

Jamaican Symmetry Project: long-term study of fluctuating asymmetry in rural Jamaican children.

Fluctuating asymmetry, small deviations from perfect bilateral symmetry, is negatively correlated with health and positively correlated with sexual selection in human adults, but the accumulation, persistence, and fitness implications of asymmetries during childhood are largely unknown. Here, we introduce the Jamaican Symmetry Project, a long-term study of fluctuating asymmetry and its physical and behavioral correlates in rural Jamaican children. The project is based on an initial sample of 285 children (156 boys and 129 girls), aged 5 to 11 years. We describe the design of the project and the methodology of measuring 10 paired morphometric traits. All traits except hand width showed fluctuating asymmetry. Fluctuating asymmetries of the legs tended to be related and were less than half as great as fluctuating asymmetries of the arms and ears. Therefore the legs may show high developmental stability resulting from selection for mechanical efficiency. A fluctuating asymmetry composite score revealed that boys have significantly lower fluctuating asymmetry than girls and that this effect resides mainly in the elbows. There were significant positive relationships between composite fluctuating asymmetry and age, height, and weight, but multiple regression analyses showed that age was negatively related to fluctuating asymmetry, whereas body size was positively correlated. These findings are compared with results from recent English studies.     

Brothers delay menarche and the onset of sexual activity in their sisters

The higher costs of sons compared with daughters extends to a negative effect of brothers on the lifetime reproductive success of their siblings in subsistence and preindustrial societies. In societies with fewer resource constraints, one might expect that these effects would be limited or non-existent. This study investigates the costs of brothers and sisters in a contemporary western society of adult Australians. Girls with elder brothers had a delayed age at menarche. Younger brothers were associated with delayed onset of sexual activity in sisters, but not in brothers. Neither younger nor elder brothers influenced fitness parameters (number of pregnancies, number of children, age at first pregnancy or age at first birth) in siblings of either sex. This study provides evidence that brothers negatively affect their sisters'' onset of reproductive maturity and sexual activity; however, this delay is not associated with a fitness cost in contemporary Australia. We suggest this is due to the long period of independence prior to child bearing.     

Exaggerated asymmetric head morphology of female Doubledaya bucculenta (Coleoptera: Erotylidae: Languriinae) and ovipositional preference for bamboo internodes

The lizard beetle Doubledaya bucculenta (Coleoptera: Erotylidae: Languriinae) female has evident asymmetric head morphology. The females excavate small holes in host bamboo internodes for the deposition of eggs. To understand the asymmetry and allometry of male and female adults and larvae of D. bucculenta, mandibular length, genal and head width, and elytral length were measured, and the oviposition preference for different-sized internodes of the bamboo Pleioblastus simonii and the relationship between internode size and emerging adult size were examined. Larval mandibles exhibited no clear asymmetry pattern, and genae showed fluctuating asymmetry in length. Adult male mandibles showed left-directional asymmetry, but genae showed fluctuating asymmetry. Adult female mandibles and genae exhibited marked left-directional asymmetry. The degree of asymmetry of mandibles and genae remained constant regardless of body size. Large females tended to choose large-diameter internodes of P. simonii and to lay eggs successfully, whereas small females tended to choose intermediate-diameter internodes, but to fail in oviposition, suggesting that small females pay a high cost on oviposition. There was a positive correlation between internode size and emerging adult size. Marked directional asymmetry of female mandibles and genae are discussed in relation to greater frequency of cutting bamboo fibers compared with adult males, and the traits of bamboo internode.