On the impact of sex and birth order on contact with kin

Previous research indicates that birth order is a strong predictor of familial sentiments, with middleborns less family-oriented than first- or last-borns. In this research, effects of sex and birth order on the actual frequency of contact with maternal and paternal kin were examined in two studies. In Study 1, one hundred and forty undergraduates completed a questionnaire relating to the amount of time they spent in contact with specific relatives, while in Study 2, one hundred and twelve undergraduates completed the same questionnaire with the addition of two questions relating to the subjects’ parents’ birth orders. Subjects were more likely to have frequent contact with maternal, as opposed to paternal, kin and women experienced more frequent contact than men with relatives in general. The birth order of subjects did not appear to have a significant influence on contact but the birth order of the subjects’ parents did, with the offspring of middleborn mothers having relatively little contact with maternal grandparents and the offspring of middleborn fathers having relatively little contact with paternal grandparents. These sex and birth order differences are discussed in relation to possible differences in how women and men use kinship ties and in terms of how birth order may influence parental solicitude. Catherine Salmon recently received her Ph.D. in psychology from McMaster University in Hamilton Ontario. Her interest in kinship and family relationships has grown out of her own large extended family and many visits to Utah as well as exposure to evolutionary thinking about the family in the lab of Martin Daly and Margo Wilson. Her other current research interests focus on female sexuality and the evolutionary study of literature.     

Determinants of postnatal weight in infant rhesus monkeys: Implications for the study of interindividual differences in neonatal growth

This paper provides an analysis of infant body weights obtained from a sample of 38 rhesus monkey infants (Macaco mulatta) aged 29–165 days, i.e., animals still nutritionally dependent on their mothers. We examine the data on neonatal weights in relation to a number of factors, most notably, the sex of the infants, and the age and adiposity of their mothers. The infant body weights represent cross-sectional rather than longitudinal data; because they were mostly free-ranging animals, the infants were weighed just once each. Nevertheless, the results of our analysis strongly suggest that early postnatal growth in free-ranging rhesus is dependent on both maternal fatness and age. They also suggest that, although male infants are generally heavier than like-aged female infants, they do not grow any faster during the early postnatal period. Here, we speculate that the associations between infant size and both maternal age and adiposity are the result of between-mother differences in lactational output. © 1995 Wiley-Liss, Inc.     

Age-related body weight constraints on prenatal and milk provisioning in Iberian red deer (Cervus elaphus hispanicus) affect allocation of maternal resources

Maternal phenotypic characteristics can influence key life history variables of their offspring through maternal effects. In this study, we examined how body size constraints on maternal weight in yearling and subadult compared to adult hinds (age class effects) affected prenatal (calf birth weight, calf to hind weight ratio) and postnatal (milk) provisioning of Iberian red deer calves. Age correlated with all prenatal and postnatal investment traits except calf gains, although correlations were weaker than those with maternal weight. Once the effect of linear increase in weight with age was removed from models, yearlings showed additional reductions in calf birth weight, calf gains, and milk provisioning. The low-calf birth weight might increase the risk of calf mortality during lactation, as this occurs primarily during the first day of life and is strongly related to birth weight. Yearlings showed a greater prenatal allocation of resources in terms of greater calf to hind weight ratio probably as an extra effort by yearling mothers to balance calf neonatal mortality. It might compensate young mothers to produce low-quality calves while still growing rather than waiting for the uncertain possibility of surviving to the next reproductive season.     

Early Life Experiences Affect Adult Delayed-Type Hypersensitivity in Short and Long Photoperiods

Environmental experiences during development provide animals with important information about future conditions. Siberian hamsters are photoperiodic rodents that dramatically adjust their physiology and behavior to adapt to seasonal changes. For example, during short winter-like days, hamsters enhance some components of immune function putatively to cope with increasing environmental challenges. Furthermore, early life stress alters the developmental course of the immune system. Overall, immune function is typically suppressed in response to chronic stress, but responses vary depending on the type of stress and components of immune function assessed. This led us to hypothesize that delayed-type hypersensitivity (DTH), an antigen-specific, cell-mediated immune response, would be differentially modulated in hamsters that underwent early life maternal separation (MS) in either short or long photoperiods. At birth, hamsters were assigned to either short (SD; 8?h light/day) or long (LD; 16?h light/day) photoperiods and either daily 3?h MS, daily 15-min brief maternal separation (BMS), or no manipulation from postnatal day 2 through 14. In adulthood DTH was assessed. Hamsters reared in short days enhanced DTH responses. MS and BMS attenuated DTH responses in both short and long days. However, BMS long-day female hamsters did not suppress pinna swelling, suggesting a protective effect of female sex steroids on immune function. As is typical in short days, reproductive tissue was regressed. Reproductive tissue mass was also decreased in long-day MS female hamsters. Furthermore, MS altered photoperiod-induced changes in body mass. Taken together, these findings suggest that manipulations of early life mother-pup interactions in Siberian hamsters result in physiological changes and suppressed cell-mediated immunity. (Author correspondence: fonken.1@osu.edu).     

Energetics of lactation in harp seals (Phoca groenlandica) from the gulf of St. Lawrence,Canada

 This study reports the findings of an integrated, comprehensive analysis of lactation energetics in harp seals conducted using longitudinal measurements of mass, body composition and milk composition from mother-pup pairs in conjunction with water flux measurements in pups. The nursing period of harp seals is a short, intense and relatively efficient period of energy transfer from mothers to pups. The average daily milk intake for pups was 3.65±0.24 kg which is equivalent to 79.5 MJ of energy. Eighty-one per cent of the energy received in the milk was metabolisable and 66% of the energy was stored by the pups as body tissue. The field metabolic rate of pups was 3.9±0.4 time basal metabolic rate. The pups were growing at a rate of 2.2 kg per day during the nursing period. The distribution of this mass gain varied in terms of tissue composition, depending on the age of the pups, but over the whole nursing period approximately half of the tissue was stored as fat. Harp seal mothers lost an average of 3.1 kg per day during lactation which was composed of 37% water, 50% fat, 11% protein and 2% ash. Mothers spent half of their time during the lactation period actively diving and only one-third of their time on the surface of the ice. Milk compositional changes followed the normal phocid pattern with increasing fat content and decreasing water content as lactation progressed. The mean mass transfer efficiency was 73%. However, this value cannot be used without qualification because female harp seals in this study fed to varying degrees, consuming an estimated 0–4.8 kg of fish per day. Feeding does not appear to be required in order to achieve the energy requirements for lactation, given the energy stores possessed by females, and some females do fast through the entire period so feeding may be considered opportunistic in nature. Accepted: 25 April 1996     

Sexual differentiation of motivation: A novel mechanism?

Sex differences in motivation are apparent for the motivation to engage in sexual behavior, the motivation to take drugs of abuse, and the motivation to engage in parental behavior. In both males and females there is an increase in NAcc DA associated with motivated behaviors. Here it proposed that sex differences in the regulation of DA activity in the ascending mesolimbic projections may underlie sex differences in motivation. In particular, sex differences in the neuroendocrine regulation of this brain system play a role in the expression of sex differences in motivated behaviors. Here it is proposed that sexual differentiation of motivation is mediated, at least in part, by a novel mechanism in which ovarian hormones secreted at puberty in the female actively feminize the DA system.     

Reproductive parameters of female Japanese macaques: Thirty years data from the arashiyama troops,Japan

Over a 30-year period from 1954 to 1983, 975 live births were recorded for Japanese macaque females at the Iwatayama Monkey Park, Arashiyama, Japan. Excluding unknown birth dates, primiparous mothers gave birth to 185 infants (182 cases with age of mother known) and multiparous mothers gave birth to 723 infants (603 cases with age of mother known). The peak month of birth was May with 52.3% of the total births occurring during the period. Multiparous females who had not given birth the previous year did so earlier than multiparous females who had given birth the previous year and also earlier than primiparous females. Among the females who had given birth the previous year, females whose infant had died gave birth earlier than females who had reared an infant the previous year. The offspring sex ratio (1:0.97) was not significantly different from 1:1, and revealed no consistent association with mother's age. Age-fecundity exhibited a humped curve. The annual birth rate was low at the age of 4 years but increased thereafter, ranging between 46.7% and 69.0%, at between 5 and 19 years of age, but again decreased for females between 20 and 25 years of age. Some old females displayed clear reproductive senescence. The infant mortality within the first year of age was quite low (10.3%) and the neonatal (less than 1 month old) mortality rate accounted for 49.0% of all infant deaths. There was no significant difference between the mortality rates of male and female infants. A female's rank-class had no apparent effect on the annual birth rate, infant mortality, and offspring sex ratio. These long-term data are compared with those from other primate populations.     

Sex hormones, central nervous system and pain

The aim of the present review, which highlights some relationships between sex hormones, the CNS and pain, is to provide reference points for discussion on one of the most intriguing aspects of pain pathophysiology: the presence of sex differences in the response threshold to phasic painful stimuli and in the incidence of chronic pain syndromes. The first part of the review deals with sex steroids and their mechanisms of action. In the second part, the connections between sex steroids, the CNS and pain are illustrated to introduce possible areas of discussion in the study of sex differences in experimental and clinical pain.     

Nepotistic nosiness: inclusive fitness and vigilance of kin members' romantic relationships

The logic of inclusive fitness suggests that people should be attentive to the mating relationships of their kin—especially their genetically closest kin. This logic further suggests that people will be especially attentive to close kin members' relationships when a greater indirect fitness benefit is at stake. Three studies tested implications of this analysis. The primary results were that (a) people maintain greater vigilance over (and attempt greater influence on) the romantic relationships of genetically closer kin; (b) this effect is largely mediated by feelings of emotional closeness and perceptions of physical similarity; (c) women are more vigilant than men over their kin members' relationships; (d) people are more vigilant over the relationships of female kin, as compared to male kin, but only under conditions with especially clear implications for indirect fitness; and (e) people are more vigilant over kin members' long-term committed relationships, as compared to their casual relationships. These results indicate that a subtle form of nepotism is manifest in people's concern with their kin members' romantic relationships.     

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.     

Maternal glucocorticoid deficit affects hypothalamic-pituitary-adrenal function and behavior of rat offspring

Detrimental consequences of prenatal stress include increased hypothalamic-pituitary-adrenal (HPA) function, anxiety and depression-like behavior in adult offspring. To identify the role of maternal corticosterone milieu in the fetal programming of adult function, we measured these same behavioral and hormonal endpoints after maternal adrenalectomy (ADX) and replacement with normal or moderately high levels of corticosterone (CORT). Adult male and female offspring exhibited differing HPA responses to maternal ADX. In female offspring of ADX mothers, exaggerated plasma ACTH stress responses were reversed by the higher, but not the lower, dose of maternal CORT. In contrast, male offspring of both ADX and ADX dams with higher CORT replacement showed exaggerated ACTH stress responses. Hypothalamic glucocorticoid receptor (GR) expression was decreased in these latter groups, while hippocampal GR increased only in the ADX offspring. Activity of young offspring of ADX dams replaced with the higher dose of CORT decreased in the open field test of exploration/anxiety, while immobility behavior of adult offspring in the forced swim test of depression increased following maternal ADX or higher levels of CORT replacement. Interestingly, for some measures, none or moderately high CORT replacement resulted in similar deficits in this study. These findings are in accord with consequences of prenatal stress or prenatal dexamethasone exposure, suggesting that a common mechanism may underlie the effects of too low or too high maternal glucocorticoids on adult HPA function and behavior.     

Estrogens and relationship jealousy

The relation between sex hormones and responses to partner infidelity was explored in two studies reported here. The first confirmed the standard sex difference in relationship jealousy, that males (n=133) are relatively more distressed by a partner’s sexual infidelity and females (n=159) by a partner’s emotional infidelity. The study also revealed that females using hormone-based birth control (n=61) tended more toward sexual jealousy than did other females, and reported more intense affective responses to partner infidelity (n=77). In study two, 47 females were assessed four times across one month. Patterns of response to partner infidelity did not vary by week of menstrual cycle, but significant relations between salivary estradiol level and jealousy responses were obtained during the time of rising and high fertility risk. The implications, at least for females, are that any evolved psychological, affective, or behavioral dispositions regarding reproduction-related relationships are potentially moderated by estradiol, and that the use of synthetic hormones may disrupt this relation. David C. Geary is the Frederick A. Middlebush Professor of Psychological Sciences at the University of Missouri at Columbia. He has published nearly 100 articles and chapters across a wide range of topics, including cognitive and developmental psychology, education, evolutionary biology, and medicine. His two books, Children’s Mathematical Development (1984) and Male, Female: The Evolution of Human Sex Differences (1988), have been published by the American Psychological Association. M. Catherine DeSoto recently completed her Ph.D. in psychological sciences at the University of Missouri at Columbia and is currently assistant professor of psychobiology at the University of Northern Iowa. Her research primarily focuses on the interface between biology and behavior, including the relation between sex hormones and personality disorders. Mary K. Hoard is completing her Ph.D. studies in psychological sciences at the University of Missouri at Columbia and is currently a research specialist in the Department of Psychological Sciences. Her research interests focus on children’s cognitive development, as well as the relation between sleep and cognitive and psychological functioning. Melanie Skaggs Sheldon is a graduate student in the Department of Psychological Sciences at the University of Missouri at Columbia. Her research interests include social cooperation, sexual behavior, and personality, as understood from an evolutionary perspective. M. Lynne Cooper is a professor of psychological sciences and director of the training program in social psychology at the University of Missouri at Columbia. She is an associate editor of the Journal of Personality and the author of more than 60 articles and chapters in the areas of personality and social psychology. Her primary research efforts involve directing a longitudinal study of risky sexual behavior of adolescents and young adults.     

The influence of infant facial cues on adoption preferences

Trivers’s theory of parental investment suggests that adults should decide whether or not to invest in a given infant using a cost-benefit analysis. To make the best investment decision, adults should seek as much relevant information as possible. Infant facial cues may serve to provide information and evoke feelings of parental care in adults. Four specific infant facial cues were investigated: resemblance (as a proxy for kinship), health, happiness, and cuteness. It was predicted that these cues would influence feelings of parental care for both sexes, but that resemblance would be more important for men than women because of the importance of paternity uncertainty in the ancestral environment. Seventy-six men and 76 women participated in a hypothetical adoption task in which they made judgments of infant faces. Average zero-order, partial, and component score correlations all revealed that men placed primary emphasis on cues of resemblance, while women placed primary emphasis on cues of health and cuteness (cues of infant quality). The correlations also showed that men placed a significantly greater emphasis on cues of resemblance than did women. This research was supported by a Queen’s University Graduate Award (first author) and a Senior Research Fellowship from the Ontario Mental Health Foundation (second author). Anthony Volk is a Ph.D. candidate at Queen’s University, studying parental investment. Vernon L. Quinsey is a professor of psychology and psychiatry at Queen’s University. His research focuses on forensic and evolutionary psychology. An erratum to this article is available at .     

The emergence of gonadal hormone influences on dopaminergic function during puberty

Adolescence is the developmental epoch during which children become adults—intellectually, physically, hormonally and socially. Brain development in critical areas is ongoing. Adolescents are risk-taking and novelty-seeking and they weigh positive experiences more heavily and negative experiences less than adults. This inherent behavioral bias can lead to risky behaviors like drug taking. Most drug addictions start during adolescence and early drug-taking is associated with an increased rate of drug abuse and dependence.The hormonal changes of puberty contribute to physical, emotional, intellectual and social changes during adolescence. These hormonal events do not just cause maturation of reproductive function and the emergence of secondary sex characteristics. They contribute to the appearance of sex differences in non-reproductive behaviors as well. Sex differences in drug use behaviors are among the latter. The male predominance in overall drug use appears by the end of adolescence, while girls develop the rapid progression from first use to dependence (telescoping) that represent a female-biased vulnerability. Sex differences in many behaviors including drug use have been attributed to social and cultural factors. A narrowing gap in drug use between adolescent boys and girls supports this thesis. However, some sex differences in addiction vulnerability reflect biologic differences in brain circuits involved in addiction. The purpose of this review is to summarize the contribution of sex differences in the function of ascending dopamine systems that are critical to reinforcement, to briefly summarize the behavioral, neurochemical and anatomical changes in brain dopaminergic functions related to addiction that occur during adolescence and to present new findings about the emergence of sex differences in dopaminergic function during adolescence.     

Could maternal testosterone levels govern mammalian sex ratio deviations?

Although maternal dominance and good condition are frequently associated with raised offspring sex ratios in mammals, the key factor may be female testosterone, which not only underpins the behavioural indicators but could also provide a pathway to a possible proximate mechanism for sex determination. By taking into account the fact that female testosterone levels rise in response to environmental stressors, it is possible to re-interpret the findings of atypical sex ratios in mammals in a way that reconciles seemingly conflicting results and reveals instead what could be a coherent, adaptive system of sex allocation in mammals.