Sex difference in travel is concentrated in adolescence and tracks reproductive interests

Sexual selection theory suggests that the sex with a higher potential reproductive rate will compete more strongly for access to mates. Stronger intra-sexual competition for mates may explain why males travel more extensively than females in many terrestrial vertebrates. A male-bias in lifetime distance travelled is a purported human universal, although this claim is based primarily on anecdotes. Following sexual maturity, motivation to travel outside the natal territory may vary over the life course for both sexes. Here, we test whether travel behaviour among Tsimane forager–horticulturalists is associated with shifting reproductive priorities across the lifespan. Using structured interviews, we find that sex differences in travel peak during adolescence when men and women are most intensively searching for mates. Among married adults, we find that greater offspring dependency load is associated with reduced travel among women, but not men. Married men are more likely to travel alone than women, but only to the nearest market town and not to other Tsimane villages. We conclude that men''s and women''s travel behaviour reflects differential gains from mate search and parenting across the life course.     

Mobility and Navigation among the Yucatec Maya

Sex differences in range size and navigation are widely reported, with males traveling farther than females, being less spatially anxious, and in many studies navigating more effectively. One explanation holds that these differences are the result of sexual selection, with larger ranges conferring mating benefits on males, while another explanation focuses on greater parenting costs that large ranges impose on reproductive-aged females. We evaluated these arguments with data from a community of highly monogamous Maya farmers. Maya men and women do not differ in distance traveled over the region during the mate-seeking years, suggesting that mating competition does not affect range size in this monogamous population. However, men’s regional and daily travel increases after marriage, apparently in pursuit of resources that benefit families, whereas women reduce their daily travel after marriage. This suggests that parental effort is more important than mating effort in this population. Despite the relatively modest overall sex difference in mobility, Maya men were less spatially anxious than women, thought themselves to be better navigators, and pointed more accurately to distant locations. A structural equation model showed that the sex by marital status interaction had a direct effect on mobility, with a weaker indirect effect of sex on mobility mediated by navigational ability.     

Fitness and fertility among Kalahari !Kung

In this paper we develop a model that examines fertility and childhood mortality patterns and their relationship to environmental variables. Interactions among environmental variables can account for different fertility patterns and different mixes of these variables can produce similar patterns of fertility. Our model attempts to quantify the idea that there is a trade-off between producing a few children likely to survive to reproductive age and producing a greater number of children with lower chances for survival. The optimum mix of these strategies depends on environmental characteristics. We use the model to make predictions about fertility and mortality patterns among two Bushmen populations of southern Africa--the Ghanzi and Ngamiland !Kung--using data collected by Harpending in 1967-1968. The results do not support explanations of the low fertilities observed among !Kung Bushmen women, in whom it is thought that fitness is maximized by limiting fertility, and show no relationship between mortality and family size in either !Kung population. Instead, the number of offspring reaching reproductive age in both populations increases as their completed family size increases. We examine the effects of sex, birth order, and paternal investment on mortality. No sex ratio differences and no differences in mortality by sex or birth order are present. Infant mortality among women who married more than once is significantly higher than among women who married once, suggesting that paternal care has a significant effect.     

Sex differences in spatial cognition among Hadza foragers

This paper describes sex differences in spatial competencies among the Hadza, a mobile hunter–gatherer population in Tanzania. It addresses the following questions: (a) Is the usual male advantage in Euclidean spatial abilities found in this population, where both women and men are highly mobile? (b) Do Hadza women have better object location memory than men, as the gathering hypothesis predicts? (c) Do women who are nominated by others as being good at finding bushfoods excel at the object location memory task? We tested object location memory with a version of the memory game using cards of local plants and animals. This allowed us to also ask whether women and men would have better spatial memory for the plant and animal cards, respectively. We found that Hadza men were significantly better than women in three tests of spatial ability: the water-level test, targeting, and the ability to point accurately to distant locations (the latter only in the less mobile groups). There was a trend toward a male advantage at the object location memory task, in contrast to results found previously in nonforaging populations, and women's performance at the task deteriorated with age, while that of men did not. The women who were nominated by peers as being good at finding bushfoods were consistently older women. We discuss the probable hormonal causes and functional consequences of age changes in the spatial competencies of female foragers.     

The social biology of human populations: Spontaneous group formation conforms to evolutionary predictions of adaptive aggregation patterns

Two experiments tested whether city commuter and pedestrian groups conformed with ecological predictions of adaptive group size, age, and sex composition. It was predicted that individuals with the greatest reproductive potential would optimize mating contacts and minimize competitive interference by segregating into small, mixed-sex groups, whereas more vulnerable individuals, too young or old for optimum fertility, would tend to aggregate independently of gender in larger groups, which may offer advantages for foraging and predator avoidance. To maximize reproductive potential, mature males should prefer grouping with young women of high fertility, whereas older women should prefer younger men. In Experiment 1, 2,432 persons in three cities were observed on buses, automobiles, and sidewalks, in yoked observations. As predicted, reproductively immature preschool and grade-school children formed larger groups than mature adults (p < 0.0001) and showed no sex differences in grouping. Elderly adults (with decreased reproductive potential) were also found in larger groups than younger adults (p < 0.01) and also showed no sex differences. Sexually mature men and women grouped more with the opposite sex than in all-male or all-female groups (p < 0.00001). Mature adult men were the least aggregative category (p < 0.01): they tended to be alone or in pairs with women, and all-male groups were conspicuously absent (p < 0.005). These findings were consistent across different socioeconomic levels, ethnicities, and subcultures in our samples (p = NS). In Experiment 2, 475 bus passengers in three cities were observed selecting seatmates in a naturalistic choice paradigm. Only sexually mature adults exhibited a significant preference for the sex of their seatmates (p < 0.0005): Young women chose most often to sit with other women, whereas young adult men more frequently chose seatmates of the opposite sex. Young women were chosen more often as seat partners than all age/sex categories combined (p < 0.0005). Young women chose older partners (p < 0.05), while middle-aged women preferred younger women and men (p < 0.0005) as companions. Results are explained in the context of canalized behaviors arising early in human evolution.     

Spousal age-gaps,partner preferences,and consequences for well-being in four Colombian communities

Across cultures, women tend to marry older men. This phenomenon is commonly described as the result of evolved mate choice preferences, which cause men to base reproductive decisions on cues of youth and fertility in women, and women to base such decisions on cues of wealth and status in men. Other researchers have challenged this idea, arguing that husband-older spousal age-gaps might not be consistent with the joint preferences of men and women; age-gaps might instead arise as the result of sexual conflict. In such cases, the realized age-gap could benefit one party at a cost to the other. In practice, large age-gaps may result in negative outcomes for younger women married to older men—e.g., because of power differences in these relationships. Testing for the existence of sexual conflict over the size of the spousal age-gap in Tanzania, researchers found no evidence that large age-gaps are harmful to women. Here, we conceptually replicate this previous work, and assess the relationship between spousal age-gaps, partner preferences, and individual well-being in four communities in Colombia. We extend prior methods by inferring the size of idealized age-gaps using network structured mate-choice games, and measuring the realized age-gap directly among ever-married partners. Our analyses suggest that there is limited evidence of sexual conflict over the size of the spousal age-gap in these communities. First, realized age-gaps are not large on average—around 1–7 years across communities. Second, the age-gaps that do exist are consistent with the preferences of both men and women—at least during their early to middle reproductive periods. And, lastly, large age-gaps are not negatively associated with measures of fertility or well-being for either sex. Our results underscore the importance of appreciating the cultural context within which behavioral practices with potentially negative consequences are situated.     

Ecological demography: A synthetic focus in evolutionary anthropology

The interests of evolutionary anthropologists, behavioral ecologists, and demographers converge on the ecology of human fertility. Ecological conditions influence the optimum pattern of maternal effort. Patterns of abortion, neglect, and infanticide vary with mothers' ability to invest in their children and children's ability to use that investment. As in most other mammals, the ecology of human fertility varies between the sexes: status and resource control are important for males, whereas reproductive value is crucial for females. In pre-industrial societies, and even in monogamous societies in demographic transition, wealthy men had more children than did poorer men. This correlation, often assumed to have disappeared, persists today, with richer men still having more sexual access than others. Sex differences in the ecology of fertility mean that sex of the offspring, as well as birth order, influences parental investment. Because individual fertility varies with environment, it is not surprising that “natural” (uncontrolled) fertility varies across societies or that demographic transitions proceed locally, with occasional reverses, as individuals strive to maximize their lifetime reproductive success in changing, competitive, conditions.     

How conception risk affects competition and cooperation with attractive women and men

We investigated competition and cooperation for resources across the menstrual cycle in the context of bargaining games. Although bargaining has been studied within an evolutionary framework, little attention has been paid specifically to the role of mating motives in economic behavior. To investigate how motives related to reproductive success affect bargaining, participants at high or low risk for conception or who were on oral contraceptives played ultimatum and dictator games with partners who varied in sex and facial attractiveness. In ultimatum games, women in the fertile phase were more competitive over resources with attractive women than with less attractive women. Intrasexual competition was not observed in dictator games. Women were more cooperative with attractive men than with less attractive men in both games, regardless of fertility status. Low fertility women were more cooperative with attractive members than with less attractive members of both sexes in both games. Results support the view that, during periods of high fertility, when women are most intrasexually competitive for mates, withholding resources from potential rivals would enable women to gain the means to enhance their attractiveness and weaken competitors' abilities to do the same at a time when relative advantages in appearance are most crucial to reproductive success. The lack of a fertility effect for cooperation with potential mates supports the view that displays of generosity accrue benefits for women across the cycle in their efforts to attract men who will invest in relationships.     

Salivary testosterone does not predict mental rotation performance in men or women

Multiple studies report relationships between circulating androgens and performance on sexually differentiated spatial cognitive tasks in human adults, yet other studies find no such relationships. Relatively small sample sizes are a likely source of some of these discrepancies. The present study thus tests for activational effects of testosterone (T) using a within-participants design by examining relationships between diurnal fluctuations in salivary T and performance on a male-biased spatial cognitive task (Mental Rotation Task) in the largest sample yet collected: 160 women and 177 men. T concentrations were unrelated to within-sex variation in mental rotation performance in both sexes. Further, between-session learning-related changes in performance were unrelated to T levels, and circadian changes in T were unrelated to changes in spatial performance in either sex. These results suggest that circulating T does not contribute substantially to sex differences in spatial ability in young men and women. By elimination, the contribution of androgens to sex differences in human performance on these tasks may be limited to earlier, organizational periods.     

Men in the demographic transition

Women’s fertility is the focus of most demographic analyses, for in most mammals, and in many preindustrial societies, variance in male fertility, while an interesting biological phenomenon, is irrelevant. Yet in monogamous societies, the reproductive ecology of men, as well as that of women, is important is creating reproductive patterns. In nineteenth-century Sweden, the focus of this study, male reproductive ecology responded to resource conditions: richer men had more children than poorer men. Men’s fertility also interacted with local and historical factors in complex ways to have significant impact on population growth. As a result, “the” demographic transition was local, and locally reversible, in Sweden. Results cannot be simply translated from nineteenth-century studies to current attempts to promote fertility decline, because today, male and female resource-fertility curves differ in shape, not only in magnitude. When we translate studies of fertility decline, it is important to study individual fertility and to discern whether, in any particular case, male and female patterns are similar. Bobbi S. Low is Professor of Resource Ecology at the School of Natural Resources and Environment. Her background is in behavioral and evolutionary ecology and her current research interests are human sex differences in risk-taking and resource use.     

Women Who Know Their Place

Differences between men and women in the performance of tests designed to measure spatial abilities are explained by evolutionary psychologists in terms of adaptive design. The Hunter-Gatherer Theory of Spatial Ability suggests that the adoption of a hunter-gatherer lifestyle (assuming a sexual division of labor) created differential selective pressure on the development of spatial skills in men and women and, therefore, cognitive differences between the sexes. Here, we examine a basic spatial skill-wayfinding (the ability to plan routes and navigate a landscape)-in men and women in a natural, real-world setting as a means of testing the proposition that sex-based differences in spatial ability exist outside of the laboratory. Our results indicate that when physical differences are accounted for, men and women with equivalent experience perform equally well at complex navigation tasks in a real-world setting. We conclude that experience, gendered patterns of activity, and self-assessment are contributing factors in producing previously reported differences in spatial ability.     

Sex differences in visual attention toward infant faces

Parental care and alloparental care are major evolutionary dimensions of the biobehavioral repertoire of many species, including human beings. Despite their importance in the course of human evolution and the likelihood that they have significantly shaped human cognition, the nature of the cognitive mechanisms underlying alloparental care is still largely unexplored. In this study, we examined whether one such cognitive mechanism is a visual attentional bias toward infant features, and if so, whether and how it is related to the sex of the adult and the adult’s self-reported interest in infants. We used eye-tracking to measure the eye movements of nulliparous undergraduates while they viewed pairs of faces consisting of one adult face (a man or woman) and one infant face (a boy or girl). Subjects then completed two questionnaires designed to measure their interest in infants. Results showed, consistent with the significance of alloparental care in human evolution, that nulliparous adults have an attentional bias toward infants. Results also showed that women’s interest in and attentional bias towards infants were stronger and more stable than men’s. These findings are consistent with the hypothesis that, due to their central role in infant care, women have evolved a greater and more stable sensitivity to infants. The results also show that eye movements can be successfully used to assess individual differences in interest in infants.     

Age-Related Differences in Multiple Task Monitoring

Coordinating multiple tasks with narrow deadlines is particularly challenging for older adults because of age related decline in cognitive control functions. We tested the hypothesis that multiple task performance reflects age- and gender-related differences in executive functioning and spatial ability. Young and older adults completed a multitasking session with four monitoring tasks as well as separate tasks measuring executive functioning and spatial ability. For both age groups, men exceeded women in multitasking, measured as monitoring accuracy. Individual differences in executive functioning and spatial ability were independent predictors of young adults'' monitoring accuracy, but only spatial ability was related to sex differences. For older adults, age and executive functioning, but not spatial ability, predicted multitasking performance. These results suggest that executive functions contribute to multiple task performance across the adult life span and that reliance on spatial skills for coordinating deadlines is modulated by age.     

Reproduction is adapted to survival characteristics across geographically isolated medfly populations

We propose the hypothesis that individual longitudinal trajectories of fertility are closely coupled to varying survival schedules across geographically isolated populations of the same species, in such a way that peak reproduction takes place before substantial increases in mortality are observed. This reproductive adaptation hypothesis is investigated for medflies through a statistical analysis of biodemographic data that were obtained for female medflies from six geographically far apart regions. The following results support the hypothesis: (i) both survival and reproductive schedules differ substantially between these populations, where early peaks and subsequently fast declining reproduction are observed for short-lived and protracted reproductive schedules for long-lived flies; (ii) when statistically adjusting reproduction for the observed differences in survival, the differences in reproductive schedules largely vanish, and thus the observed differences in fertility across the populations can be explained by differences in population-specific longevity; and (iii) specific survival patterns of the medflies belonging to a specific population predict the individual reproductive schedule for the flies in this population. The analysis is based on innovative statistical tools from functional data analysis. Our findings are consistent with an adaptive mechanism whereby trajectories of fertility evolve in response to specific constraints inherent in the population survival schedules.     

Sex differences in mushroom gathering: men expend more energy to obtain equivalent benefits

Some of the strongest evidence for sex differences in human cognition relate to spatial abilities, with men traditionally reported to outperform women. Recently, however, such differences have been shown to be task dependent. Supporting the argument that a critical factor selecting for sex differences in spatial abilities during human evolution is likely to have been the division of labor during the Pleistocene, evidence is accumulating that women excel on tasks appropriate to gathering immobile plant resources, while men excel on tasks appropriate to hunting mobile, unpredictable prey. Most research, with the exception of some recent experimental field studies, has been conducted in the laboratory, with little information available on how men and women actually forage under natural conditions. In a first study, we GPS-tracked the foraging pathways of 21 pairs of men and women from an indigenous Mexican community searching for mushrooms in a natural environment. Measures of costs, benefits and general search efficiency were analyzed and related to differences between the two sexes in foraging patterns. Although men and women collected similar quantities of mushrooms, men did so at significantly higher cost. They traveled further, to greater altitudes, and had higher mean heart rates and energy expenditure (kcal). They also collected fewer species and visited fewer collection sites. These findings are consistent with arguments in the literature that differences in spatial ability between the sexes are domain dependent, with women performing better and more readily adopting search strategies appropriate to a gathering lifestyle than men.     

Sex ratios and mental health: Evidence from China

While sex ratios (i.e., relative numbers of men and women) have been linked to various economic and social outcomes, how sex ratios affect mental health is underexplored. Using nationally representative data from the China Family Panel studies (CFPS) and Population Census, we evaluate the impact of sex ratios on mental health among Chinese men and explore potential mechanisms. Employing the instrumental variables (IV) approach where the One-Child Policy’s mandated fertility limits and implementation are used as exogenous variations in local sex ratios, we find that higher local sex ratios increase depressive symptoms and probability of depression among Chinese men. The impact is stronger for men with lower levels of education and living in rural areas. Analyses of potential mechanisms show that higher sex ratios increase the likelihood of marriage delay and unemployment for men, and prolong working hours for the employed men. The findings are of direct relevance to the health and population policy in China.     

Sex differences in monocyte expression of IL-6: role of autonomic mechanisms

Sex differences in the prevalence of inflammatory disorders exist, perhaps due to sex differences in cellular mechanisms that contribute to proinflammatory cytokine activity. This study analyzed sex differences of monocyte intracellular expression of IL-6 and its associations with reproductive hormones and autonomic mechanisms in 14 matched pairs of men and women (n = 28). Monocyte intracellular IL-6 production was repeatedly assessed over two circadian periods. Sympathetic balance was estimated by heart rate variability and the ratio of power in the low-frequency (LF) to high-frequency (HF); vagal tone was indexed by the power of HF component. As compared to men, women showed greater monocyte expression of IL-6 across the circadian period. In addition, women showed lower sympathetic balance (LF/HF ratio), and greater levels of vagal tone (HF power). In women, but not men, sympathovagal balance was negatively associated with monocyte IL-6 expression, whereas vagal tone was positively associated with production of this cytokine. Levels of reproductive hormones were not related to monocyte IL-6 expression. The marked increase in monocyte expression of interleukin-6 in women has implications for understanding sex differences in risk of inflammatory disorders. Additionally, these data suggest that sex differences in sympathovagal balance or vagal tone may be a pathway to explain sex differences in IL-6 expression. Interventions that target autonomic mechanisms might constitute new strategies to constrain IL-6 production with impacts on inflammatory disease risk in women.     

Women’s work,child care,and helpers-at-the-nest in a hunter-gatherer society

Considerable research on helpers-at-the-nest demonstrates the positive effects of firstborn daughters on a mother’s reproductive success and the survival of her children compared with women who have firstborn sons. This research is largely restricted to agricultural settings. In the present study we ask: “Does ‘daughter first’ improve mothers’ reproductive success in a hunting and gathering context?” Through an analysis of 84 postreproductive women in this population we find that the sex of the first- or second-born child has no effect on a mother’s fertility or the survival of her offspring. We conclude that specific environmental and economic factors underlay the helpers-at-the-nest phenomenon.     

Fitness,reproduction and longevity among European aristocratic and rural Finnish families in the 1700s and 1800s

The life histories of two socio-economically different groups of humans comprising birth cohorts from the 1700s and 1800s were investigated. It was discovered that fertility selection was greater among European aristocrats and mortality selection greater among rural Finns. The life history of the rural Finns involved shorter female life spans, a considerably longer period of reproduction, higher juvenile mortality, a greater total production of offspring and slightly higher individual fitness. In a comparison of parental cohorts, it was discovered that longevity and progeny survival improved significantly from the 1700s to the 1800s. Out of the three factors investigated, longevity was found to influence reproduction and fitness more than socio-economic group or birth cohort. The reproductive efficacy and fitness of women increased along with their life span. However, reproductive success and fitness were lower among women with the longest life span (over 80 years). Among men, reproductive success improved consistently along with the increase in longevity. When birth intervals were examined, it was discovered that the sex of previous offspring did not influence the interval between births.     

What do men want? Re-examining whether men benefit from higher fertility than is optimal for women

Several empirical observations suggest that when women have more autonomy over their reproductive decisions, fertility is lower. Some evolutionary theorists have interpreted this as evidence for sexual conflicts of interest, arguing that higher fertility is more adaptive for men than women. We suggest the assumptions underlying these arguments are problematic: assuming that women suffer higher costs of reproduction than men neglects the (different) costs of reproduction for men; the assumption that men can repartner is often false. We use simple models to illustrate that (i) men or women can prefer longer interbirth intervals (IBIs), (ii) if men can only partner with wives sequentially they may favour shorter IBIs than women, but such a strategy would only be optimal for a few men who can repartner. This suggests that an evolved universal male preference for higher fertility than women prefer is implausible and is unlikely to fully account for the empirical data. This further implies that if women have more reproductive autonomy, populations should grow, not decline. More precise theoretical explanations with clearly stated assumptions, and data that better address both ultimate fitness consequences and proximate psychological motivations, are needed to understand under which conditions sexual conflict over reproductive timing should arise.