The relationship between cumulative number of cohabiting partners and number of children for men and women in modern Sweden

Societies with institutional monogamy may be effectively polygynous through some males having several wives in succession: serial monogamy. This study investigates the relationship between the number of cohabiting partners and the number of children in a modern society where serial monogamy is common in both sexes. Data from an investigation on cohabitation and reproduction were provided by the Swedish Statistics Bureau, where the oldest cohort were used to estimated “lifetime” number of partners and children. The analyses were repeated for the next-to-oldest cohort. About 78% males and 79% females had one partner, and about 15% of both sexes had more than one partner during their reproductive lifespan in the oldest cohort. Thus, monogamy was predominant, and serial monogamy was equally common among men and women. Serial monogamy was somewhat more frequent in the next-to-oldest cohort. Remating increased the number of offspring for males, but not for females, in both cohorts. This is in accordance with the idea of serial monogamy being a conditional reproductive strategy for males in a society with institutional monogamy. However, the reproductive costs and benefits for females of remating are obscure. In the oldest cohort of males there were significantly more unskilled laborers with no partner and no children compared to other socioeconomic categories. The same pattern, however nonsignificant, was found for the next-to-oldest cohort and, according to SSB data, is strong when even younger age groups are included. Data on socioeconomic status for females were not provided.     

Resources,attractiveness, family commitment; reproductive decisions in human mate choice

This study of reproductive decisions in human mate selection used data from "lonely hearts" advertisements to examine a series of predictions based on the mate preferences of male and females relating to age; physical appearance; financial condition and socioeconomic status; family commitment and personal traits; short- and long-term mating; and marital status and preexisting children. The sample consisted of 1000 personal advertisements (500 male) placed in two daily, national papers between February and October 1994 in Hungary. The research procedure included a pilot study of 150 advertisers (75 male) to refine the categories examined. Analysis was performed using 1) a matrix with one axis referring to offers and the other to demands of males and females separately; 2) a matrix of offers only to derive correlated traits of claims by males and females; and 3) a matrix with columns describing sex, offers, demands, advertiser's age, and required age and a row for each of the 1000 samples. It was found that men preferred younger mates, while women preferred older ones. Men were more likely to seek physical attractiveness, while women were more likely to seek financial resources (ranked 7th) and high status (ranked 6th). Women strongly preferred male domestic virtue and family commitment, and twice as many women as men demanded long-term relationships. Women more frequently declared preexisting children, and men exhibited a reluctance to accept these children. Both males and females employed "trade-off" strategies, making greater demands if they felt they had attractive offers.     

Energetic Consequences of Human Sociality: Walking Speed Choices among Friendly Dyads

Research has shown that individuals have an optimal walking speed–a speed which minimizes energy expenditure for a given distance. Because the optimal walking speed varies with mass and lower limb length, it also varies with sex, with males in any given population tending to have faster optimal walking speeds. This potentially creates an energetic dilemma for mixed-sex walking groups. Here we examine speed choices made by individuals of varying stature, mass, and sex walking together. Individuals (N = 22) walked around a track alone, with a significant other (with and without holding hands), and with friends of the same and opposite sex while their speeds were recorded every 100 m. Our findings show that males walk at a significantly slower pace to match the females’ paces (p = 0.009), when the female is their romantic partner. The paces of friends of either same or mixed sex walking together did not significantly change (p>0.05). Thus significant pace adjustment appears to be limited to romantic partners. These findings have implications for both mobility and reproductive strategies of groups. Because the male carries the energetic burden by adjusting his pace (slowing down 7%), the female is spared the potentially increased caloric cost required to walk together. In energetically demanding environments, we will expect to find gender segregation in group composition, particularly when travelling longer distances.     

Effects of obesity and sex on the energetic cost and preferred speed of walking.

The metabolic energy cost of walking is determined, to a large degree, by body mass, but it is not clear how body composition and mass distribution influence this cost. We tested the hypothesis that walking would be most expensive for obese women compared with obese men and normal-weight women and men. Furthermore, we hypothesized that for all groups, preferred walking speed would correspond to the speed that minimized the gross energy cost per distance. We measured body composition, maximal oxygen consumption, and preferred walking speed of 39 (19 class II obese, 20 normal weight) women and men. We also measured oxygen consumption and carbon dioxide production while the subjects walked on a level treadmill at six speeds (0.50-1.75 m/s). Both obesity and sex affected the net metabolic rate (W/kg) of walking. Net metabolic rates of obese subjects were only approximately 10% greater (per kg) than for normal-weight subjects, and net metabolic rates for women were approximately 10% greater than for men. The increase in net metabolic rate at faster walking speeds was greatest in obese women compared with the other groups. Preferred walking speed was not different across groups (1.42 m/s) and was near the speed that minimized gross energy cost per distance. Surprisingly, mass distribution (thigh mass/body mass) was not related to net metabolic rate, but body composition (% fat) was (r2= 0.43). Detailed biomechanical studies of walking are needed to investigate whether obese individuals adopt novel energy saving mechanisms during walking.     

Male traits associated with attractiveness in Conambo,Ecuador

This study investigated male attractiveness rankings in a small-scale Amazonian society. In the rural community of Conambo, Ecuador, men and women practice self-sufficient horticulture, men hunt, and, traditionally, men have experienced a high rate of mortality due to homicide. We tested whether male attractiveness rankings would be related to male age, warriorship, hunting ability, status, coalitional affiliation, and female age. Twenty-five women aged between 14 and 78 years ranked photographs of 29 local men aged between 16 and 74 years for attractiveness in addition to warriorship, hunting ability, and status. Results revealed that male age was negatively correlated (r=?.683, p=.01) with female rankings of male attractiveness. Warriorship (r=.517, p=.005), status (r=.489, p=.008), and hunting ability (r=.577, p=.001) were found to be positively correlated with attractiveness, after controlling for age. Additionally, females showed a bias for males in their in-group when ranking attractiveness (one-sample t test: T₂₉=16.727, p<.001). Attention is given to male age and coalitional affiliation as factors important in attractiveness rankings; warriorship and hunting ability also serve as ecologically salient features of male social worth. These findings contribute to a broader understanding of human attractiveness research by adding a new case study to the literature that documents previously unreported findings from a cultural context that is significantly different from the standard university-level student sample.     

Serial Monogamy as Polygyny or Polyandry?

Applications of sexual selection theory to humans lead us to expect that because of mammalian sex differences in obligate parental investment there will be gender differences in fitness variances, and males will benefit more than females from multiple mates. Recent theoretical work in behavioral ecology suggests reality is more complex. In this paper, focused on humans, predictions are derived from conventional parental investment theory regarding expected outcomes associated with serial monogamy and are tested with new data from a postreproductive cohort of men and women in a primarily horticultural population in western Tanzania (Pimbwe). Several predictions derived from the view that serial monogamy is a reproductive strategy from which males benefit are not supported. Furthermore, Pimbwe women are the primary beneficiaries of multiple marriages. The implications for applications of sexual selection theory to humans are discussed, in particular the fact that in some populations women lead sexual and reproductive lives that are very different from those derived from a simple Bateman-Trivers model.     

Optimal running speed and the evolution of hominin hunting strategies

Recent discussion of the selective pressures leading to the evolution of modern human postcranial morphology, seen as early as Homo erectus, has focused on the relative importance of walking versus running. Specifically, these conversations have centered on which gait may have been used by early Homo to acquire prey. An element of the debate is the widespread belief that quadrupeds are constrained to run at optimally efficient speeds within each gait, whereas humans are equally efficient at all running speeds. The belief in the lack of optimal running speeds in humans is based, however, on a number of early studies with experimental designs inadequate for the purpose of evaluating optimality. Here we measured the energetic cost of human running (n = 9) at six different speeds for five minutes at each speed, with careful replicates and controls. We then compared the fit of linear versus curvilinear models to the data within each subject. We found that individual humans do, in fact, have speeds at which running is significantly less costly than at other speeds (i.e., an optimal running speed). In addition, we demonstrate that the use of persistence hunting methods to gain access to prey at any running speed, even the optimum, would be extremely costly energetically, more so than a persistence hunt at optimal walking speed. We argue that neither extinct nor extant hominin populations are as flexible in the chosen speeds of persistence hunting pursuits as other researchers have suggested. Variations in the efficiency of human locomotion appear to be similar to those of terrestrial quadrupeds.     

Kin-directed altruism and the evolution of male androphilia among Istmo Zapotec Muxes

Male androphilia (i.e., sexual attraction and arousal to adult males) is considered an evolutionary puzzle because it reduces direct reproduction, but is influenced by genetic factors, reliably occurs across cultures, and has persisted over evolutionary time. The kin selection hypothesis states that genes for male androphilia can be maintained in a population if the costs of not reproducing directly are offset by enhancing the reproduction of kin. We tested this hypothesis among the Istmo Zapotec of Oaxaca, Mexico, where transgender and cisgender androphilic males are known as muxe gunaa and muxe nguiiu, respectively. We compared altruistic tendencies towards kin and non-kin children between muxe nguiiu (n = 106), muxe gunaa (n = 106), gynephilic men (i.e., men sexually attracted and aroused to adult females; n = 172), and androphilic women (n = 130). We also assessed whether the sisters of muxes (n = 96) reported receiving more childcare support from their muxe sibling compared to women with only gynephilic brothers (n = 65). The results showed that cisgender and transgender muxes reported more kin-directed altruistic tendencies than men. Muxe nguiiu also reported more kin-directed altruistic tendencies than women. When controlling for altruistic tendencies towards non-kin children, both muxe types exhibited more kin-directed altruistic tendencies than men and women. Women with muxe siblings reported receiving more childcare support from these relatives compared to women with only gynephilic brothers. These findings provide support for the kin selection hypothesis and highlight its potential role in elucidating the evolutionary paradox of male androphilia.     

Gait patterns of Japanese pedestrians

Gait patterns of adult Japanese were observed while walking on a sidewalk at Fukuoka city under natural conditions on clear days during December 1987. Pedestrians were recorded by means of video recorder for eleven 1-hr observations. From the measurements of the number of steps and the time required for each subject to traverse 50 m, walking speed was calculated for 479 women and 139 men, and step length and cadence for 292 women and 117 men. The following factors were taken into account in the data analysis: footwear, accompanist, time of day, estimated age and sex. Normal values for walking speed, step length, and cadence information on present-day adult Japanese were documented. The range for the total sample was from 49 to 119 m/min in walking speed, from 51 to 91 cm in step length, and from 95 to 160 steps/min in cadence. There were no significant differences in walking speed, step length, and cadence of the women between wearing flat shoes and heeled shoes. Walking velocity and step length were greater in single walking than in group walking. As compared with the values in the daytime (10:00-15:30) velocity, step length, and cadence tended to be higher in the morning (7:30-10:00) and lower in the evening (15:30-18:00). Older people walked at a slower speed with shorter steps than younger people. For younger pedestrians no significant sex difference was observed in walking speed. Women required a higher cadence than men because of their shorter step lengths. Older men walked faster than older women.     

Size-dependent selection on arrival times in sticklebacks: why small males arrive first

Abstract Studies on arrival time to breeding areas show that high-quality males usually arrive first and gain the highest reproductive success. This is generally assumed to be due to phenotype-dependent costs and benefits of early arrival. We show that the opposite arrival order can occur, probably due to selection on poor-quality males to increase their chances of reproduction. In a fish species, the threespine stickleback, Gasterosteus aculeatus , small males arrived before larger males at the breeding grounds. Early arrival was costly because predation risk was at its highest at the start of the season and early territory establishment was selected against, as demonstrated by selection coefficients for territory maintenance and hatching success. Large males probably postponed arrival until females were available to decrease predation risk costs and increase offspring production. An experimental study showed that a delay in arrival of large males does not decrease their probability of reproduction, because large males are able to take over nest sites from small males. Small males, on the other hand, are less likely to establish territories in competition with large males but can pay the costs of early arrival in exchange for the benefit of access to territories. Thus, whereas natural selection favors later arrival, sexual selection through competition for breeding territories favors early arrival in small, competitively inferior males. This results in the benefits of early arrival depending on the competitive ability of the male, which favors size-dependent optimal arrival times.     

THE EFFECT OF MATERNAL AGE AND REPRODUCTIVE HISTORY ON OFFSPRING SURVIVAL AND LIFETIME REPRODUCTION IN PREINDUSTRIAL HUMANS

Senescence is one of the least understood aspects of organism life history. In part, this stems from the relatively late advent of complete individual‐level datasets and appropriate statistical tools. In addition, selection against senescence should depend on the contribution to population growth arising from physiological investment in offspring at given ages, but offspring are rarely tracked over their entire lives. Here, we use a multigenerational dataset of preindustrial (1732–1860) Finns to describe the association of maternal age at offspring birth with offspring survival and lifetime reproduction. We then conduct longitudinal analyses to understand the drivers of this association. At the population level, offspring lifetime reproductive success (LRS) declined by 22% and individual λ, which falls with delays to reproduction, declined by 45% as maternal age at offspring birth increased from 16 to 50 years. These results were mediated by within‐mother declines in offspring survival and lifetime reproduction. We also found evidence for modifying effects of offspring sex and maternal socioeconomic status. We suggest that our results emerge from the interaction of physiological with social drivers of offspring LRS, which further weakens selection on late‐age reproduction and potentially molds the rate of senescence in humans.     

Are reproductive and somatic senescence coupled in humans? Late, but not early, reproduction correlated with longevity in historical Sami women

Evolutionary theory of senescence emphasizes the importance of intense selection on early reproduction owing to the declining force of natural selection with age that constrains lifespan. In humans, recent studies have, however, suggested that late-life mortality might be more closely related to late rather than early reproduction, although the role of late reproduction on fitness remains unclear. We examined the association between early and late reproduction with longevity in historical post-reproductive Sami women. We also estimated the strength of natural selection on early and late reproduction using path analysis, and the effect of reproductive timing on offspring survival to adulthood and maternal risk of dying at childbirth. We found that natural selection favoured both earlier start and later cessation of reproduction, and higher total fecundity. Maternal age at childbirth was not related to offspring or maternal survival. Interestingly, females who produced their last offspring at advanced age also lived longest, while age at first reproduction and total fecundity were unrelated to female longevity. Our results thus suggest that reproductive and somatic senescence may have been coupled in these human populations, and that selection could have favoured late reproduction. We discuss alternative hypotheses for the mechanisms which might have promoted the association between late reproduction and longevity.     

Handedness and socioeconomic status in an urban population in Uzbekistan

The persistence of left-handers in every human population studied to date is an evolutionary puzzle in light of evidence of survival costs associated with left-handedness. Associations between left-handedness and socioeconomic advantages have been observed in Western countries and could provide left-handers fitness benefits through higher survival chances and greater reproductive success. We aimed to explore the generality of this result in another culture. For this purpose, we investigated several socioeconomic status indicators and the number of children alive for 917 men and women in Uzbekistan and compared results for two different measures of handedness: hand preferences for writing and for knife use. Among both men and women, left-handed writers were significantly more likely to own a car, own a washing machine and have a bank account. Left-handed women (using both measures) had a higher income than right-handed women. Among men, left-handers for knife use had a higher income than right-handers. The results of our study suggest that the previously observed socioeconomic advantage of left-handers in Western populations also applies to non-Western populations, at least in the urban environment studied. However, we did not detect any difference in the number of children. We discussed how the frequency-dependent socioeconomic status advantage could be responsible for the persistence of left-handers throughout human evolution.     

Different impacts of resources on opposite sex ratings of physical attractiveness by males and females

Parental investment hypotheses regarding mate selection suggest that human males should seek partners featured by youth and high fertility. However, females should be more sensitive to resources that can be invested on themselves and their offspring. Previous studies indicate that economic status is indeed important in male attractiveness. However, no previous study has quantified and compared the impact of equivalent resources on male and female attractiveness. Annual salary is a direct way to evaluate economic status. Here, we combined images of male and female body shape with information on annual salary to elucidate the influence of economic status on the attractiveness ratings by opposite sex raters in American, Chinese and European populations. We found that ratings of attractiveness were around 1000 times more sensitive to salary for females rating males, compared to males rating females. These results indicate that higher economic status can offset lower physical attractiveness in men much more easily than in women. Neither raters' BMI nor age influenced this effect for females rating male attractiveness. This difference explains many features of human mating behavior and may pose a barrier for male engagement in low-consumption lifestyles.     

Female choice of sexually antagonistic male adaptations: a critical review of some current research

We contrast some recent uses of the concept of male-female conflict, with the type of conflict that is inherent in traditional Darwinian female choice. Females in apparent conflict situations with males may suffer reduced lifetime reproduction, but nevertheless benefit because they obtain sons with superior manipulative abilities. Female defences against male manipulations may not be 'imperfect' because of inability to keep pace with male evolution, but in order to screen males and favour those that are especially good manipulators. We examine the consequences of these ideas, and of the difficulties of obtaining biologically realistic measures of female costs, for some recent theoretical and empirical presentations of male-female conflict ideas, and find that male-female conflict in the new sense is less certain than has been commonly supposed. Disentangling previous sexual selection ideas and the new conflict of interest models will probably often be difficult, because the two types of payoffs are not mutually exclusive.     

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.     

Beauty and the beast: mechanisms of sexual selection in humans

Literature in evolutionary psychology suggests that mate choice has been the primary mechanism of sexual selection in humans, but this conclusion conforms neither to theoretical predictions nor available evidence. Contests override other mechanisms of sexual selection; that is, when individuals can exclude their competitors by force or threat of force, mate choice, sperm competition, and other mechanisms are impossible. Mates are easier to monopolize in two dimensional mating environments, such as land, than in three-dimensional environments, such as air, water, and trees. Thus, two-dimensional mating environments may tend to favor the evolution of contests. The two-dimensionality of the human mating environment, along with phylogeny, the spatial and temporal clustering of mates and competitors, and anatomical considerations, predict that contest competition should have been the primary mechanism of sexual selection in men. A functional analysis supports this prediction. Men's traits are better designed for contest competition than for other sexual selection mechanisms; size, muscularity, strength, aggression, and the manufacture and use of weapons probably helped ancestral males win contests directly, and deep voices and facial hair signal dominance more effectively than they increase attractiveness. However, male monopolization of females was imperfect, and female mate choice, sperm competition, and sexual coercion also likely shaped men's traits. In contrast, male mate choice was probably central in women's mating competition because ancestral females could not constrain the choices of larger and more aggressive males through force, and attractive women could obtain greater male investment. Neotenous female features and body fat deposition on the breasts and hips appear to have been shaped by male mate choice.     

Mate choice differences according to sex and age

We used 7,415 advertisements published in Spain to analyze traits sought/offered by men and women from different age groups. Findings regarding age, socioeconomic status, and physical attractiveness requirements support evolutionary predictions about mate preferences. However, changes in trait preferences among women under 40 appear to be contingent on Spain’s socioeconomic transformation. Women under 40 seek mainly physical attractiveness in men, whereas those over 40 seek mainly socioeconomic status. The trait most sought by men in all age groups is physical attractiveness. Traits sought and offered by advertisers may be conditioned by the personal situation of the advertiser. Mean age of advertisers (around their forties) and Spain’s social indicators suggest that the majority of advertisers have been unsuccessful in the mating arena at the conventional age.     

Testosterone levels and their associations with lifetime number of opposite sex partners and remarriage in a large sample of American elderly men and women

Testosterone (T) has been argued to modulate mating and parenting behavior in many species, including humans. The role of T for these behaviors has been framed as the challenge hypothesis. Following this hypothesis, T should be positively associated with the number of opposite sex partners a male has. Indeed research in humans has shown that T is positively related to the number of opposite sex partners a young man has had. Here we test, in both men and women, whether this relationship extends to the lifetime number of sex partners. We also explored whether or not T was associated with current marital status, partnership status and whether or not the participant remarried. Using a large sample of elderly men and women (each sample n > 700), we show that T is positively and sizably associated with the number of opposite sex partners in men. When controlling for potential confounding variables such as educational attainment, age, BMI, ethnicity, specific use of a medication and time of sampling this effect remained. For women, the relationship between T and number of opposite sex partners was positive but did not prove to be robust. In both men and women there was no evidence for an association between T and current marital status and partnership status (being in a relationship or not). However, remarriage was positively associated with T, but only in males. Results are discussed with reference to the literature on T and sex partners, remarriage and more broadly the challenge hypothesis.     

Sex effect on habitat selection in response to hunting disturbance: the study of wild boar

Spatial variation of the ‘predation risk’ due to human activities or distribution may increase the sexual difference in habitat selection. Indeed, females with offspring are usually more risk adverse than males. Based on a long-term wild boar study, we analysed the diurnal distribution of female and male wild boar before, during and after the hunting period. Hunting, food and foliation were investigated as factors affecting patterns of forest parcel selection. As expected, dense vegetative covers were selected during resting periods, but wild boar decreased this pattern of habitat selection in response to hunting disturbance. Moreover, the habitat selection of wild boar did not fit with the variation of food availability (presence or absence of mast) and the vegetation cycle. As expected, sows responded more to the hunting disturbance than males, leading to a more pronounced sexual difference during the riskier season. The unexpected decrease of bush use may be explained either by the increased hunting effort in this habitat or by the increased movements between resting sites due to disturbance, leading to a more random habitat selection pattern. The observed difference between sexes could result from a higher response of females with offspring to hunting, leading to an increased frequentation of secondary habitats, whereas males can tolerate more risks and remain hidden in thicket plots. Our results highlight how hunting disturbance can lead game species to change their patterns of refuge habitat selection and may affect the habitat segregation between the sexes.