Are elder siblings helpers or competitors? Antagonistic fitness effects of sibling interactions in humans

The impact of welfare on maternal investment and sibling competition: evidence from Serbian Roma communities

Siblings compete for limited parental resources, which can result in a trade-off between family size and child growth outcomes. Welfare incentives may improve parental circumstances in large families by compensating for the additional costs of an extra child and increasing the resources available to a family. The improvements in conditions may influence parents to increase their investment, expecting greater returns from the investment in child survival and development, while in turn increase sibling competition for the investment. This study assessed whether welfare benefits have influenced parental investment trade-offs and competition between siblings among Serbian Roma, a population largely dependent on welfare. Using data from the UNICEF Multiple Indicator Cluster Survey 6, this study assessed the associations between maternal investment, child cash benefits, sibship size, and child anthropometry, as an indicator of health, among 1096 Serbian Roma children aged 0 to 59 months. Living in a small family benefited Roma children, while the incentives increased competition between siblings. Maternal investment was negatively associated with incentives, as the improvements brought about were insufficient to influence a change in maternal perceptions about the local setting uncertainty and thus promote an increase in investment.

The long-lasting legacy of reproduction: lifetime reproductive success shapes expected genetic contributions of humans after 10 generations

An individual's lifetime reproductive success (LRS) measures its realized genetic contributions to the next generation, but how well does it predict this over longer periods? Here we use human genealogical data to estimate expected individual genetic contributions (IGC) and quantify the degree to which LRS, relative to other fitness proxies, predicts IGC over longer periods. This allows an identification of the life-history stages that are most important in shaping variation in IGC. We use historical genealogical data from two non-isolated local populations in Switzerland to estimate the stabilized IGC for 2230 individuals approximately 10 generations after they were born. We find that LRS explains 30% less variation in IGC than the best predictor of IGC, the number of grandoffspring. However, albeit less precise than the number of grandoffspring, we show that LRS does provide an unbiased prediction of IGC. Furthermore, it predicts IGC better than lifespan, and accounting for offspring survival to adulthood does not improve the explanatory power. Overall, our findings demonstrate the value of human genealogical data to evolutionary biology and suggest that reproduction-more than lifespan or offspring survival-impacts the long-term genetic contributions of historic humans, even in a population with appreciable migration.

Sibling competition, dispersal and fitness outcomes in humans

Determining how sibling interactions alter the fitness outcomes of dispersal is pivotal for the understanding of family living, but such studies are currently scarce. Using a large demographic dataset on pre-industrial humans from Finland, we studied dispersal consequences on different indicators of lifetime reproductive success according to sex-specific birth rank (a strong determinant of dispersal in our population). Contrary to the predictions of the leading hypotheses, we found no support for differential fitness benefits of dispersal for either males or females undergoing low vs. high sibling competition. Our results are inconsistent with both hypotheses that family members could have different fitness maximizing strategies depending on birth rank, and that dispersal could be mainly driven by indirect fitness benefits for philopatric family members. Our study stresses the need for studying the relative outcomes of dispersal at the family level in order to understand the evolution of family living and dispersal behaviour.

Post-marital residence patterns and the timing of reproduction: evidence from a matrilineal society

Humans exhibit a broad range of post-marital residence patterns and there is growing recognition that post-marital residence predicts women's reproductive success; however, the nature of the relationship is probably dependent on whether co-resident kin are cooperators or competitors. Here, we explore this relationship in a Tibetan population, where couples practice a mixture of post-marital residence patterns, co-residing in the same village with the wife's parents, the husband's parents or endogamously with both sets of parents. Using detailed demographic data from 17 villages we find that women who live with only their own parents have an earlier age at first birth (AFB) and age at last birth (ALB) than women who live with only their parents-in-law. Women who co-reside with both sets of parents have the earliest AFB and ALB. However, those with co-resident older siblings postponed reproduction, suggestive of competition-related delay. Shifts to earlier reproductive timing were also observed in relation to the imposition of family planning policies, in line with Fisherian expectations. Our study provides evidence of the costs and benefits to women's direct fitness of co-residing with different kin, against a backdrop of adaptive responses to cultural constraints on completed fertility.

Transition to siblinghood causes a substantial and long-lasting increase in urinary cortisol levels in wild bonobos

In animals with slow ontogeny and long-term maternal investment, immatures are likely to experience the birth of a younger sibling before reaching maturity. In these species, the birth of a sibling marks a major event in an offspring’s early life as the older siblings experience a decrease in maternal support. The transition to siblinghood (TTS) is often considered to be stressful for the older offspring, but physiological evidence is lacking. To explore the TTS in wild bonobos, we investigated physiological changes in urinary cortisol (stress response), neopterin (cell-mediated immunity), and total triiodothyronine (T3, metabolic rate), as well as changes in behaviors that reflect the mother–offspring relationship. Following a sibling’s birth, urinary cortisol levels of the older offspring increased fivefold, independent of their age, and remained elevated for 7 months. The cortisol level increase was associated with declining neopterin levels; however, T3 levels and behavioral measures did not change. Our results indicate that the TTS is accompanied by elevated cortisol levels and that this change does not coincide with nutritional weaning and attainment of physical independence. Our results suggest that bonobos and humans experience TTS in similar ways and that this developmental event may have emerged in the last common ancestor.

The smell of cooperativeness: Do human body odours advertise cooperative behaviours?

Several physical features influence the perception of how cooperative a potential partner is. While previous work focused on face and voice, it remains unknown whether body odours influence judgements of cooperativeness and if odour-based judgements are accurate. Here, we first collected axillary odours of cooperative and uncooperative male donors through a public good game and used them as olfactory stimuli in a series of tasks examining whether and how they influence cooperative decision-making in an incentivized economic game and ratings of cooperativeness. Our results show that having access to the donor's body odours provided a strategic advantage to women during economic decisions (but not to men): with age, women were more likely to cooperate with cooperative men and to avoid interacting with uncooperative men. Ratings of cooperativeness were nonetheless unrelated to the donors' actual cooperativeness. Finally, while men with masculine and intense body odours were judged less cooperative, we found no evidence that donors' actual cooperativeness was associated with less masculine or less intense body odour. Overall, our findings suggest that, as faces and voices, body odours influence perceived cooperativeness and might be used accurately and in a non-aware manner as olfactory cues of cooperativeness, at least by women.

The elephant in the family: Costs and benefits of elder siblings on younger offspring life-history trajectory in a matrilineal mammal

Many mammals grow up with siblings, and interactions between them can influence offspring phenotype and fitness. Among these interactions, sibling competition between different-age offspring should lead to reproductive and survival costs on the younger sibling, while sibling cooperation should improve younger sibling's reproductive potential and survival. However, little is known about the consequences of sibling effects on younger offspring life-history trajectory, especially in long-lived mammals. We take advantage of a large, multigenerational demographic dataset from semi-captive Asian elephants to investigate how the presence and sex of elder siblings influence the sex, survival until 5 years old, body condition, reproductive success (i.e. age at first reproduction and lifetime reproductive success) and long-term survival of subsequent offspring. We find that elder siblings have heterogeneous effects on subsequent offspring life-history traits depending on their presence, their sex and the sex of the subsequent offspring (named focal calf). Overall, the presence of an elder sibling (either sex) strongly increased focal calf long-term survival (either sex) compared to sibling absence. However, elder sisters had higher impact on the focal sibling than elder brothers. Focal females born after a female display higher long-term survival, and decreased age at first reproduction when raised together with an elder sister rather than a brother. Focal males born after a female rather than a male showed lower survival but higher body weight when both were raised together. We did not detect any sibling effects on the sex of the focal calf sex, survival until 5 years old and lifetime reproductive success. Our results highlight the general complexity of sibling effects, but broadly that elder siblings can influence the life-history trajectory of subsequent offspring. We also stress the importance of considering all life stages when evaluating sibling effects on life trajectories.

Hired helpers at the nest: The association between life-cycle servants and net fertility in North Orkney, 1851-1911

OBJECTIVES

The presence of kin is often, but not always, associated with higher fertility in historical populations. However, the effect of other household members on fertility is less frequently studied. While not genetically related, life-cycle servants lived and worked alongside household members and may have provided assistance to reproducing families. Female servants in particular may have helped mothers with small children through direct help with childcare activities or by replacing the economic effort of mothers whose work was not compatible with childcare. This study examines the presence of servants in the households of married women of reproductive age to assess whether households with young children are more likely to also have servants.

MATERIALS AND METHODS

This study uses individual-level census data from North Orkney, Scotland (1851-1911) to investigate the relationship between the presence of servants in households and a measure of recent net marital fertility, the number of women's own-children under age 5, using logistic regression models.

RESULTS

Households with young children were more likely to have a female, but not male, servant in the household after controlling for the effects of other possible helpers, including older children, mothers, and mothers-in-law.

DISCUSSION

These findings are consistent with prior research that indicates the importance of female labor to smallholder agricultural households and suggests that female servants may have provided support to reproducing families. Life-cycle servants should be considered one component of biocultural reproduction in historical Northwest Europe. The use of hired help is not restricted to contemporary or elite groups.

Evolutionary significance of maternal kinship in a long-lived mammal

Preferential treatment of kin is widespread across social species and is considered a central prerequisite to the evolution of cooperation through kin selection. Though it is well known that, among most social mammals, females will remain within their natal group and often bias social behaviour towards female maternal kin, less is known about the fitness consequences of these relationships. We test the fitness benefits of living with maternal sisters, measured by age-specific female reproduction, using an unusually large database of a semi-captive Asian elephant ( Elephas maximus) population. This study system is particularly valuable to an exploration of reproductive trends in a long-lived mammal, because it includes life-history data that span multiple generations, enabling a study of the effects of kinship across a female's lifespan. We find that living near a sister significantly increased the likelihood of annual reproduction among young female elephants, and this effect was strongest when living near a sister 0-5 years younger. Our results show that fitness benefits gained from relationships with kin are age-specific, establish the basis necessary for the formation and maintenance of close social relationships with female kin, and highlight the adaptive importance of matriliny in a long-lived mammal. This article is part of the theme issue 'The evolution of female-biased kinship in humans and other mammals'.

A comparative study of litter size and sex composition in a large dataset of callitrichine monkeys

In many birds and mammals, the size and sex composition of litters can have important downstream effects for individual offspring. Primates are model organisms for questions of cooperation and conflict, but the factors shaping interactions among same-age siblings have been less-studied in primates because most species bear single young. However, callitrichines (marmosets, tamarins, and lion tamarins) frequently bear litters of two or more, thereby providing the opportunity to ask whether variation in the size and sex composition of litters affects development, survival, and reproduction. To investigate these questions, we compiled a large dataset of nine species of callitrichines (n = 27,080 individuals; Callithrix geoffroyi, Callithrix jacchus, Cebuella pygmaea, Saguinus imperator, Saguinus oedipus, Leontopithecus chrysomelas, Leontopithecus chrysopygus, Leontopithecus rosalia, and Callimico goeldii) from zoo and laboratory populations spanning 80 years (1938-2018). Through this comparative approach, we found several lines of evidence that litter size and sex composition may impact fitness. Singletons have higher survivorship than litter-born peers and they significantly outperform litter-born individuals on two measures of reproductive performance. Further, for some species, individuals born in a mixed-sex litter outperform isosexually-born individuals (i.e., those born in all-male or all-female litters), suggesting that same-sex competition may limit reproductive performance. We also document several interesting demographic trends. All but one species (C. pygmaea) has a male-biased birth sex ratio with higher survivorship from birth to sexual maturity among females (although this was significant in only two species). Isosexual litters occurred at the expected frequency (with one exception: C. pygmaea), unlike other animals, where isosexual litters are typically overrepresented. Taken together, our results indicate a modest negative effect of same-age sibling competition on reproductive output in captive callitrichines. This study also serves to illustrate the value of zoo and laboratory records for biological inquiry.

Limits to Fitness Benefits of Prolonged Post-reproductive Lifespan in Women

Recent advances in medicine and life-expectancy gains have fueled multidisciplinary research into the limits of human lifespan [1-3]. Ultimately, how long humans can live for may depend on selection favoring extended longevity in our evolutionary past [4]. Human females have an unusually extended post-reproductive lifespan, which has been explained by the fitness benefits provided from helping to raise grandchildren following menopause [5, 6]. However, formal tests of whether such grandmothering benefits wane with grandmother age and explain the observed length of post-reproductive lifespan are missing. This is critical for understanding prevailing selection pressures on longevity but to date has been overlooked as a possible mechanism driving the evolution of lifespan. Here, we use extensive data from pre-industrial humans to show that fitness gains from grandmothering are dependent on grandmother age, affecting selection on the length of post-reproductive lifespan. We find both opportunities and ability to help grandchildren declined with age, while the hazard of death of women increased greatly in their late 60s and 70s compared to menopausal ages, together implying waning selection on subsequent longevity. The presence of maternal grandmothers aged 50-75 increased grandchild survival after weaning, confirming the fitness advantage of post-reproductive lifespan. However, co-residence with paternal grandmothers aged 75+ was detrimental to grandchild survival, with those grandmothers close to death and presumably in poorer health particularly associated with lower grandchild survival. The age limitations of gaining inclusive fitness from grandmothering suggests that grandmothering can select for post-reproductive longevity only up to a certain point.

Using Geographic Distance as a Potential Proxy for Help in the Assessment of the Grandmother Hypothesis

Life-history theory predicts that selection could favor the decoupling of somatic and reproductive senescence if post-reproductive lifespan (PRLS) provides additional indirect fitness benefits [1, 2]. The grandmother hypothesis proposes that prolonged PRLS evolved because post-reproductive grandmothers gain inclusive fitness benefits by helping their daughters and grandchildren [3, 4]. Because most historical human data do not report direct evidence of help, we hypothesized that geographic distance between individuals may be inversely related to their capacity to help. Using an exceptionally detailed dataset of pre-industrial French settlers in the St. Lawrence Valley during the 17^th and 18^th centuries, we assessed the potential for grandmothers to improve their inclusive fitness by helping their descendants, and we evaluated how this effect varied with geographic distance, ranging between 0 and 325 km, while accounting for potential familial genetic and environmental effects [5-9]. Grandmothers (F0) who were alive allowed their daughters (F1) to increase their number of offspring (F2) born by 2.1 and to increase their number of offspring surviving to 15 years of age by 1.1 compared to when grandmothers were dead. However, the age at first reproduction was not influenced by the life status (alive or dead) of grandmothers. As geographic distance increased, the number of offspring born and lifetime reproductive success decreased, while the age at first reproduction increased, despite the grandmother being alive in these analyses. Our study suggests that geographic proximity has the potential to modulate inclusive fitness, supporting the grandmother hypothesis, and to contribute to our understanding of the evolution of PRLS.

Maternal age at birth and daughters' subsequent childlessness

STUDY QUESTION

Does maternal age at a daughter's birth predict her subsequent probability of lifelong childlessness?

SUMMARY ANSWER

In this study population, women born to older mothers were more likely to be childless.

WHAT IS KNOWN ALREADY

Although maternal age at childbearing is increasing in many countries, there is limited evidence on whether being born to older parents may influence offspring fertility.

STUDY DESIGN SIZE AND DURATION

This analysis included 43 135 women from the US-based Sister Study, a cohort study of 50 884 sisters of women with breast cancer recruited between 2003 and 2009.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Participants had no breast cancer at baseline. Women were included in the analytic sample if they were born between 1930 and 1964 and were at least 44 years old at enrolment. Median age when reproductive history was last ascertained was 63.8 years. We estimated relative risks (RR) and 95% CI of lifelong childlessness as a function of maternal age at birth, using multivariable log-binomial models, including total number of siblings, birth order, socioeconomic indicators of the family of origin, race and birth cohort. We examined the association in different subgroups and in a sibling-matched analysis including 802 sister pairs discordant for childlessness.

MAIN RESULTS AND ROLE OF CHANCE

Compared with women born to 20-24-year-old mothers, those born to mothers aged 25-29, 30-34 and ≥35 years were more likely to be childless [RR (95% CI): 1.21 (1.14-1.29), 1.30 (1.22-1.39) and 1.40 (1.31-1.50), respectively]. The association was consistent in strata defined by birth cohort, number of siblings, birth order, and participant's educational level, as well as within sister pairs. Overall, we found weak evidence for an independent contribution of paternal age at birth to the daughter's probability of childlessness.

LIMITATIONS REASONS FOR CAUTION

All participants had at least one sister, and all information was self-reported. We had no knowledge of whether childlessness was intentional and found only a modest association between maternal age at birth and self-reported indicators of infertility. Still, the association with childlessness was highly consistent.

WIDER IMPLICATIONS OF THE FINDING

Given the widespread tendency to delay childbearing, evaluating the influence of maternal age at birth on offspring fertility is a public health priority.

STUDY FUNDING/COMPETING INTERESTS

This research was supported in part by the Intramural Research Programme of the NIH, National Institute of Environmental Health Sciences (Z01-ES044005). The authors report no conflict of interest.

Grandmotherhood across the demographic transition

Grandmothers provide key care to their grandchildren in both contemporary and historic human populations. The length of the grandmother-grandchild relationship provides a basis for such interactions, but its variation and determinants have rarely been studied in different contexts, despite changes in age-specific mortality and fertility rates likely having affected grandmotherhood patterns across the demographic transition. Understanding how often and long grandmothers have been available for their grandchildren in different conditions may help explain the large differences between grandmaternal effects found in different societies, and is vital for developing theories concerning the evolution of menopause, post-reproductive longevity, and family living. Using an extensive genealogical dataset from Finland spanning the demographic transition, we quantify the length of grandmotherhood and its determinants from 1790–1959. We found that shared time between grandmothers and grandchildren was consistently low before the demographic transition, only increasing greatly during the 20^th century. Whilst reduced childhood mortality and increasing adult longevity had a role in this change, grandmaternal age at birth remained consistent across the study period. Our findings further understanding of the temporal context of grandmother-grandchild relationships, and emphasise the need to consider the demography of grandmotherhood in a number of disciplines, including biology (e.g. evolution of the family), sociology (e.g. changing family structures), population health (e.g. changing age structures), and economics (e.g. workforce retention).

Long-lasting effects of family-related factors on adults' ability to recognise brief facial expressions of emotion

This study investigated whether adults' ability to attribute emotions to brief facial expressions (microexpressions) is associated with family-related environmental factors (FrFs) such as one's number of siblings (Experiment 1), attachment style (Experiment 2), or perceived parental authority style (Experiment 3). Participants' accuracy and reaction time (RT) to the recognition of anger, contempt, disgust, fear, happiness, and sadness to facial microexpressions (exposure: 100 ms) were measured with a six-alternative forced choice computerised method (6AFC). The attachment style and the authority style of the participants' parents were accessed using questionnaires. The findings revealed that up to 13% of the variance in participants' responses could be explained by FrFs, with modest to moderate effect sizes. Microexpressions linked to signs of hostility or threat (i.e., contempt and fear) were decoded faster and/or more accurately by adults with few or no siblings or with a fearful attachment. Conversely, participants who recalled their fathers as authoritarian were worse at recognising contempt and fear than participants who perceived them as permissive or authoritative. The findings suggest that early FrFs may still be involved in the fine-tuning of responses to signs of contextual danger when the time for cognitive processing of facial expressions is severely restricted.

The tempo of human childhood: a maternal foot on the accelerator, a paternal foot on the brake

Relative to the life history of other great apes, that of humans is characterized by early weaning and short interbirth intervals (IBIs). We propose that in modern humans, birth until adrenarche, or the rise in adrenal androgens, developmentally corresponds to the period from birth until weaning in great apes and ancestral hominins. According to this hypothesis, humans achieved short IBIs by subdividing ancestral infancy into a nurseling phase, during which offspring fed at the breast, and a weanling phase, during which offspring fed specially prepared foods. Imprinted genes influence the timing of human weaning and adrenarche, with paternally expressed genes promoting delays in childhood maturation and maternally expressed genes promoting accelerated maturation. These observations suggest that the tempo of human development has been shaped by consequences for the fitness of kin, with faster development increasing maternal fitness at a cost to child fitness. The effects of imprinted genes suggest that the duration of the juvenile period (adrenarche until puberty) has also been shaped by evolutionary conflicts within the family.

When the mother-in-law is just as good-Differential mortality of reproductive females by family network composition

Motivated by the cooperative breeding hypothesis, we investigate the effect of having kin on the mortality of reproductive women based on family reconstitutions for the Krummhörn region (East Frisia, Germany, 1720-1874). We rely on a combination of Cox clustered hazard models and hazard models stratified at the family level. In order to study behavior-related effects, we run a series of models in which only kin who lived in the same parish are considered. To investigate structural, non-behavior-related effects, we run a different model series that include all living kin, regardless their spatial proximity. We find that women of reproductive age who had a living mother had a reduced mortality risk. It appears that having living sisters had an ambivalent impact on women's mortality: i.e., depending on the socioeconomic status of the family, the effect of having living sisters ranged between representing a source of competition and representing a source of support. Models which are clustered at the family level suggest that the presence of a living mother-in-law was associated with reduced mortality among her daughters-in-law especially among larger-scale farm families. We interpret this finding as a consequence of augmented consanguineous marriages among individuals of higher social strata. For instance, in first cousin marriages, the mother-in-law could also be a biological aunt. Thus, it appears that among the wealthy elite, the genetic in-law conflict was neutralized to some extent by family solidarity. This result further suggests that the tipping point of the female trade-off between staying with the natal family and leaving the natal family to join an economically well-established in-law family might have been reached very quickly among women living under the socioeconomic conditions of the Krummhörn region.

Do juveniles help or hinder? Influence of juvenile offspring on maternal behavior and reproductive outcomes in wild chimpanzees (Pan troglodytes)

Compared to great apes, humans maintain a relatively rapid reproductive pace despite long periods of dependency. This seemingly contradictory set of traits is made possible by weaning offspring before nutritional independence and alloparents who help provide care. In traditional societies, this help may be provided to mothers in part by their juvenile offspring who carry, supervise, or provision younger siblings. In contrast to humans, chimpanzees (Pan troglodytes) are nutritionally independent after weaning, yet juveniles continue to travel with their mother and younger sibling for an additional 4-5 years. This continued association could be costly to the mother if she continues to invest in weaned offspring. Alternately, while juvenile chimpanzees do not typically provision younger siblings, their presence and social interaction with infants may allow mothers to focus on other tasks. In this study, we investigate the costs and benefits to mothers of continued association with juveniles in wild chimpanzees. Using 26 years of long-term behavioral data we examined how maternal activity budgets varied based on the presence of a dependent juvenile offspring. We found that continued social interaction between mothers and juveniles does not influence the mother's time allocated to interacting with the younger infant, her feeding, resting, or travel time, or time socializing with other community members. Instead, mothers may benefit from the additional social interaction and/or relationship with their older offspring. Using 45 years of demographic data we found that those offspring who had an older sibling tended to be more likely to survive each year from birth to 8 years than those without an older sibling. Additionally, interbirth intervals were more likely to end when the female had an older offspring present. A mutually beneficial mother-juvenile dynamic in great apes provides insight into continued association between mothers and offspring after nutritional independence and the emergence of juvenile helping during hominin evolution.

More Unintended Injuries in Half Sibling Than Full Sibling Households in the UK

Sibling relationships can be a source of both support and conflict. Due to more severe sibling competition, childhood injuries are predicted to increase as genetic relatedness decreases. Here we use the British Millennium Cohort Study (n = 7,143 children) to analyze the risk of accidents for small children in different types of households. Results show that, after controlling for several potential confounding factors, 3-year-old children who lived in same household with their full siblings had a significantly lower risk of injuries (predicted probability 25.3%) than did children who lived with both their full and half siblings (29.6%) or only with their half siblings (29.1%). We conclude that efforts to prevent child maltreatment should pay attention to sibling relations and family composition.

Fitness Consequences of Advanced Ancestral Age over Three Generations in Humans

A rapid rise in age at parenthood in contemporary societies has increased interest in reports of higher prevalence of de novo mutations and health problems in individuals with older fathers, but the fitness consequences of such age effects over several generations remain untested. Here, we use extensive pedigree data on seven pre-industrial Finnish populations to show how the ages of ancestors for up to three generations are associated with fitness traits. Individuals whose fathers, grandfathers and great-grandfathers fathered their lineage on average under age 30 were ~13% more likely to survive to adulthood than those whose ancestors fathered their lineage at over 40 years. In addition, females had a lower probability of marriage if their male ancestors were older. These findings are consistent with an increase of the number of accumulated de novo mutations with male age, suggesting that deleterious mutations acquired from recent ancestors may be a substantial burden to fitness in humans. However, possible non-mutational explanations for the observed associations are also discussed.

Within-litter differences in personality and physiology relate to size differences among siblings in cavies

Many aspects of an animal's early life potentially contribute to long-term individual differences in physiology and behaviour. From several studies on birds and mammals it is known that the early family environment is one of the most prominent factors influencing early development. Most of these studies were conducted on highly altricial species. Here we asked whether in the highly precocial cavy (Cavia aperea) the size rank within a litter, i.e. whether an individual is born as the heaviest, the lightest or an intermediate sibling, affects personality traits directly after birth and after independence. Furthermore, we investigated whether individual states (early growth, baseline cortisol and resting metabolic rate) differ between siblings of different size ranks and assessed their relation to personality traits. Siblings of the same litter differed in personality traits as early as three days after birth. Pups born heaviest in the litter were more explorative and in general more risk-prone than their smaller siblings. Physiological state variables were tightly correlated with personality traits and also influenced by the size rank within litter, suggesting that the size relative to littermates constitutes an important factor in shaping an individual's developmental trajectory. Our data add valuable information on how personalities are shaped during early phases of life and indicate the stability of developmentally influenced behavioural and physiological traits.

The biological evolution of conscience – from parent-offspring conflict to morality

In this paper, two theories regarding the biological evolution of morality with conscience as its central regulatory agency are compared and contrasted. One theory (“navigator theory”) interprets conscience as a strategically operating agency for the optimum balance between selfish and altruistic behavioral tendencies to maximize gains in cooperation in view of social complexity. From this standpoint, conscience serves the evolved self-interest of the person having a conscience. In contrast hereto, the second theory (“helper theory”) locates the evolutionary origins of conscience on the battlefield of the parent-offspring conflict through intrafamilial demands for altruism. Functions of conscience, and thus human morality in a narrower sense, evolved during the transition of hominines to cooperative breeding and the novel helper conflict emerging through this evolution. The “helper theory” of the evolution of conscience can resolve some of the theoretical and empirical inconsistencies of the conventional “navigator theory”, in particular, the contradiction between the consequentialistic regulation of altruistic behavior and the non-consequentialistic nature of the judgment of conscience. And in contrast to the “navigator theory”, it is compatible with the observation that behavior guided by a conscience is not infrequently disastrous for one’s own fitness outcome.

Cooperation and conflict between women in the family

Here I review recent research on reproductive conflict between females in families and how it influences their reproductive behaviour. Kin selection can favor cooperation between parent and offspring, siblings, or unrelated co-residents who share interests in other family members such as grand-offspring. However, these are also the individuals most likely to be sharing resources, and so conflict can also emerge. While substantial interest has arisen in evolutionary anthropology, especially over the last two decades, in the possibility of cooperative breeding in humans, less attention has been paid to reproductive conflict among female kin. Communal breeding in animals is generally understood as emerging from competition over the resources needed to breed. Competition for household resources is a problem that also faces human families. Models suggest that in some circumstances, inclusive fitness can be maximized by sharing reproduction rather than harming relatives by fighting with them, even if the shares that emerge are not equal. Thus, competition and cooperation turn out to be strongly related to each other. Reproductive competition within and between families may have underpinned the biological evolution of fertility patterns (such as menopause) and the cultural evolution of marriage, residence, and inheritance norms (such as late male marriage or primogeniture), which can enhance cooperation and minimize the observed incidence of such conflicts.

Female competition and aggression: interdisciplinary perspectives

This paper introduces a Theme Issue combining interdisciplinary perspectives in the study of female competition and aggression. Despite a history of being largely overlooked, evidence is now accumulating for the widespread evolutionary significance of female competition. Here, we provide a synthesis of contributions to this Theme Issue on humans and other vertebrates, and highlight directions for future research. Females compete for resources needed to survive and reproduce, and for preferred mates. Although female aggression takes diverse forms, under most circumstances relatively low-risk competitive strategies are favoured, most probably due to constraints of offspring production and care. In social species, dominance relationships and threats of punishment can resolve social conflict without resort to direct aggression, and coalitions or alliances may reduce risk of retaliation. Consistent with these trends, indirect aggression is a low cost but effective form of competition among young women. Costs are also minimized by flexibility in expression of competitive traits, with aggressive behaviour and competitive signalling tailored to social and ecological conditions. Future research on female competition and the proximate mediators of female aggression will be greatly enhanced by opportunities for interdisciplinary exchange, as evidenced by contributions to this Theme Issue.

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.