44 research outputs found
Natural Selection on Female Life-History Traits in Relation to Socio-Economic Class in Pre-Industrial Human Populations
Life-history theory predicts that resource scarcity constrains individual optimal reproductive strategies and shapes the evolution of life-history traits. In species where the inherited structure of social class may lead to consistent resource differences among family lines, between-class variation in resource availability should select for divergence in optimal reproductive strategies. Evaluating this prediction requires information on the phenotypic selection and quantitative genetics of life-history trait variation in relation to individual lifetime access to resources. Here, we show using path analysis how resource availability, measured as the wealth class of the family, affected the opportunity and intensity of phenotypic selection on the key life-history traits of women living in pre-industrial Finland during the 1800s and 1900s. We found the highest opportunity for total selection and the strongest selection on earlier age at first reproduction in women of the poorest wealth class, whereas selection favoured older age at reproductive cessation in mothers of the wealthier classes. We also found clear differences in female life-history traits across wealth classes: the poorest women had the lowest age-specific survival throughout their lives, they started reproduction later, delivered fewer offspring during their lifetime, ceased reproduction younger, had poorer offspring survival to adulthood and, hence, had lower fitness compared to the wealthier women. Our results show that the amount of wealth affected the selection pressure on female life-history in a pre-industrial human population
Changes in Length of Grandparenthood in Finland 1790-1959
The importance of grandparents for their grandchildren is well-studied in several disciplines, and studies are now also addressing the potential effects of grandchildren on grandparental wellbeing. Any such effects are limited by the time grandparents share with their grandchildren. Changing child mortality rates, grandparental longevity, and childbearing patterns may have profoundly altered the length of grandparenthood across the demographic transition, but this has received little scientific attention. Using a genealogical dataset from Finland, we investigate changes in this shared time, from the late 18th to mid-20th century. We found the number of shared years between grandparents and grandchildren was low until roughly the onset of industrialisation in Finland, after which point shared time increased rapidly, from both the grandchild and grandparent perspectives. Understanding changing patterns in the opportunity for intergenerational transfers between grandparents and grandchildren has implications for several fields of study, including biology, demography, sociology, health studies, and economics
Effects of female reproductive competition on birth rate and reproductive scheduling in a historical human population
Costly reproductive competition among females is predicted to lead to
strategies that reduce these costs, such as reproductive schedules.
Simultaneous births of coresident women in human families can reduce
their infant survival, but whether such competition also affects overall
birth rates and whether females time their pregnancies to avoid
simultaneous births remain unknown, despite being key questions for
understanding how intrafemale competition affects reproductive
strategies. Here, we used detailed parish registers to study female
reproductive competition in historical Finnish joint-families, where
brothers stayed on their natal farms and sisters married out, and
consequently unrelated daughters-in-law often coresided and competed for
household resources. We quantified the time-varying effects of having
reproductive-aged competitor(s) on a woman’s interval from marriage to
first childbirth, on age-specific fertility, and on birth scheduling.
Contrary to our hypothesis, the presence of one or several potential
female competitors did not lead to longer first birth intervals or lower
age-specific probability of reproduction. We also found no evidence
that women would schedule their reproduction to avoid the real cost of
simultaneous births on their offspring mortality risk; age-specific
reproductive rates were unaltered by changes in the presence of other
infants in the household. These results raise interesting questions
regarding the evolution of fertility suppression in social mammals in
different contexts, the costs and benefits of extended families for
female reproductive success and strategies deployed, and the cultural
practices that may help to avoid the negative outcomes of female
reproductive competition in human families.</p
Female-biased sex ratios in urban centers create a “fertility trap” in post-war Finland
Because sex ratios are a key factor regulating mating success and
subsequent fitness both across and within species, there is widespread
interest in how population-wide sex ratio imbalances affect marriage
markets and the formation of families in human societies. Although most
modern cities have more women than men and suffer from low fertility
rates, the effects of female-biased sex ratios have garnered less
attention than male-biased ratios. Here, we analyze how sex ratios are
linked to marriages, reproductive histories, dispersal, and urbanization
by taking advantage of a natural experiment in which an entire
population was forcibly displaced during World War II to other local
Finnish populations of varying sizes and sex ratios. Using a discrete
time-event generalized linear mixed-effects model, and including factors
that change across time, such as annual sex ratio, we show how sex
ratios, reproduction, and migration are connected in a female-dominated
environment. Young childless women migrated toward urban centers where
work was available to women, and away from male-biased rural areas. In
such areas where there were more females, women were less likely to
start reproduction. Despite this constraint, women showed little
flexibility in mate choice, with no evidence for an increase in partner
age difference in female-biased areas. We propose that together these
behaviors and conditions combine to generate an “urban fertility trap”
which may have important consequences for our understanding of the
fertility dynamics of today including the current fertility decline
across the developed world.</p
Lack of anti-predator recognition in a marine isopod under the threat of an invasive predatory crab
The prey naivete hypothesis suggests that the failure of prey to recognize novel predators as a threat is caused by a lack of anti-predator adaptations. We tested this hypothesis in a unique natural setting, where the isopod Idotea balthica encountered the rapidly spreading invasive crab, Rhithropanopeus harrissii. Earlier research had indicated high mortality of the isopods during exposure to R. harrissii. The isopod exerted no co-evolutionary history with any littoral crabs and thus the strong impact could be caused by lack of pre-adaptations towards the new predator species. We tested this hypothesis by studying the anti-predator responses of the isopods with water-born cues of R. harrissii and of the native predatory fish Perca fluviatilis. Compared to control water, the isopods lowered their activity when exposed to the fish cue. Instead crab cue did not induce anti-predator behaviour. We also tested the hypothesis that mortality caused by novel predator, similar to predation by P. fluviatilis, would result in differential selection for the two sexes and contribute to the evolution of personalities. However, we found no differences in anti-predator behaviour nor in mortality between the sexes or personalities of the isopods. The outcomes reveal an interesting evolutionary scenario, where predation by a local predator induce soft selection on prey characteristics, but an invasive species cause hard selection without differentiating between prey individuals. Our study-conducted in the dawn of the population outbreak of R. harrissii-provides an excellent reference point for studies resolving the evolutionary impacts of invasive predators on naive prey
The interplay of grandparental investment according to the survival status of other grandparent types
Inclusive fitness theory predicts that grandparental investment in grandchildren aims to maximise their inclusive fitness. Owing to an increasing overlap between successive generations in modern affluent populations, the importance of grandparental investment remains high. Despite the growing literature, there is limited knowledge regarding how the survival status of different grandparent types influences each other's investment in grandchildren. This question was studied by using the Involved Grandparenting and Child Well-Being Survey, which provided nationally representative data of English and Welsh adolescents aged 11-16-years. We applied Bayesian structural equation modeling (BSEM) where grandparental investment in grandchildren was modelled using multi-indicator unobserved latent variable. Our results showed that maternal grandmothers' investment was increased by having a living maternal grandfather but not vice versa. Having a living maternal grandmother was also associated with decreased investment of paternal grandparents while the opposite was not found. These findings indicate that the association between the survival status of other grandparents and the focal grandparents' investment varies between grandparent types
Will granny save me? Birth status, survival, and the role of grandmothers in historical Finland
Grandmothers play a crucial role in families enhancing grandchild
wellbeing and survival but their effects can be context-dependent, and
the children born in poor conditions are most likely to benefit from the
investments made by helping grandmothers. In this study, we examined,
for the first time, whether grandmothers' presence modified associations
between adverse birth status and survival up to 5Â years of age. In
detail, we verified, whether (i) firstborns, (ii) twins, (iii) children born within 24Â months after their sibling, and (iv)
children followed by short interval (i.e. their younger sibling was
born within 24Â months) survived better when either their maternal,
paternal, or both grandmothers were present. Moreover, we evaluated
whether illegitimate children survived better when the maternal
grandmother was present. We used an extensive and largely pre-industrial
demographic dataset collected from parish population registers kept by
the Lutheran Church of Finland from years 1730–1895. We show that
although grandmother presence cannot mitigate adverse effects of many
poorer birth conditions, grandchildren whose next sibling was born after
a short interval survived better when the maternal grandmother was
present. Taken together, these findings highlight an important role of
grandmothers in compensating the mother's investment in the new baby,
thus enabling overall faster successful reproductive rate of mothers.
Whilst the opportunity for grandmothers to mitigate the risks of adverse
birth statuses is limited, this study does show - through the
beneficial effect on survival for those with a short subsequent birth
interval - that grandmothers can increase their daughters' and their own
reproductive success.</p
Offspring fertility and grandchild survival enhanced by maternal grandmothers in a pre-industrial human society
Help is directed towards kin in many cooperative species, but its nature and intensity can vary by context. Humans are one of few species in which grandmothers invest in grandchildren, and this may have served as an important driver of our unusual life history. But helping behaviour is hardly uniform, and insight into the importance of grandmothering in human evolution depends on understanding the contextual expression of helping benefits. Here, we use an eighteenth-nineteenth century pre-industrial genealogical dataset from Finland to investigate whether maternal or paternal grandmother presence (lineage relative to focal individuals) differentially affects two key fitness outcomes of descendants: fertility and survival. We found grandmother presence shortened spacing between births, particularly at younger mother ages and earlier birth orders. Maternal grandmother presence increased the likelihood of focal grandchild survival, regardless of whether grandmothers had grandchildren only through daughters, sons, or both. In contrast, paternal grandmother presence was not associated with descendants’ fertility or survival. We discuss these results in terms of current hypotheses for lineage differences in helping outcomes.</p