Original ArticleElder brothers affect the life history of younger siblings in preindustrial humans: social consequence or biological cost?
Introduction
Sex-specific sibling interactions have been shown to influence variation in offspring fitness throughout vertebrates (Uller, 2006). In humans, several anthropological studies have described how the number and sex of (particularly elder) siblings can influence an individual's survival and reproductive success. A recurring pattern is that the presence of elder brothers decreases the marriage prospects, resources and reproductive success of younger male siblings. Findings along these lines have been reported among 19th century Swedes (Low, 1991), as well as the Kenyan Gabbra pastoralists (Mace, 1996) and Kipsigis (Borgerhoff Mulder, 1998). The most likely explanation for such patterns is that they are due to competition driven by patrilineal inheritance and the corresponding bias of parental investment in elder sons (Borgerhoff Mulder, 1998, Low, 1991, Mace, 1996). In contrast, strong parental preferences against daughters on the Indian subcontinent has been shown to substantially increase the mortality risk of girls with elder sisters (Muhuri & Preston, 1991).
In contrast to the consequences of sex-specific sibling competition, evidence from industrialised societies suggests that being born to a mother who has previously produced a son can have negative consequence in both males and females. Of six studies documenting the association between elder sibling sex and birthweight, four found that both male and female newborns were lighter at birth when born following an elder brother versus an elder sister (Trotnow et al., 1976, Magnus et al., 1985, Nielsen et al., 2008, Rickard, 2008), while two found that only the birthweight of boys was affected by the sex of their elder sibling (Blanchard & Ellis, 2001, Côté et al., 2003). Furthermore, a study also able to record adult size for the same individuals found that both males and females with elder brothers achieved a shorter final height on average than those with elder sisters, suggesting that the apparent birthweight difference may have important long-term consequences for phenotype in adulthood (Rickard, 2008).
That being born after a male sibling can have long-term phenotypic consequences for the subsequent offspring of both sexes led us to hypothesise that differences between the success of children can be biological in origin. In particular, if mothers incur a higher cost as a result of producing a male versus female offspring, then the fitness of offspring born after an elder brother might be reduced irrespective of the sex of the current offspring. In a previous study, we utilized a large sample of church record data from preindustrial Finland to show that individuals born to a mother who had previously produced a son had reduced lifetime fecundity and eventual lifetime reproductive success compared with those born following a daughter (Rickard, Russell, & Lummaa, 2007).
In the present study, we first explore which of the four fecundity-increasing life-history traits are influenced by the sex of the elder sibling: probability of reproducing (i.e., being recruited into the breeding population), age at first reproduction, mean length of interbirth intervals between offspring and reproductive lifespan. Second, we determine the life-history trait through which elder sibling sex most compromises the lifetime fecundity of the subsequent offspring. Finally, we investigate whether or not the cost of sons is acquired cumulatively by examining whether each elder brother preceding the focal individual has an additional effect on reproductive success or whether only the sex of the immediately elder sibling matters.
In all analyses, we pay particular attention to the task of differentiating between ‘biological' and ‘social' explanations. There are a number of modifiers which can be used in analysis to give an accurate indication of whether the elder sibling sex effect is driven by biological or social factors. First, if there is a tendency to bias investment in sons, we would expect it to only affect younger female siblings. Second, if the investment bias is subject to family resources, we would expect the effect to be strongest in the very richest families (the landowners). Third, if there is bias in investment towards sons who would be more likely to receive the majority of their parents' inheritance, then the effect might be stronger in those born after an elder male sibling from a high birth order. Finally, if the effect is driven by high preferential investment, then we would not expect to see the pattern in those whose elder sibling died in infancy. Accordingly, we pay particular attention to testing these predictions in all analyses in the paper, by examining the effect of interactions between elder sibling sex and (a) the sex of the focal (younger) sibling, (b) family social class, (c) an individual's birth order, and (d) elder sibling death before 6 months of age.
Section snippets
Methods
To investigate which younger sibling life-history traits were associated with elder sibling sex, we analysed data from preindustrial Finnish farming and fishing communities (Rickard, Russell, & Lummaa, 2007). These data were collected from population registers maintained by the Lutheran church in the 18th and 19th centuries. The church systematically recorded all births, deaths, and marriages within parishes, as well as migrations between them (Luther, 1993). This allows for most individuals to
Effect of elder sibling sex on life-history traits
First, approximately 65% of individuals who survived to adulthood produced at least one child. The probability of reproducing varied with parish and relatively fewer poor individuals reproduced than rich and middle-class individuals. The probability of reproducing also declined with increasing birth order (survived birth order effect stronger). After controlling for these associations, we found that individuals whose mother had previously produced a male versus a female >offspring experienced a
Discussion
In humans, variation in individual development is associated with the sex and age of siblings. Such patterns can often be explained by inheritance practices or bias in the economic value of the two sexes causing parental investment to vary with sex and birth order of children. However, evidence from studies of wild animals as well as studies of human birthweight suggest there may also be a higher physiological cost of producing sons versus daughters (Rickard, Russell, & Lummaa, 2007). In
Acknowledgments
We are grateful to Natural Environmental Research Council, UK (IJR), The Royal Society of London (V.L., A.F.R.) and The Academy of Finland (V.L.) for funding, to Aino Siitonen, Kimmo Pokkinen, and Timo Verho for collection of the church record material, and to two anonymous reviewers for their helpful suggestions and contributions.
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