Interbirth intervals: Intrafamilial, intragenomic and intrasomatic conflict

Background and objectives: Interbirth intervals (IBIs) mediate a trade-off between child number and child survival. Life history theory predicts that the evolutionarily optimal IBI differs for different individuals whose fitness is affected by how closely a mother spaces her children. The objective of the article is to clarify these conflicts and explore their implications for public health. Methodology: Simple models of inclusive fitness and kin conflict address the evolution of human birthspacing. Results: Genes of infants generally favor longer intervals than genes of mothers, and infant genes of paternal origin generally favor longer IBIs than genes of maternal origin. Conclusions and implications: The colonization of maternal bodies by offspring cells (fetal microchimerism) raises the possibility that cells of older offspring could extend IBIs by interfering with the implantation of subsequent embryos. K E Y W O R D S : parent–offspring conflict; interbirth interval; genomic imprinting; microchimerism; secondary infertility BACKGROUND AND OBJECTIVES The time between the birth of one child and the birth of the next shapes family structures and has implications for public health. Short interbirth intervals (IBIs) are associated with increased risk of death in childhood, both for the child whose birth begins the interval and for the child whose birth ends it [1–3]. Long IBIs, when these are not planned or the result of sexual abstinence, may provide evidence of underlying infertility [4]. IBI is a key variable in life-history theory [5]. Mothers on limited budgets face an evolutionary trade-off between investing less in each of a larger number of offspring or more in each of a smaller number [6]. This trade-off implies evolutionary conflict between genes of mothers and offspring because maternal fitness will have been maximized by less investment per child than maximizes each child’s fitness [7]. Offspring fitness will have been maximized by longer IBIs than optimal for maternal fitness [8, 9]. Genetic boundaries within families are less clear cut than was once thought. Cells move in both directions across the placenta, from mother to fetus and from fetus to mother. These self-transplanted cells can maintain themselves for decades in their original research article 12 The Author(s) 2014. Published by Oxford University Press on behalf of the Foundation for Evolution, Medicine, and Public Health. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. new locations [10]. This phenomenon has attracted recent medical interest but has been largely ignored by evolutionary biologists, even though ubiquitous microchimerism suggests unexplored possibilities of mother–offspring conflict and sibling rivalry within ‘individual’ bodies. This article has two major parts. The first presents simple heuristic models of evolutionary conflict over optimal IBIs with the intent of clarifying factors that should be considered in understanding the evolution of human birth-spacing. The second considers the implications, for IBIs and maternal fertility, of the colonization of the mother’s body by cells from each of her offspring.