Interbirth intervals: Intrafamilial, intragenomic and intrasomatic conflict

D Haig

doi:10.1093/emph/eou002

Abstract

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. Simple models of inclusive fitness and kin conflict address the evolution of human birth-spacing. 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. 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.

Highlights

Background and objectivesInterbirth intervals (IBIs) mediate a trade-off between child number and child survival
The models of this article apply to all sexual organisms that produce offspring one at a time, but I will employ the terminology of husbands and wives because I need a simple way to distinguish between mother’s partners who may be the father of a child and the genetic father of the child
The four right-hand terms represent marginal effects of longer interbirth intervals (IBIs): the first is a positive contribution from increased fitness of the infant, the second a negative contribution from a decrease in the mother’s residual reproductive value, the third a negative contribution from the increased probability that some other male will father the mother’s future offspring, the fourth summarizes ‘social’ effects via the husband’s kin, including effects on his production of offspring with other mothers