Abstract
Variation in parental care by child's sex is evident across cultures. Evolutionary theory provides a functional explanation for this phenomenon, predicting that parents will favour specific children if this results in greater fitness payoffs. Here, we explore evidence for sex-biased parental care in a high-fertility, patriarchal and polygynous population in Tanzania, predicting that both mothers and fathers will favour sons in this cultural setting. Our data come from a cross-sectional study in rural northwestern Tanzania, which included surveys with mothers/guardians of 808 children under age 5. We focus on early childhood, a period with high mortality risk which is fundamental in establishing later-life physical and cognitive development. Examining multiple measures of direct/physical care provision (washing, feeding, playing with, supervising, co-sleeping and caring when sick), we demonstrate that fathers favour sons for washing, feeding and supervising, while maternal care is both more intensive and unrelated to child sex. We find no difference in parental care between girls and boys regarding the allocation of material resources and the duration of breastfeeding; or in terms of parental marital and co-residence status. This bias towards sons may result from higher returns to investment for fathers than mothers, and local gender norms about physical care provision.
Highlights
A broad principle of parental investment theory posits that natural selection will favour equal parental care for sons and daughters if rearing both sexes is costly, as each sex provides exactly half the genes for all future descendants (Fisher 1930)
Parental investment is defined as any allocation of resources which benefits offspring at a cost to a parent’s ability to invest in other components of fitness, while parental care more broadly refers to any parental trait that enhances the fitness of offspring, and is likely to have originated and/or to be maintained for that function, without necessarily being costly to the parent (Royle et al 2012; Trivers, 1972)
In this rural Tanzanian context, analysing reports of parental care behaviour from children’s mothers or guardians, we find that fathers favour sons in several measures of direct/physical parental care, but mothers do not discriminate their care in any form – resource provisioning, direct/physical care, or breastfeeding duration – based on their child’s sex
Summary
A broad principle of parental investment theory posits that natural selection will favour equal parental care for sons and daughters if rearing both sexes is costly, as each sex provides exactly half the genes for all future descendants (Fisher 1930). The costs and benefits of investment in each sex are rarely uniform (Hamilton 1967; Trivers and Willard 1973), and discriminatory parental care by offspring sex is observed across human cultures. Parental investment is defined as any allocation of resources which benefits offspring at a cost to a parent’s ability to invest in other components of fitness, while parental care more broadly refers to any parental trait that enhances the fitness of offspring, and is likely to have originated and/or to be maintained for that function, without necessarily being costly to the parent (Royle et al 2012; Trivers, 1972). The focus of this paper is on post-natal parental care, as opposed to biases in sex ratio at birth. Sex-biases in post-natal care may include such factors as discriminatory feeding, supervision, expenditure on health care and schooling, along with differential allocation of resources throughout life, including the transfer of inheritance
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