A dynamic framework for the study of optimal birth intervals reveals the importance of sibling competition and mortality risks

Abstract

Human reproductive patterns have been well studied, but the mechanisms by which physiology, ecology and existing kin interact to affect the life history need quantification. Here, we create a model to investigate how agespecific interbirth intervals adapt to environmental and intrinsic mortality, and how birth patterns can be shaped by competition and help between siblings. The model provides a flexible framework for studying the processes underlying human reproductive scheduling. We developed a state-based optimality model to determine age-dependent and family-dependent sets of reproductive strategies, including the state of the mother and her offspring. We parameterized the model with realistic mortality curves derived from five human populations. Overall, optimal birth intervals increase until the age of 30 after which they remain relatively constant until the end of the reproductive lifespan. Offspring helping each other does not have much effect on birth intervals. Increasing infant and senescent mortality in different populations decreases interbirth intervals. We show that sibling competition and infant mortality interact to lengthen interbirth intervals. In lower-mortality populations, intense sibling competition pushes births further apart. Varying the adult risk of mortality alone has no effect on birth intervals between populations; competition between offspring drives the differences in birth intervals only when infant mortality is low. These results are relevant to understanding the demographic transition, because our model predicts that sibling competition becomes an important determinant of optimal interbirth intervals only when mortality is low, as in post-transition societies. We do not predict that these effects alone can select for menopause.

Effect of Birth Interval Upon Neonatal Deaths in Indonesia: 5-Year-Based Calendar Data

“…The risk of neonatal mortality decreases with rising birth spacing lengths up to 24 months (Ezeh et al, 2019;Hossain et al, 2019). Shorter birth intervals related to a higher risk of infant and child mortality may indicate maternal depletion syndrome and sibling competition for parental resources, and infection transmission (Dhingra & Pingali, 2021;Molitoris, 2017;Nisha et al, 2019;Onwuka et al, 2020;Thomas et al, 2015). Moreover, birth spacing between two pregnancies allows mothers to prepare their physical and mental for the next pregnancy, such as the supply of essential nutrients, iron, and folic acid restoration (Afeworki et al, 2015).…”

8. Why Do We Do What We Do?

“…This optimality framework shows how high costs of raising children co-evolve with low fertility. Thomas et al used this approach to investigate the relationships between sibling helping and sibling conflict, and their effect on birth intervals (Thomas, Shanley et al, 2015); they show that sibling competition and mortality risk interact, in that sibling competition only has a serious influence on optimal birth intervals when mortality is low. This captures one of the essential features of the demographic transition to low fertility, which is that, as mortality has declined, parents appear to invest more in each offspring at the expense of high fertility (a trend that has occurred worldwide over the last 200 years).…”