Quality–quantity trade‐off of human offspring under adverse environmental conditions

Abstract

A central paradigm in life‐history theory is the trade‐off between offspring number and quality. Several studies have investigated this trade‐off in humans, but data are inconclusive, perhaps because prosperous socio‐cultural factors mask the trade‐off. Therefore, we studied 2461 offspring groups in an area under adverse conditions in northern Ghana with high fertility and mortality rates. In a linear mixed model controlling for differences in age and tribe of the mother and socioeconomic status, each additional child in the offspring group resulted in a 2.3% (95% CI 1.9–2.6%, P < 0.001) lower proportional survival of the offspring. Furthermore, we made use of the polygamous population structure and compared offspring of co‐wives in 388 households, thus controlling for variation in resources between compounds. Here, offspring survival decreased 2.8% (95% CI 2.3–4.0%, P < 0.001) for each increase in offspring number. We interpret these data as an apparent quality–quantity trade‐off in human offspring.

Parental investment and the optimization of human family size

“…Life-history models have emphasized that negative effects of resource competition between offspring will be most severe when resources are relatively scarce, as this reinforces the assumption of finite parental resources [28,30,31,34,38,69]. This position is also empirically supported by a number of studies [31,34,69].…”

“…of offspring born, except for Borgerhoff Mulder [33,69] and Gibson [41] [ 36] and were actually associated with higher fertility for males [28,30]. Predicted negative relationships between family size and child survival have been more successfully demonstrated in a number of contemporary African agriculturalist societies ( [31,32]; but see [33]) and historical European and American populations [34 -36]. It should also be noted that a number of related studies have presented evidence of an association between family size and child anthropometric status which probably predicts future survival.…”

“…Some studies have made attempts to adjust models for this possibility. For example, Strassmann & Gillespie [32] exclude cases of very early death from their model of family size and child mortality in the Dogon (see also [31]). Concerns about causality need to be more explicitly addressed in future research by testing whether a division of parental investment truly drives quantity -quality trade-off effects.…”

Polygynous marriage and child health in sub-Saharan Africa: What is the evidence for harm?

“…Studies from East Africa (Tanzania, Uganda, Kenya, Ethiopia), West Africa (Gambia, Ghana, Sierra Leone, Mali, Burkina Faso, Nigeria), Central Africa (Chad), and Southern Africa (Zambia) are included. Several of these 'small-scale' studies suggest detrimental impacts of polygyny on child health, in that children of polygynously married women are at an apparent health disadvantage compared to children of monogamously married women (Begin, Frongillo, and Delisle 1998;Leroy, Razak, and Habicht 2008;Meij et al 2009;Strassmann 1997). However, in the majority of studies the authors conclude that statistically significant negative relationships between polygyny and child health are restricted to certain children or contexts (Borgerhoff Mulder 1997Brahmbhatt et al 2002;Gibson and Mace 2007;Gillett-Netting and Perry 2005;Diallo et al 2012;Sellen, Borgerhoff Mulder, and Sieff 2000;Sellen 1999;Strassmann 2011), and some studies report no overall statistically significant association between polygyny and child health (Arthi and Fenske 2018;Issac and Fienberg 1982;Sear et al 2002).…”

Faster reproductive rates trade off against offspring growth in wild chimpanzees

“…Many studies find the predicted negative relationship between reproductive rates and offspring growth or health (14)(15)(16)(17)(18) or demonstrate fitness maximization at intermediate levels of fertility (19)(20)(21). For example, when a water tap was installed to reduce women's workload in a rural Ethiopian community, birth rates increased, but child growth rates decreased, and more children were clinically stunted (22).…”