The evolution of class-dependent reproductive effort in humans and other animals

Reproductive effort is a major life history trait that largely determines an organisms reproductive and survival schedule, and therefore it has a significant impact on lifetime fitness. A wealth of theoretical models have identified a wide range of factors that provide adaptive explanations for reproductive effort, including senescence, differential adult and offspring survival, and inter-generational competition. This work, however, is inadequate for explaining the levels of variation in reproductive effort found in stratified societies characterised by complex social dynamics. Rank and class-based societies are widespread in the natural world and common in social species, from insects and birds to humans and other mammals. In this article, I investigate how class and intra-generational social mobility influence the allocation of resources between fecundity and somatic tissue. I find that social mobility causes lower-class mothers to preferentially invest in survival, but only if class is associated with additional reproductive resources. If, by contrast, class is associated with extra survival resources, then upper-class mothers are always favoured to invest more in somatic maintenance, whilst lower-class mothers are always favoured to invest less in somatic maintenance, irrespective of social mobility. Moreover, I find that class-dependent reproductive effort leads to the emergence of distinct class-specific life-history syndromes, with each syndrome being associated with a suite of contrasting life-history traits. Finally, I find that these life-history syndromes are in close agreement with those observed in a human contemporary population. These findings lend support to the idea that evolutionary models can bridge the gap between the animal-human divide, and therefore be a valuable tool for public health decision-making and other human affairs.