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

Abstract

Reproductive effort is a major life history trait that largely determines an organism’s 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.

Reproductive value and the evolution of altruism

“…For instance, when the relative quality of high-quality individuals (H) increases, their predisposition for helping individuals of inferior quality (M and L) decreases (Figure II, third column). Note that fertility effects require the weighting of costs and benefits by the reproductive values of the offspring [54,65]. By contrast, if the behavior entails survival effects, the weighting is given by the residual reproductive values of the actors and recipients (e.g., [65]).…”

“…Note that fertility effects require the weighting of costs and benefits by the reproductive values of the offspring [54,65]. By contrast, if the behavior entails survival effects, the weighting is given by the residual reproductive values of the actors and recipients (e.g., [65]). In such cases, individuals with lower residual reproductive value are more predisposed to altruism.…”