Reproduction and production in a social context: Group size, reproductive skew and increasing returns

Abstract

Evolutionary success requires both production (acquisition of food, protection and warmth) and reproduction. We suggest that both may increase disproportionately as group size grows, reflecting ‘increasing returns’ or ‘group augmentation benefits’, raising fitness in groups that cooperate in production and limit reproduction to one or a few high fertility females supported by non‐reproductives, with high reproductive skew. In our optimisation theory both Allee effects (when individual fitness increases with group size or density) and reproductive skew arise when increasing returns determine optimal group size and proportion of reproductive females. Depending on which of food or maternal time is more important for reproduction, evolutionary trajectories of lineages may (1) reach a boundary constraint where only one female reproduces in a period (as with African wild dogs) or (2) reach a boundary where all females reproduce during their lifetimes but only during an early life stage (human menopause) or a late life stage (birds with non‐dispersing helpers), where stage length optimises the proportion of females that is reproductive at any time or (3) reach the intersection of these boundary constraints where a single reproductive female is fully specialised in reproduction (as with eusocial insects). We end with some testable hypotheses.

Abstract

A female's average cost per birth declines as her number of births rises, so a cooperative group achieves more births by limiting reproduction to a single female with others supporting. This reproductive skew benefits the group and generates high relatedness within it, raising indirect fitness benefits. Direct benefits arise through increasing returns to group ‘production’ of food, protection and warmth. Evolution can lead to a single lifetime reproductive female or to stage‐based cooperative reproduction. p: proportion of females in group that are reproductive. x: degree of specialization of reproductive females in reproduction. Line indicates evolutionary trajectory from solitary to stage‐based reproductive limitation in group.