Early-season helping yields increasing returns to scale at the onset of eusociality

The evolution of eusociality, characterized by cooperative brood care, reproductive division of labor, and overlapping generations, represents a major evolutionary transition. A central paradox is that early models suggested diminishing returns to helping, making solitary reproduction seemingly more efficient. Here, we experimentally demonstrate increasing returns to scale from early-season helping in the primitively eusocial wasp <i>Polistes gallicus</i>. By manipulating the proportion of females allowed to remain as helpers, we show that while reproductive output scales linearly with worker numbers, total sexual productivity increases convexly with helper probability, thereby showing that early-season helping leads to compounding effects on reproductive output. Using these data, we parameterize a dynamic population genetic model and show that this convex relationship facilitates the spread of eusociality alleles more readily under monandry than polyandry, contrary to the conclusions of some prior models. Importantly, we show that compounding effects can cause eusociality to evolve even when helpers are no more efficient at rearing brood than solitary breeders. Our findings emphasize the value of integrating experimental data with mechanistically motivated theoretical models to study social evolution.