The evolution of simultaneous progressive provisioning revisited: extending the model to overlapping generations

Abstract

“Simultaneous progressive provisioners” feed their offspring gradually as they develop—and typically feed more than one offspring simultaneously (SIM) at a time. In contrast, “sequential mass provisioners” supply offspring one after another (SEQ). Utilizing individual-based simulations, Field (Nature 404:869–871, 2005) compared the lifetime reproductive success of these strategies in different scenarios. Accordingly, SEQ should evolve in the majority of cases—SIM only has an evolutionary benefit if offspring depend on their mothers’ protection until adulthood even past the provisioning period. However, this is only one potential explanation for the evolution of SIM. Here, we present an alternative mechanism for solitary individuals with overlapping generations. We propose an analytical model (comprising Field’s former approach) utilizing growth rate instead of lifetime reproductive success as a measure of fitness. Our model shows that multiplicative geometric effects on fitness would typically compensate for the demographic disadvantages of SIM (due to prolonged dependency) and consequently support the evolution of SIM over SEQ for a wide range of life history parameters. The optimal level of SIM (i.e., the optimal number of eggs laid simultaneously) is determined by offspring development time, survival rates, and foraging efficiency of the mother. Only extreme values of these demographic parameters would favor a transition to SEQ behavior. Our model provides a coherent explanation of selective favoring of SIM over SEQ that may also contribute to understanding why SIM is the dominant strategy among social insect species. Workers in social insects typically feed several offspring simultaneously while solitary species with parental care—apart from a few exceptions—provision brood cells one after another. The provisioning pattern might play a prominent role in the evolutionary pathway to higher social organization. Based on a novel theoretical approach, we show that geometric growth benefits increase selection pressure towards simultaneous progressive provisioning in species with generation overlap. Such geometric benefits may specifically emerge in seasonal eusocial species. This result alters former assessment of causal mechanisms and extends findings focusing on solitary insects. It adds a new and reasonable explanation for the dominance of simultaneous provisioning among social species.