The evolution of progressive provisioning

Abstract

Most nonsocial wasps and bees are mass provisioners (MP), sealing each egg into its own cell containing all of the food required to reach maturity. Other species instead provision progressively, feeding their offspring only gradually as they develop and usually provisioning more than one offspring simultaneously (SPP). The evolution of progressive provisioning is interesting because it has obvious drawbacks. In particular, it could prolong the period of offspring dependency, and hence increase the chance that a mother will die before her offspring reach independence. Prolonged dependency could in turn facilitate the evolution of helping through insurance-based mechanisms. In this paper, I outline two ecologically extreme models of how reproductive success is accrued during provisioning. In Model 1, immature offspring become independent as soon as they are fully provisioned. SPP is then disadvantageous because it prolongs the provisioning period compared with MP. If SPP does evolve, Model 1 predicts that brood size, the number of offspring provisioned simultaneously in a batch, should be minimized. Model 2 differs from Model 1 in that offspring become independent only at adulthood. SPP can then be advantageous because investment is converted into independent offspring more quickly than under MP. Model 2 predicts an intermediate brood size, positively correlated with larval development time and the abundance of provisions. Ammophiline digger wasps and eusocial vespids may correspond to Models 1 and 2, respectively, whereas ground-nesting wasps and bees with multicellular nests may be intermediate.