Optimal resource allocation, maternal investment, and body size in a solitary bee, Osmia bicornis

Resources (energy expenses) that can be devoted to progeny are always limited. Optimal resource allocation theory predicts that parents should allocate resources to their offspring in portions that maximise their own fitness returns per unit invested, not the fitness of the offspring. Females of solitary, nest-constructing bees control the key features of offspring fitness by determining both body size and sex of each individual. Therefore the sexual size dimorphic red mason bee, Osmia bicornis (L.) [= Osmia rufa (L.)] (Hymenoptera: Megachilidae), constitutes a suitable model system to test optimal resource allocation theory. Body size range was analysed by food stock manipulation and hedged against possible associated allometric divergences of flight parameters. In harmony with the central prediction of the theory, O.bicornis females ordinarily provisioned offspring with food that did not enable maximal body size, but ensured successful development to adulthood. On average, only three-quarters of the actually required food to growth to full body size was supplied. Although females fundamentally balanced size vs. number of offspring according to the optimal allocation theory, the variability in body size was very high and the mean body mass of both sexes differed in the field between years. These fluctuations in body size over space and time indicate that various environmental factors are of prime importance in the actual resource assessment decision in addition to the core parental resource allocation strategy. As a physiological consequence of optimal resource allocation, the sex-specific critical weight of larvae to induce metamorphosis is ordinarily not reached at field conditions. Instead, metamorphosis is induced by a bail-out process when larvae run out of the food provisioned by their mother.