Production of undersized offspring in a solitary bee

Abstract

As predicted by Conditional Sex Allocation Theory, females of the solitary bee Osmia cornuta exposed to scarce floral resources biased their progeny sex ratio towards males, the least costly sex, and produced smaller-than-average females. Surprisingly, nesting females also produced a number of ‘tiny’ offspring, which contrasted with regular-sized offspring within the same nest. Developmental and wintering mortality are strongly size dependent in O. cornuta , and a high proportion of tiny offspring did not survive. This result is in disagreement with Optimal Allocation Theory, according to which resources should be allocated in portions that maximize fitness returns. I ask why did O. cornuta females build tiny provisions and why did they lay female eggs (with lower survival probability than male eggs) on these provisions. I argue that egg maturation rates and selective pressure to avoid kleptoparasitism and provision desiccation in cells left unsealed for long periods may impose a limit to the time available for cell provisioning. Under low food availability, this limit will be reached before provision sizes resulting in maximum fitness returns are attained. I also argue that the decision to fertilize an egg (and thus produce a female) is made at the beginning of the cell-provisioning process, so that females cannot adjust offspring sex to provision size. At the same time, altering the female–male cell sequence within a nest would result in fratricide because of protandric emergence. I provide evidence supporting these ecological and physiological constraints on resource allocation decisions in O. cornuta .