SEX-RATIOS AND MULTIFACETED PARENTAL INVESTMENT

Although theories of parental investment and sex ratio generally assum e that a single resource limits reproduction, many organisms invest tw o or more qualitatively different types of resources in the production of offspring. We examine the consequences of multifaceted parental in vestment for offspring provisioning and sex allocation, building our a rgument around a study of the nest-building Hymenoptera (wasps, bees, and ants). We review empirical studies that demonstrate that lifetime reproductive success may be constrained not only by resources used to provision offspring but also by the supply of mature oocytes or, in so me cases, by the availability of space within nest sites or the time r equired to defend nests. Under multifaceted parental investment, the f actor limiting parental fitness determines the currency of the optimiz ation problem; parents are predicted to adjust reproductive behavior t o maximize fitness returns per unit of the limiting resource. We devel op simple models that predict that a greater availability of resources used for provisions will lead to an increase in the amount provisione d per offspring and an increase in the numerical or biomass proportion of females produced. These predictions explain widely observed patter ns of variation in offspring provisioning and sex allocation in the ne st-building Hymenoptera.