Why make daughters larger? Maternal sex-allocation and sex-dependent selection for body size in a mass-provisioning wasp, Trypoxylon politum

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Abstract
Mass-provisioning wasps package maternal investment into brood cells, sealed structures that contain all the provisions necessary for an offspring's growth and development. Optimal sex-allocation theory predicts that if maternal provisions determine the size of each offspring, and the amount of provisions available to each offspring varies, females should allocate well-stocked brood cells to the sex that benefits most from being large. I tested this hypothesis using observations of organ-pipe wasps, Trypoxylon politum, and dissections of their nests. A Mississippi population of T. politum was intensively studied from 1993 to 1995. This population fit the assumptions of optimal sex-allocation models by Green and Brockmann and Grafen. Female weight at emergence was 1.29 times that of males, and wing length was 1.15 times that of males. This discrepancy in size occurred because the volume of parental provisions strongly influenced adult body size, and better-stocked brood cells were preferentially allocated to daughters. Brood-cell volume correlated with both wing length and weight at emergence in both sexes, and the chance that a given brood cell contained a female offspring increased with increasing brood-cell volume. Fitness was positively related to body size for females, but I found no evidence of an advantage to large males. Although there was evidence of stabilizing selection for male wing length in one year, there was no evidence of an increasing relationship between body size and fitness (directional selection) for males in either 1993 or 1994. Female fecundity was positively related to body size in both years, indicating that larger females have increased reproductive success. The rate at which females provisioned brood cells was also correlated with body size. Observed patterns of investment in brood cells are quantitatively consistent with the predictions of optimal sex-allocation theory, but certain aspects of female provisioning behavior suggest females are not following a single “optimal” strategy. Patterns of provisioning were variable among different females at the study site during the same year. Large females tended to produce larger offspring. Although Brockmann and Grafen's model predicts a single, population wide “switchpoint” from the production of male to female offspring, there was no evidence for such a switchpoint

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