INTERNEST SEX-RATIO VARIATION AND MALE BROOD SURVIVAL IN THE ANT PHEIDOLE PALLIDULA

Sex allocation in social insects has become a general model in tests o f inclusive fitness theory sex-ratio theory, and parent-offspring conf lict. Several studies have shown that colony sex ratios are often bimo dally distributed, with some colonies producing mainly females and oth ers mainly males. Sex specialization may result from workers assessing their relatedness to male brood versus female brood, relative to the average worker relatedness asymmetry in other colonies of their popula tion. Workers then adjust the sex ratio in their own interest. This hy pothesis assumes that workers can recognize the sex of the brood in th eir colony and selectively eliminate males. We compared the primary se x ratio (at the egg stage) and secondary sex ratio (reproductive pupae and adults) of colonies in the ant Pheidole pallidula. There was a st rong bimodal distribution of secondary sex ratios, with most colonies producing mainly reproductives of one sex. In contrast, there was no e vidence of a bimodal distribution of primary sex ratios. The proportio n of haploid eggs produced by queens was 0.35 in early spring and decr eased to about 0.1 in summer. Male eggs also were present in virtually all field colonies sampled in July, although eggs laid at this time o f year never give rise to males. All male brood is, therefore, selecti vely eliminated beginning in July and continue to be eliminated throug h the rest of the year. Finally the population sex-ratio investment wa s female-biased. Together, these results are consistent with the hypot hesis that workers control the secondary sex ratio by selectively elim inating male brood in about half the colonies, perhaps those with high relatedness asymmetry.