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.