The success of an ant colony depends on the
simultaneous presence of reproducing queens and nonreproducing
workers in a ratio that will maximize colony
growth and reproduction. Despite its presumably crucial
role, queen–worker caste ratios (the ratio of adult queens
to workers) and the factors affecting this variable remain
scarcely studied. Maintaining polygynous pharaoh ant
(Monomorium pharaonis) colonies in the laboratory has
provided us with the opportunity to experimentally manipulate
colony size, one of the key factors that can be expected
to affect colony level queen–worker caste ratios and body
size of eclosing workers, gynes and males. We found that
smaller colonies produced more new queens relative to
workers, and that these queens and workers both tended to
be larger. However, colony size had no effect on the size
of males or on the sex ratio of the individuals reared.
Furthermore, for the first time in a social insect, we confirmed
the general life history prediction by Smith and
Fretwell (Am Nat 108:499–506, 1974) that offspring number
varies more than offspring size. Our findings document a
high level of plasticity in energy allocation toward female
castes and suggest that polygynous species with budding
colonies may adaptively adjust caste ratios to ensure rapid
growth.