REPRODUCTIVE STRATEGY OF THE SLAVE ANT FORMICA-PODZOLICA RELATIVE TO RAIDING EFFICIENCY OF ENSLAVER SPECIES

Formica podzolica serves as host to slave-making ants in North America . We propose that F. podzolica may respond to slavery by two alternati ve colony-growth and reproductive strategies depending on the raiding ability of the slavemaker: (1) Rapid colony growth at the expense of p roducing sexuals to a stage where raiding by unspecialized, facultativ e slavemakers, capable of exploiting only small colonies, becomes unli kely owing to a strong work force and (2) Early production of sexual o ffspring at the cost of colony growth to secure some sexual production in an environment with specialized obligate enslavers, capable of rai ding large colonies. We tested the strategies by excavating 30 small t o moderately large mounds of F. podzolica and measured reproductive pa rameters of colonies in relation to mound size, worker number, and wor ker size. Mound area predicted worker number satisfactorily. Worker nu mber correlated significantly with worker head width and with number o f worker and sexual offspring. With a growing work force, the proporti on of sexual offspring increased in the total offspring. Two thirds of the colonies producing sexuals emitted single sex, sex being independ ent of colony size. Some of the large colonies produced both sexes wit h a strong bias toward either sex. The unweighted population-level sex ratio did not differ from even, being 0.52 (numerical) or 0.54 (bioma ss). Very large mounds (not excavated) had small workers and highly ma le-biased sex ratios, probably owing to energy constraints set by cent ral-place foraging. Population-level colony ontogeny data did not fit either one of the suggested strategies, but imply a mixture of the two . We discuss an alternative, still untested raid-independent explanati on to the ontogeny pattern.