Reproductive strategy of the slave antFormica podzolica relative to raiding efficiency of enslaver species

Abstract

Formica podzolica serves as host to slave-making ants in North America. We propose thatF. podzolica may respond to slavery by two alternative colony-growth and reproductive strategies depending on the raiding ability of the slavemaker: (1) Rapid colony growth at the expense of producing sexuals to a stage where raiding by unspecialized, facultative slavemakers, capable of exploiting only small colonies, becomes unlikely owing to a strong work force and (2) Early production of sexual offspring at the cost of colony growth to secure some sexual production in an environment with specialized obligate enslavers, capable of raiding large colonies. We tested the strategies by excavating 30 small to moderately large mounds ofF. podzolica and measured reproductive parameters of colonies in relation to mound size, worker number, and worker size. Mound area predicted worker number satisfactorily. Worker number correlated significantly with worker head width and with number of worker and sexual offspring. With a growing work force, the proportion of sexual offspring increased in the total offspring. Two thirds of the colonies producing sexuals emitted single sex, sex being independent of colony size. Some of the large colonies produced both sexes with a strong bias toward either sex. The unweighted population-level sex ratio did not differ from even, being 0.52 (numerical) or 0.54 (biomass). Very large mounds (not excavated) had small workers and highly male-biased sex ratios, probably owing to energy constraints set by central-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 explanation to the ontogeny pattern.