Size-dimorphism in the queens of the North American ant Leptothorax rugatulus (emery)

With recent findings of an unexpected variability in the reproductive behavior of ant sexuals, their morphology has become an area of major evolutionary interest (Heinze and Tsuji, 1995). Here we report on the occurrence of two queen morphs in Leptothorax rugatulus (Hym., Formicidae): Microgynes (small queens), exceeding worker-size only marginally, and macrogynes, which are, typically for the subgenus Myrafant, about twice as big as their workers. The frequency distribution of queen-size is clearly bimodal, in contrast to worker- and male-size. The average size of queens is highly correlated with the size of daughters in field-collected colonies, whereas within colonies no correlation between the average queen-size and the size of workers or males exists. This gives additional support that size-dimorphism is due to a specific, transmissible size reduction of the microgynes which could be based on genetics, the environment or both. This reduction is quasi-isometric, with a slightly smaller thorax-to-head ratio in microgynes, and scanning electron microscopy does not reveal any significant degeneration of the pterothorax, ocelli or number of ommatidia. The frequency of microgynes at different sample sites is highly variable, correlating well with the prevailing social structure in the respective subpopulations. Indeed, the majority of macrogynes is found in monogynous colonies, while microgynes abound in polygynous ones, which is strong evidence for an alternative dispersal tactic. However, the expected correlation to altitude or latitude was not found and further investigations are needed to reveal proximate and ultimate causes of this prevalent polymorphism between two types of female ant reproductives.