Stenamma debile (Hymenoptera, Formicidae): Facultative polygyny, and an extreme 1997 sex ratio

Summary: Monogynous and a high proportion of truly polygynous colonies of Stenamma debile were found in populations of this small and inconspicuous ant in southern Germany. Moreover, dissections revealed that queens of different age may co-occur in one nest. Thus polygyny is not only an outcome of pleometrosis. Probably young queens sometimes join the mother colonies.¶Practically no female sexuals were reared in the 1997 brood of 29 colonies, thus a population-level sex ratio of &gif1; was recorded. It is likely that such an extreme sex ratio is atypical for this species, suggesting substantial year-to-year variation in sex ratios. Variation of sex ratios over several years has been reported for other ant species, too, but rarely to such an extent. The reason for the failure of queen production in 1997 was perhaps an unusual sequence of warm and cold periods in the spring which may have caused a re-determination of the queen-destined hibernated larvae to become workers, while male larvae were reared in ordinary numbers.