Sex Ratio Strategies in Formica Ants

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Abstract
Colony and population sex ratios were studied in 13 spp. of Formica ants [Formica aquilonia, F. polyctena, F. lugubris, F. pratensis, F. truncorum, F. uralensis, F. sanguinea, F. exsecta, F. fusca, F. pressilabris, F. fusca, F. cinerea and F. rufibarbis] in southern Finland. The data are based on material mainly pupae, collected from nests. Stability of both colony and population sex ratios, and the similarity of sex ratios in conspecific populations living in similar environments, suggest that the sex ratios can be considered as adaptive strategies produced by genetic evolution. From the 23 population samples examined, 12 disagreed with the assumption of equal parental investment in the 2 sexes, there being departures in some in favor of males and in others of females. The most obvious explanation for these deviations is polydomy (multinest associations) leading to strongly male-biased ratios in 3 spp. The correlations between sex ratios and the degree of polygyny and levels of intranest genetic relatedness between the offspring and workers caring for them, and the patterns of variance of colony sex ratios are examined. The data indicate that in the mound-building red wood ants of the F. rufa group, which produce sexuals as the 1st offspring in early spring and whose gynes affect the caste determination by laying eggs predisposed to develop as sexuals, the gynes probably control the sex ratios. In the other species which produce sexuals and new workers in mid-summer, the sex ratio is controlled, or at least greatly influenced, by workers. Workers probably do not normally have offspring, but may produced males in orphaned nests.