SEX INVESTMENT RATIOS IN MONOGYNE AND POLYGYNE POPULATIONS OF THE FIRE ANT SOLENOPSIS-INVICTA

Both monogyne (single queen per colony) and polygyne (multiple queens per colony) populations of the fire ant Solenopsis invicta are good su bjects for tests of kin selection theory because their genetic and rep roductive attributes are well-characterized, permitting quantitative p redictions about the degree to which sex investment ratios should be f emale-biased if workers and not queens control reproductive allocation . In the study populations, an investment ratio of 3 females:1 male is predicted (a proportional investment in females of 0.75) in the monog yne form, whereas a proportional investment in females between 0.637 a nd 0.740 is expected in the polygyne form. To test these predictions, colonies from a single population of each social form were collected a nd censused during three different seasons. Consistent with their alte rnative modes of colony founding, monogyne colonies invested more in r eproduction (sexual production) and less in growth/maintenance (worker production) than did the polygyne colonies. Overall, the sex investme nt ratios were female-biased in both forms, although there was conside rable seasonal variation. After adjusting for sex-specific energetic c osts, the proportional investment in females was 0.607 in the monogyne population, a value in between those expected under complete control by either the queen or the workers. However, when combined with data f rom four other previously studied monogyne populations in the U.S.A., the mean investment ratio did not differ significantly from the value predicted if workers have exclusive control. In the polygyne populatio n, the proportional investment in females of 0.616 was consistent with the level of female bias expected under partial to complete worker co ntrol, although the potential influence of two confounding factors - p ossible contact with monogyne colonies and the preponderance of steril e diploid males - weakens this conclusion somewhat. Taken as a whole, the sex investment ratios of monogyne and polygyne populations of S. i nvicta are consistent with at least partial worker control. Of several ultimate and proximate explanations that have been proposed to explai n inter-colonial variation in the sex investment ratio, only the effec t of the primary sex ratio (female-determined eggs:male-determined egg s) laid by the queen appears to account for the observed variation amo ng monogyne colonies. In the polygyne population, there is limited sup port for the hypothesis that greater resource abundance favors investm ent in females.