A Comparative Analysis of Sex Ratio Investment Parameters in Ants

Abstract

1. Accurate estimates of female-to-male cost ratios are essential for testing theories about kin selection and worker control over sexual investments in ants. However, obtaining such estimates has proved to be notoriously difficult. 2. Comparative data on dry weight, energy content and respiration in ant sexuals were analysed across species, to explore biases following from the use of different female-to-male cost ratios. 3. Two groups of ants, those with claustral and independent colony foundation (without the help of workers ; usually species with single-queen colonies) and those with non-claustral, dependent colony foundation (with the help of workers ; generally species with multiple-queen colonies), showed consistent differences in weight and energetic cost ratios, with only a relatively small range of overlap. 4. The fat accumulation in females, relative to males, during maturation was a clear power function of sexual weight dimorphism when analysed across all ant species, but more like a constant fraction when ant species with independent and dependent colony founding were analysed separately. 5. Female-to-male respiration ratios were always lower than female-to-male dry weight ratios. The adult respiration ratio was a power function of adult sexual weight dimorphism but the pupal respiration ratio was found to be a constant fraction of the pupal dry weight ratio across ant species of both categories. 6. The across-species trends confirmed earlier species-specific findings. Cost ratios expressed as adult female-to-male dry weight ratios underestimate the relative energy content but overestimate the cost of respiration of females, relative to males. 7. A complete female-to-male cost ratio could be obtained for the Argentine ant, Iridomyrmex humilis, and a similar estimate was available for Lasius niger from a previous study. These estimates plus the less complete data for three other ant species agreed reasonably well with an earlier proposed 0.7 power-conversion factor, both for ants with independent and dependent colony founding.