QUEEN-WORKER CONFLICT OVER SEX-RATIO - A COMPARISON OF PRIMARY AND SECONDARY SEX-RATIOS IN THE ARGENTINE ANT, IRIDOMYRMEX-HUMILIS

We compare the primary sex ratio (proportion o haploid eggs laid by qu eens) and the secondary sex ratio (proportion of male pupae produced) in the Argentine ant Iridomyrmex humilis with the aim of investigating whether workers control the secondary sex ratio by selectively elimin ating male brood. The proportion of haploid eggs produced by queens wa s close to 0.5 in late winter, decreased to less than 0.3 in spring an d summer, and increased again to a value close to 0.5 in fall. Laborat ory experiments indicate that temperture is a proximate factor influen cing the primary sex ratio with a higher proportion of haploid eggs be ing laid at colder temperatures. Production of queen pupae ceased in m id-June, about three weeks before that of male pupae. After this time only worker pupae were produced. During the period of production of se xuals, the proportion of male pupae ranged from 0.30 to 0.38. Outside this period no males were reared although haploid eggs were produced a ll the year round by queens. Workers thus exert a control on the secon dary sex ratio by eliminating a proportion of the male brood during th e period of sexual production and eliminating all the males during the remainder of the cycle. These data are consistent with workers prefer ring a more female-biased sex ratio than queens. The evolutionary sign ificance of the production of male eggs by queens all the year round i s as yet unclear. It may be a mechanism allowing queen replacement in the case of the death of the queens in the colony.