QUEEN NUMBER INFLUENCES THE PRIMARY SEX-RATIO IN THE ARGENTINE ANT, LINEPITHEMA HUMILE (=IRIDOMYRMEX HUMILIS)

Hymenoptera are characterized by a haplodiploid sex determination syst em where females are diploid and males haploid. Because females can de cide whether they fertilize their eggs they have control over the sex of their offspring. Females are thus expected to lay haploid and diplo id eggs in a ratio that maximizes their fitness. A surprising finding of recent studies on the primary sex ratio of ants has been that queen s lay male eggs outside the time of production of sexuals, a period du ring which all haploid eggs are usually destroyed by workers. It has b een suggested that the production of a high proportion of haploid eggs by queens might be a mechanism to achieve higher reproductive success when the colony loses all its queens and rears new sexuals from the b rood present within the colony. Here it is shown that, as predicted by this hypothesis, the proportion of haploid eggs laid by Linepithema h umile queens is higher in colonies containing several queens than in s ingle-queen colonies. Because L. humile sexuals mate in the nest, the females that produce a higher proportion of haploid eggs should have h igher fitness when the colony sex ratio of the brood is female-biased and sons of several queens compete to fertilize the new queens produce d. (C) 1996 The Association for the Study of Animal Behaviour