Behav Ecol Sociobiol (2005) 58: 527–533 DOI 10.1007/s00265-005-0964-0 ORIGINAL ARTICLE Ludivine de Menten · Denis Fournier · Colin Brent · Luc Passera · Edward L. Vargo · Serge Aron Dual mechanism of queen influence over sex ratio in the ant Pheidole pallidula Received: 4 January 2005 / Revised: 15 April 2005 / Accepted: 18 April 2005 / Published online: 26 May 2005 C  Springer-Verlag 2005 Abstract Social Hymenoptera are general models for the study of parent-offspring conflict over sex ratio, be- cause queens and workers frequently have different reproductive optima. The ant Pheidole pallidula shows a split distribution of sex ratios with most of the colonies producing reproductives of a single sex. Sex ratio spe- cialization is tightly associated with the breeding system, with single-queen (monogynous) colonies producing male- biased brood and multiple-queen (polygynous) colonies female-biased brood. Here, we show that this sex special- ization is primarily determined by the queen’s influence over colony sex ratio. Queens from monogynous colonies produce a significantly more male-biased primary sex ratio than queens from polygynous colonies. Moreover, queens from monogynous colonies produce a significantly lower proportion of diploid eggs that develop into queens and this is associated with lower rate of juvenile hormone (JH) production compared to queens from polygynous colonies. These results indicate that queens regulate colony sex ratio in two complementary ways: by determining the proportion of female eggs laid and by hormonally biasing the devel- opment of female eggs into either a worker or reproductive form. This is the first time that such a dual system of queen influence over colony sex ratio is identified in an ant. Communicated by L. Sundstr ¨ om L. de Menten · D. Fournier () · S. Aron Behavioral and Evolutionary Ecology, Universit´ e Libre de Bruxelles CP 160/12, Avenue F.D. Roosevelt, 50, B-1050 Brussels, Belgium e-mail: Denis.Fournier@ulb.ac.be Tel.: +32-2-6504497 Fax: +32-2-6502445 C. Brent · E. L. Vargo Department of Entomology, North Carolina State University, Box 7613, Raleigh, NC 27695-7613, USA L. Passera Centre de Recherches sur la Cognition Animale, CNRS-Universit´ e Paul Sabatier, 118, Route de Narbonne, F-31062 Toulouse, France Keywords Conflicts . Juvenile hormone . Kin selection . Microsatellites . Primary sex ratio . Social hymenoptera . Split sex ratio Introduction Studies of queen-worker conflicts over sex ratio in social Hymenoptera have emerged as prime tests of kin selection and inclusive fitness theories (Hamilton 1964a; Hamilton 1964b). The haplodiploid sex-determining system of Hy- menoptera (males develop from unfertilized-haploid eggs, females from fertilized-diploid eggs) produces asymme- tries in genetic relatedness among colony members, with workers being more closely related to females than to males. For instance, in a colony headed by a single once- mated queen, workers are three times more related to sis- ters (‘life-for-life relatedness’: r=0.75) than to brothers (r=0.25). Workers, who reproduce indirectly by rearing the queen’s offspring, therefore increase their inclusive fitness by producing a 3:1 female-biased sex ratio among sex- ual offspring (Trivers and Hare 1976). Queens, however, are equally related to daughters and sons so that natural selection should act on queens to favor an equal invest- ment in male and female sexuals at the population level (Trivers and Hare 1976; Bourke and Franks 1995; Crozier and Pamilo 1996; Beekman and Ratnieks 2003). Although all sex ratios at the level of the colony are equally fit in large populations at equilibrium (Kolman 1960), queen-worker conflict should occur in every colony, leading queens in all colonies to produce more males than workers would pre- fer. The resulting conflict has likely promoted the evolution of behavioral and physiological mechanisms that provide both the workers and queens some control over sex ratio. Although recent studies have elucidated some of the control mechanisms employed by ant workers, the regu- lation of brood sex ratio by queens is still poorly under- stood. Intraspecific studies show that workers bias sex ra- tios adaptively by rearing mostly females in colonies with high relatedness asymmetry (e.g. colonies headed by one or