Split Sex Ratios in a Multiple-Queen Ant Population

Split sex ratio theory is an important extension of sex allocation theory. It suggests that colony sex ratios in social insects vary because workers control sex allocation and respond to variations in their comparative relatedness with females and males (relatedness asymmetry). In a population of the ant Leptothorax acervorum, 21 monogynous (single-queen) colonies produced a female-biased sex ratio (62% females), and 24 polygynous (multiple-queen) colonies produced a male-biased sex ratio (28% females). Within the polygynous colonies, queen number did not affect sex ratio (with colony productivity statistically controlled). As colony productivity rose, the sex ratio either did not change (monogynous colonies) or became more male-biased (polygynous colonies). The fraction invested in sexuals rose with increasing colony size and productivity in monogynous but not in polygynous colonies, which invested less in sexuals. These findings suggest that split sex ratios in L. acervorum stem from two processes. The first is workers’ response to the variation in relatedness asymmetry caused by variable queen numbers. The second is sex-ratio compensation by monogynous colonies for male-biased production in polygynous colonies. This arises because polygynous colonies reproduce, partly, by colony budding and so have daughter colonies subject to local resource competition.