| Summary: | Sex allocation theory predicts parents should adjust their investment in male and female offspring in a way that increases parental fitness. This has been shown in several species and selective contexts. Yet, seasonal sex ratio variation within species and its underlying causes are poorly understood. Here we study sex allocation variation in the wood ant Formica pratensis. This species displays conflict over colony sex ratio as workers and queens prefer different investment in male and female offspring, owing to haplodiploidy and relatedness asymmetries. It is unique among Formica ants because it produces two separate sexual offspring cohorts per season. We predict sex ratios to be closer to queen optimum in the early cohort but more female-biased and closer to worker optimum in the later one. This is because the power of workers to manipulate colony sex ratio varies seasonally with the availability of diploid eggs. Consistently more female-biased sex ratios in the later offspring cohort over a three-year sampling period from 93 colonies clearly support our prediction. The resulting seasonal alternation of sex ratios between queen and worker optima is a novel demonstration how understanding constraints of sex ratio adjustment increases our ability to predict sex ratio variation. Formica pratensis sex allocationNumerical sex allocation data from Formica pratensis colonies from Southern Finland, 2007-2009. Data collected from two cohorts each year, with different sex allocation strategies in the two cohorts. Columns: nest = nest ID, cluster = ID of the colony (for monodomous nests this equals the nest ID, for polydomous colonies several nests are included in the same colony, based on their close physical proximity in the field), year = year, cohort: 1 = early, 2 = late, totalm = number of males in the sample, totalf = number of females in the sample, propmales = totalm /(totalm + totalf), r = relatedness among nestmate workers, calculated based on the genotypes in the pratensis_genotypes.txt file. Note that there ...
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