Temporal Variability in the Sex Ratio of a Non-Social Bee, Osmia Lignaria Propinqua: Extrinsic Determination or the Tracking of an Optimum?

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Abstract
Six years of data on the sex ratio of O. lignaria obtained from several sites in northern Utah and southern Idaho [USA], were used to test Fisher''s theory of an equilibrium sex ratio. Several years of data are necessary to adequately test the predictions of sex-ratio theory. There was considerable variability in the sex ratio for both between-site within-year comparisons and for within-site between-year comparisons. The sex ratio appeared sensitive to both site-specific factors and to year-year changes in external agents such as climate. Despite this variability, patterns consistent with Fisher''s theory were evident. The sex ratio at all sites changed from a significant male bias in 1973 to a significant female bias in 1975 and subsequently returned to the area of equilibrium. Data from another study are cited to support the contention that variation in the sex of offspring produced by adult females has a genetic basis. The results are interpreted as support for Fisher''s theory.