Maternally derived hormones are known to influence the growth and development of offspring. The differential deposition of these maternally derived hormones into egg yolk is one way by which females can alter their chicks' growth or survival. Yolk constituents, especially testosterone, have been described for a wide variety of species. However, few studies have focused on multiple maternally derived hormones regulated by independent axes in the endocrine system, and those studies that have, mainly focus on corticosterone and testosterone. In this study, within and among female variation in testosterone and triiodothyronine concentrations in egg yolks were measured in three free-living Franklin's Gull (Leucophaeus pipixcan) populations. Testosterone (T), but not triiodothyronine (T3), concentrations increase within a female's clutch such that eggs laid later in the clutch have increasingly higher concentrations (from a mean of 1.97 log pg T/mg yolk for the first egg to 2.98 log pg T/mg yolk for the third egg compared to a mean of 0.14 log pg T3/mg yolk for the first egg to 0.31 log pg T3/mg yolk for the third egg). Both testosterone and triiodothyronine concentrations increase among females across the breeding season such that eggs laid later in the season have significantly higher hormone concentrations (mean of 2.37 log pg T/mg yolk and mean of -0.03 log pg T3/mg yolk) than eggs laid early in the season (mean of 1.95 log pg T/mg yolk and mean of -0.17 log pg T3/mg yolk).
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Vol. 42 • No. 3