Measuring Steroid Hormones in Avian Eggs

Avian eggs contain substantial levels of various hormones of maternal origin and have recently received a lot of interest, mainly from behavioral ecologists. These studies strongly depend on the measurement of egg hormone levels, but the method of measuring these levels has received little attention. This paper describes the sampling, extraction, and assay of hormones in avian eggs and related methodological problems. The method of sampling is important because hormones are not homogeneously distributed within the egg, and after onset of embryonic development their levels may decrease and increase due to changes in egg structure and secretion or uptake of hormones by the embryo. The extraction of hormones from the yolk and chromatographic separation of different hormones for immunoassays can strongly influence the results because such procedures remove interfering substances such as proteins, lipids, and other hormones and their metabolites, which can cross-react with the antiserum used. Finally, the assay itself needs more validation than many studies report, especially with respect to the accuracy and specificity of the hormone measurements. We conclude that the addressed issues need more attention for the correct interpretation of differences in hormone levels within and between studies.