Quantification of 27 yolk steroid hormones in seven shrubland bird species: interspecific patterns of hormone deposition and links to life history, development, and predation risk

Steroid hormones play critical organizational and activational roles during vertebrate development, impacting everything from sexual differentiation to metabolic activity. For oviparous species such as birds, these hormones are transferred from female to egg during follicle maturation, and differences in relative and absolute concentrations of the steroid hormones may reflect differences in life history, developmental, and ecological conditions. Prior work on yolk steroid hormones has focused on a handful of candidate hormones (e.g., testosterone, androstenedione, and corticosterone), but we used high-performance liquid chromatography with tandem mass spectroscopy (LC–MS–MS) to quantify 27 yolk steroids from the eggs of seven shrubland bird species (American Robin, Turdus migratorius Linnaeus, 1766; Brown-headed Cowbird, Molothrus ater (Boddaert, 1783); Brown Thrasher, Toxostoma rufum (Linnaeus, 1758); Eastern Towhee, Pipilo erythrophthalmus (Linnaeus, 1758); Field Sparrow, Spizella pusilla (A. Wilson, 1810); Gray Catbird, Dumetella carolinensis (Linnaeus, 1766); Northern Cardinal, Cardinalis cardinalis (Linnaeus, 1758)). In addition to comparing steroid profiles across species, we conducted exploratory analyses to determine how the hormones clustered using a principal component (PC) approach and if PCs were correlated with aspects of egg resources (relative egg size, proportion yolk), life-history traits (embryonic and nestling development speed), and nest-predation risk (daily survival rate (DSR)). We documented substantial interspecific variation in both absolute and proportional endocrine profiles. PCAs indicated that glucocorticoids generally clustered together (PC1), but other classes of steroids did not. PC2 and PC3 strongly covaried with egg resources, DSR, and development speed, suggesting that they reflect adaptive patterns of maternal hormone deposition.

Divergent selection for inherent fearfulness leads to divergent yolk steroid levels in quail

Avian yolk steroid levels may vary according to maternal condition or environmental factors, causing epigenetic modulation of offspring phenotype. Here we test whether artificial selection based on divergent duration of tonic immobility (TI) in Japanese quail (i.e., divergent inherent fearfulness) is correlated with divergent levels of yolk steroids. We analysed yolk steroid levels and egg quality in quail selected for either long or short duration of TI. Yolk steroids, i.e., immunoreactive progesterone and androstenedione, were found to be significantly less concentrated in eggs of the high inherent fearfulness line compared to eggs of the low inherent fearfulness line. A similar trend was found with testosterone levels. Larger eggs with lighter eggshell were also found in more fearful quail. Hence, the selection for divergent fearfulness has led to correlated changes in yolk steroid levels and egg quality. These data suggest that hormones of maternal origin, egg quality and genetic background may all contribute to line differences in phenotype. A modulation of progesterone concentrations by selection for behaviour in egg yolk is reported here for the first time. Although the effect of this hormone on avian embryos remains unknown, we argue it may have significant effects on phenotypic outcome.