Nest predation risk and deposition of yolk steroids in a cavity nesting songbird: an experimental test

Extraembryonic metabolism of corticosterone protects against effects of exposure

When females experience stress during reproduction, developing embryos can be exposed to elevated levels of glucocorticoids, which can permanently affect offspring development, physiology, and behavior. However, the embryo can regulate exposure to glucocorticoids. In placental species, the placenta regulates embryonic exposure to maternal steroids via metabolism. In a comparable way, recent evidence has shown the extraembryonic membranes of avian species also regulate embryonic exposure to a number of maternal steroids deposited in the yolk via metabolism early in development. However, despite the known effects of embryonic exposure to glucocorticoids, it is not yet understood how glucocorticoids are metabolized early in development. To address this knowledge gap, we injected corticosterone into freshly laid chicken (Gallus gallus) eggs and identified corticosterone metabolites, located metabolomic enzyme transcript expression, tracked metabolomic enzyme transcript expression during the first six days of development, and determined the effect of corticosterone and metabolites on embryonic survival. We found that yolk corticosterone was metabolized before day four of development into two metabolites: 5β-corticosterone and 20β-corticosterone. The enzymes, AKR1D1 and CBR1 respectively, were expressed in the extraembryonic membranes. Expression was dynamic during early development, peaking on day two of development. Finally, we found that corticosterone exposure is lethal to the embryos, yet exposure to the metabolites is not, suggesting that metabolism protects the embryo. Ultimately, we show that the extraembryonic membranes of avian species actively regulate their endocrine environment very early in development.

Female red-winged blackbirds (Agelaius phoeniceus) do not alter nest site selection, maternal programming, or hormone-mediated maternal effects in response to perceived nest predation or brood parasitism risk

Predation or brood parasitism risks can change the behaviors and reproductive decisions in many parental animals. For oviparous species, mothers can mitigate their reproductive success in at least three ways: (1) by avoiding nest sites with high predation or parasitism risks, (2) through hormonal maternal effects that developmentally prime offspring for survival in risky environments, or (3) by investing less in reproduction when predation or parasitism risks are high. Here, we tested if perceived predation and parasitism risks can induce any of these behavioral or physiological responses by exposing female red-winged blackbirds (Agelaius phoeniceus) to playbacks of two major nest threats, a predator (Cooper's hawk, Accipiter cooperii) and an obligate brood parasite (brown-headed cowbird; Molothrus ater), as well as two controls (harmless Eastern meadowlark, Sturnella magna; and silence). We found that female blackbirds did not avoid nesting at sites treated with predator or brood parasite playbacks, nor were females more likely to abandon nesting attempts at these sites. Egg size and yolk hormone profiles, which are common proxies for maternal investment in oviparous species, were statistically similar across treatment sites. Instead, we found intraclutch variation in yolk steroid hormone profiles: concentrations of three progestogens (pregnanedione, 17α-hydroxypregnenolone, and deoxycorticosterone) and two androgens (testosterone and androstenedione) were higher in third-laid than first-laid eggs. Our study largely confirms previous findings of consistent intraclutch yolk hormone variation in this species, in birds in general, and in other oviparous lineages, but uniquely reports on several yolk steroid hormones largely overlooked in the literature on hormone-mediated maternal effects.

New insights into the relationships between egg maternal components: the interplays between albumen steroid hormones, proteins and eggshell protoporphyrin

Recent studies have shown that the egg yolk maternal components, which are a mixture of substances that can affect the developing embryo, do not act separately but are interconnected and co-adapted. Surprisingly, no study to date has focused on the associations between maternally derived albumen steroids and albumen and eggshell compounds with pleiotropic effects. Eggshell pigment protoporphyrin (PROTO IX) should provide primary antimicrobial protection for eggs, but as a proven pro-oxidant, it may compromise female fitness. Abundant albumen proteins ovotransferrin (OVOTR) and lysozyme (LSM) have been shown to have antimicrobial, antioxidant, immunoregulatory and growth-regulatory roles. To investigate associations between albumen steroids and OVOTR, LSM and eggshell cuticle PROTO IX, we used chicken eggs with differently pigmented eggshells. We found that albumen steroid hormones were strongly intercorrelated. In addition, we revealed that albumen LSM and testosterone (T) were positively associated, while a negative association was found between albumen LSM and pregnenolone (P5). Eggshell cuticle PROTO IX was negatively associated with the concentration of albumen 17α-hydroxypregnenolone (17-OHP5). Finally, of all the hormones tested, only the concentration of albumen 17-OHP5 correlated negatively with egg volume and varied with eggshell colour and chicken breed. Although experimental evidence for the effect of maternal albumen steroids on avian developing embryo is still scarce, our study is the first to highlight co-variation and potential co-adjustment of maternally derived albumen steroids, proteins and eggshell cuticle pigment suggesting similar allocation mechanisms known for yolk maternal compounds with the potential to influence the avian embryo and offspring phenotype.