Progesterone is the most concentrated maternal yolk steroid characterized to date in birds; however, no information about it is available in ratite eggs. We collected freshly laid eggs from zoo-housed Greater Rhea females (Rhea americana) bred under similar rearing conditions during two breeding seasons to characterize concentration and distribution of maternal yolk progesterone. After high-performance liquid chromatography analysis, yolk hormone was measured using a commercial electrochemiluminescence immunoassay. Progesterone concentrations were found to vary significantly among the yolk layers, supporting a follicular origin for this steroid in Greater Rhea eggs. Additionally, highly similar mean absolute yolk progesterone concentrations were detected between 2013 and 2015 breeding seasons (1,332.98 ± 82.59 and 1,313.59 ± 85.19 ng/g, respectively). These values are also comparable to those found in some domestic carinate species. Findings suggest that at population level, when rearing conditions are similar, mean absolute yolk maternal progesterone concentrations also appear bounded. Future research on the factors and mechanisms that regulate progesterone deposition in Greater Rhea eggs is needed to better understand whether its levels depend on different rearing conditions.
Keywords: Greater Rhea; ratite; steroid hormone; yolk progesterone.
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