Yolk immunoreactive corticosterone does not relate with maternal fecal glucocorticoids in a captive population of Greater Rheas ( Rhea americana )

Maternal corticosterone in avian egg yolks may signal and prepare offspring for the prevailing conditions. However, this adjustment requires flexible mechanisms of regulation that lead to the absence of a correlation between hormone levels in the mother and the yolk. In this study, we examined whether yolk immunoreactive corticosterone and maternal glucocorticoid levels are related in the Greater Rhea ( Rhea americana ), a ratite species with a communal nesting system. During the intermediate laying period of the season, we carried out daily nest-observations to collect those eggs in which the laying female was identified. We also collected fresh fecal samples from the identified laying females to quantify their fecal glucocorticoid metabolite (FGM) levels during the yolk formation period (7 days before egg-laying). Yolk immunoreactive corticosterone and FGM levels were measured using a corticosterone 125 radioimmunoassay kit, previously validated for the species. No significant relationships were found between female FGM and yolk immunoreactive corticosterone. Although futures studies are needed to elucidate the underlying mechanisms, our results suggest that Greater Rhea female may be able to control the distribution of corticosterone into their yolks, supporting the idea that this hormone may be mediating an adaptive maternal effect.