Yolk corticosterone and progesterone levels in Greater Rhea (Rhea americana) eggs vary in a changing social environment

Abstract

Maternal hormones in avian egg yolks may signal and prepare offspring for the prevailing conditions. However, this adjustment requires some degree of flexibility in regulating yolk hormone deposition. The Greater Rhea (Rhea americana) has a particular mating system that combines mixed polygyny and polyandry, communal nesting, and exclusive paternal care of chicks. In this species, we previously found that yolk hormone deposition varies among eggs of different captive populations and could influence chicks' physiology and behavior. However, it is still unknown whether females can modify yolk hormone deposition in a changing social environment. Using a captive population of Greater Rheas, in this study, we quantified yolk hormone levels before and after a reduction in the number of females present in the population. We found that females deposited on average higher yolk corticosterone and lower yolk progesterone after the change in their social environment. Since corticosterone deposited into the yolk comes exclusively from the female's plasma, our results suggest that females had, on average, higher plasma corticosterone levels. The change in the number of females may increase the events of male-male competitions, courtships, and matings, leading to an increase of corticosterone in the females' plasma and then into their eggs. Since we previously found that higher yolk corticosterone and lower yolk progesterone were associated with the production of chicks that have an attenuated stress response, the present study results suggest that yolk hormone deposition is mediated by flexible mechanisms that could adjust development to the prevailing conditions.

The Effect of Maternal Hormones on the Avian Offspring: A Review

“…For example, using lipids supplements during avian feeding may have an effect on the hormone levels in eggs [12]. Environmental Factors: Stressors, temperature changes, and other environmental elements may have an impact on a bird's hormonal balance, which in turn may have an impact on the hormone levels in eggs [13]. The hen's age has also an impact on the hormones that are deposited in her eggs.…”

“…According to Hsu et al [26], precocial birds deposited more total thyroid hormones (T3 and T4) in their yolks than in altricial species. According to Della Costa et al [13], the development of chicks with a reduced stress was linked to higher yolk corticosterone and lower yolk progesterone levels.…”