Changes in the sex ratio of the breeding stock modify hormone levels in rhea eggs: its usefulness in captive management.

DELLA COSTA N. S.; MARIN R. H.; MARTELLA M. B.; NAVARRO J. L.

Paracas

Congreso; XIV Congreso Internacional de manejo de fauna silvestre de la amazonia y Latinoamérica; 2021

Avian eggs contain different steroids of maternal origin that could modify offspring phenotype. It has been therefore proposed that these hormones could be mediating an adaptive maternal effect that allows a flexible adjustment of development to the prevailing conditions. However, this adjustment requires some degree of flexibility in regulating yolk hormone deposition according to the offspring?s environmental conditions. In this study, we worked with the Greater rhea (Rhea americana), a ratite species endemic to South America that is classified as ?Near Threatened? by the International Union for Conservation of Nature and Natural Resources (IUCN). Its mating system combines simultaneous polygyny and simultaneous and serial polyandry, with a high degree of promiscuity. As the male constructs a communal nest, incubates the eggs and cares for the precocial chicks, the maternal influence appears to end once the eggs have been laid. This fact makes Greater rheas an excellent model to reveal the effects of yolk hormone deposition on embryo development, without the interference of other maternal effects during incubation and after hatching. In this species, we previously found that yolk hormone deposition varies among eggs of different captive populations and could influence the chicks? physiology and behaviour. However, it is unknown if females could modify yolk hormone deposition in a changing environment. Using a captive population of Greater rheas, we quantified yolk hormone levels before and afterchanging the breeding sex ratio from 1:1 to one female for every two males. We found that females deposited on average higher yolk corticosterone and lower yolk progesterone levels after the change in the sex ratio. Since corticosterone deposited into the yolk comes exclusively from the female?s plasma, our results suggest that females have on average higher plasma levels of this hormone. The change in the sex ratio may increase the events in which females were exposed to male-male competitions, courtships, and matings, leading to corticosterone levels increasing in their plasma and being transferred to their eggs. Previous studies showed that higher yolk corticosterone and lower yolk progesterone had been associated with the production of chicks that have attenuated stress responses. Therefore, in a captive environment perceived as stressful by the females, where individuals cannot escape, an attenuated stress response in the chicks could improve their survival chances. Our results suggest that the management of the sex ratio can have implications not only in the Greater rhea females but also in their offspring?s quality.