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

DELLA COSTA, N.; MARIN, R. H.; MARTELLA M.B.; NAVARRO JL.

Congreso; 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. Ithas been therefore proposed that these hormones could be mediating an adaptive maternaleffect that allows a flexible adjustment of development to the prevailing conditions. However,this adjustment requires some degree of flexibility in regulating yolk hormone depositionaccording to the offspring´s environmental conditions. In this study, we worked with the Greaterrhea (Rhea americana), a ratite species endemic to South America that is classified as “NearThreatened” by the International Union for Conservation of Nature and Natural Resources(IUCN). Its mating system combines simultaneous polygyny and simultaneous and serialpolyandry, 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 endonce the eggs have been laid.Natalia S. Della278This fact makes Greater rheas an excellent model to reveal the effects of yolk hormonedeposition on embryo development, without the interference of other maternal effects duringincubation and after hatching. In this species, we previously found that yolk hormone depositionvaries among eggs of different captive populations and could influence the chicks´ physiologyand behaviour. However, it is unknown if females could modify yolk hormone deposition in achanging environment. Using a captive population of Greater rheas, we quantified yolkhormone levels before and after changing the breeding sex ratio from 1:1 to one female forevery two males. We found that females deposited on average higher yolk corticosterone andlower yolk progesterone levels after the change in the sex ratio. Since corticosterone depositedinto the yolk comes exclusively from the female´s plasma, our results suggest that females haveon average higher plasma levels of this hormone. The change in the sex ratio may increase theevents 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 beenassociated with the production of chicks that have attenuated stress responses. Therefore, in acaptive environment perceived as stressful by the females, where individuals cannot escape,an attenuated stress response in the chicks could improve their survival chances. Our resultssuggest that the management of the sex ratio can have implications not only in the Greater rheafemales but also in their offspring´s quality.