Non-invasive monitoring of adrenocortical activity in Greater Rhea (Rhea americana) by measuring fecal glucocorticoid metabolites

Abstract

The Greater Rhea (Rhea americana) is a near-threatened species. Wild populations are affected by human activities, such as illegal hunting, egg harvesting and conversion of natural habitats to croplands. An indicator of disturbances is the increase of the glucocorticoid corticosterone, a stress hormone that helps to cope with life-threatening situations. Here, we evaluate and characterize adrenocortical function in the Greater Rhea by validating the use of a radioimmunoassay (RIA) to assess fecal glucocorticoid metabolite concentrations, and by comparing the time course of plasma corticosterone and fecal immunoreactive glucocorticoid metabolite excretion. An adrenocorticotropic hormone (ACTH) challenge was performed and serial blood and fecal (cecal and rectal) samples were collected and analyzed by a corticosterone RIA. High-performance liquid chromatography (HPLC) was employed to characterize fecal immunoreactive glucocorticoid metabolites. HPLC analysis revealed the presence of two immunoreactive glucocorticoid metabolites in rectal and cecal Rhea feces. Greater Rheas responded to the ACTH challenge with a 30-fold increase in plasma corticosterone concentration 2 h post-injection, and a 30- to 40-fold increase in concentrations of glucocorticoid metabolites from rectal feces 4–6 h post-injection. In cecal feces, ACTH challenge produced an 11-fold increase. Therefore, collection of rectal feces would be more appropriated to ensure detection of minor increases in adrenocortical activity in this species. The adrenocortical response to ACTH found in Rhea was higher than what is usually observed in other birds and might be associated with their flightless condition and the use of running as an antipredator strategy.