Seasonal changes in plasma levels of sex hormones in the greater Rhea (Rhea americana), a South American Ratite with a complex mating system.

Diego J Valdez,Natalia S Della Costa,Alvina Lèche,Cristian Hansen,Marilina Vera Cortez,Mónica B Martella,Joaquín L Navarro,Paul A Bartell

doi:10.1371/journal.pone.0097334

Diego J Valdez, Natalia S Della Costa + Show 6 more

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https://doi.org/10.1371/journal.pone.0097334

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Abstract

Seasonal rhythm in sex hormones has been extensively studied in birds, as well as its relationship with the type of mating system. The Greater Rhea (Rhea americana), a South American ratite species, reproduces seasonally and has a complex mating system: female-defense polygyny and sequential polyandry. The present study aimed at analyzing the endocrine basis of reproduction in this species and its relationship with its mating system. We used HPLC and electrochemiluminescence techniques to identify and measure plasma testosterone and estradiol levels. Annual oscillations in sex hormones, testosterone and estradiol, in adult males and females were observed. Lower levels of these hormones were exhibited during the non reproductive season (February to July), whereas their maximum values were reached in September for males and November-December for females. These fluctuations reflect the seasonal changes in gonadal function. By contrast, no significant sex hormones oscillations were observed in juvenile males and females (negative control of seasonal changes). Greater rheas maintain high testosterone and estradiol levels throughout the reproductive period. The high testosterone levels during incubation and chick rearing did not inhibit parental behavior in males, which appears not to conform to the “Challenge Hypothesis”. In females, the high estradiol levels throughout the reproductive season would be needed to sustain their long egg-laying period.

Highlights

The occurrence of seasonal oscillations in both plasma and fecal levels of sex hormones has been widely reported in birds
The seasonal pattern of testosterone in males has been related to mating systems for different species in the context of ‘‘The Challenge Hypothesis’’ [3], which postulates that high testosterone levels throughout the breeding season inhibit parental behavior in polygamous birds
High performance liquid chromatography (HPLC) Chromatographic separation of pools of Greater Rhea plasma samples revealed that a peak co-eluted with the reference external standards of testosterone and 17b-estradiol, respectively

Summary

Introduction

The occurrence of seasonal oscillations in both plasma and fecal levels of sex hormones (testosterone and estradiol) has been widely reported in birds. The seasonal pattern of testosterone in males has been related to mating systems for different species in the context of ‘‘The Challenge Hypothesis’’ [3], which postulates that high testosterone levels throughout the breeding season inhibit parental behavior in polygamous birds. This hormonal pattern allows polygamous males to copulate with all receptive females during the entire reproductive period. Females move in non-cohesive groups (Martella et al unpubl. data) and copulate and lay eggs in communal nests throughout the entire reproductive season [10,11]

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