Hormonal and behavioural symmetries between the sexes in the Northern bald ibis

Seasonal patterns of fecal androgen and corticosteroid levels were measured in a semi-tame colony of Northern bald ibis (Geronticus eremita) and put in perspective with socio-sexual patterns. Northern bald ibis are monomorphic, monogamous, year-round social colonial breeders. Steroids levels were assessed from the pre-mating until the end of the mating season in the sexes, age groups, and social categories. Male immunoreactive androgen concentrations, determined by enzyme immuno-assay from feces, were relatively low, even during the mating phase, and were close to the female levels. Sexual monomorphy and symmetries in socio-sexual and parental roles may account for this similarity. Males and females responded in a similar way to social challenges from conspecifics, particularly during pair-formation. Singletons showed higher androgen metabolites levels than paired individuals. Corticosterone metabolites were relatively high during periods of social instability in the mating phase and were highest in low-ranking, young individuals. We suggest that the challenge hypothesis may even apply to females in this non-passerine bird.