Dose-dependent effect of testosterone on the induction of eclipse coloration in castrated wild mallard drakes (Anas platyrhynchos L.)

Abstract

Four groups of adult wild mallard drakes that had been previously castrated were injected daily with 5, 1, or 0.2 mg testosterone dissolved in propanediol or with the vehicle alone. The injections started on February 29 and continued for 64 days during the natural breeding season of the species. The birds were kept in outdoor aviaries in a seminatural environment in Kiel (54°N). By the end of the postnuptial molt in mid-July the groups differed markedly in coloration. The vehicle-injected control group had regained an almost typical nuptial plumage, whereas the castrates that had received the highest daily dose of testosterone exhibited an almost complete eclipse plumage. The two other groups showed an intermediate pattern, with about 40% eclipse coloration in the group injected with 1 mg testosterone/day and about 10% eclipse coloration in the group that had received 0.2 mg testosterone/day. The possible role of the aromatization of testosterone to estrogens in the induction of the eclipse coloration is discussed.