INFLUENCES OF SEX, CASTRATION, AND ANDROGENS ON THE EUMELANIN AND PHEOMELANIN CONTENTS OF DIFFERENT FEATHERS IN WILD MALLARDS

In mallards the bright nuptial plumage of the drake represents the neu tral, sex hormone-independent coloration of the species that both sexe s eventually exhibit after castration. We compared the pheo- and eumel anin contents of feathers from the head, breast, flank, and under-tail coverts in five groups of mallards after the post-nuptial molt in sum mer: intact hens, intact drakes, castrated drakes, castrated drakes in jected with testosterone during the spring, and castrated drakes injec ted with 5 alpha-dihydrotestosterone during the spring. In the head fe athers and under-tail coverts, the gonadal hormones of the intact bird s and the testosterone injections into castrates significantly reduced the eumelanin content, tended to increase the pheomelanin content, an d, thereby, changed the melanin type from eumelanic in the untreated c astrates to mixed melanic in the other three groups. The eumelanin con tents of the flank feathers did not differ among the groups, but the p heomelanin contents at this site was significantly elevated in the two intact groups and the testosterone-treated compared to the uninjected castrates. Again, the melanin type changed from eumelanic in the cast rates to mixed melanic in the other three groups. The high pheomelanin content of the breast feathers in the castrated birds was significant ly reduced in the hens, intact drakes, and testosterone-injected castr ates with a concomitant tendency for elevated eumelanin contents. At t his site, a change occurred from pheomelanic to mixed melanic. 5 alpha -dihydrotestosterone was dearly less effective than testosterone in af fecting the melanin contents in castrates and resulted in an intermedi ate coloration. The differing effects of the two androgens might be a result of differences in their conversion to estrogens.