We investigated the question of whether weight gain is an essential part of the moulting process in penguins by attempting to stimulate appetite with medroxyprogesterone acetate. Beginning on September 4, 1992, six partially moulting or non-moulting chinstrap penguins (Pygoscelis antarctica) were given two series of five weekly intramuscular injections at a dose of 30 mg/kg. After the first five injections, five of six penguins had gained an average of 22%. Five weeks after the initial series, they had lost an average of 20% and had shown no evidence of moulting. Another round of injections was given between November 6 and December 4, 1992, producing a similar weight gain and loss profile. On January 7, 1993, all six began moulting, which required approximately two weeks to complete.

Five individuals showed very significant weight gain and loss profiles; four had complete moults, and one barely completed a 25% moult. The sixth bird, who had not moulted during the previous five years, showed insignificant weight gains during the injections, but completed a 50% moult.

The fact that there were significant weight gains and only minor feather changes during the first set of injections, leads us to believe that the second set was the most important for inducing a complete moult. Significant weight gain and loss was a common feature of all the complete moults, but the incomplete moults of two of the penguins suggest the presence of additional variables.