Endocrine and Body Weight Changes of Nesting and Non-Nesting Canada Geese

Seasonal changes in circulating steroid hormones, body weights, and social behaviors were measured in a captive flock of Canada geese (Branta canadensis moffitti). The adaptive significance of hormonal fluctuation was interpreted in relation to age of reproductive maturity, social status, timing of nesting events, and reproductive performance. Androgen levels of reproductively active males rose 4 weeks before their mates began egg-laying and remained high (>2 ng/ml) during the period of territorial defense, i.e., through the incubation period. There was a seasonal peak of estrogens associated with egg-laying, and estrogen fluctuations were significantly correlated with changes in body weights. Corticosterone levels did not vary significantly in breeding males, but among breeding females corticosterone levels declined around the time of egg-laying, rose during incubation, and declined again after the time of the hatch. Androgen levels in immature males remained low (<1 ng/ml). Yearling and nonbreeding adult males experienced high androgen levels, but late in the season compared with reproductively active males. Estrogen levels in immature, yearling, and nonbreeding adult females remained low. Corticosterone titers declined in nonbreeding adult males, but did not vary significantly in nonbreeding adult females during the study period. The development of reproductive capacity in male geese appeared to precede that of females, and high levels of androgens occurred in unpaired males, suggesting that androgen secretion in male geese may be more dependent on environmental cues than stimulation provided by the mate. Peaks of estrogens and body weights and a characteristic pattern of corticosterone changes were clearly identifiable in reproductively active female geese, and lacking in unpaired females, suggesting that development of reproductive capacity in the female goose was intimately tied to stimulation provided by the gander.