Sex-related differences in the effects of late winter pairing activity andseasonal influences on neuroendocrinology and gonadal development of mallards

The importance of late winter pairing activity on the neuroendocrinology of wild waterfowl is unknown. In this study, we examined the sex-related diff erences in the roles of late winter pairing activity and seasonal influence s on neuroendocrine and reproductive physiology in both male and female mal lards. Our main goals were to determine (1) which physiological responses w ere influenced by pairing status or by seasonal changes and (2) whether res ponses differed between the sexes. Thus, physiological responses of mallard s in different pairing status categories were assessed at two times: Januar y 28 to February 5 and February 24 to March 3. Ducks were assigned to one o f the following pairing status categories: strong pairs, temporary pairs, u npaired or lone birds within the flock, or birds isolated in same-sex group s. Seasonal changes correlated with increases in both gonadal mass and hypo thalamic content of vasoactive intestinal peptide in both sexes, whereas on ly pairing status correlated with changes in body mass in both sexes. The m ain sex-related differences were the following: (1) Seasonal decreases in h ypothalamic gonadotrophin releasing hormone II content occurred only in fem ales. (2) Seasonal. increases in serum prolactin occurred only in males, wh ereas levels in females were low throughout the study. (3) Both male and fe male gonadal masses increased seasonally, but male gonadal mass was initial ly mice that of females. (4) Body mass of both sexes was influenced by pair ing status correlations (i.e., all paired or lone birds were heavier than i solated birds), but body mass in males decreased seasonally No sex-related differences occurred in hypothalamic gonadotrophin releasing hormone I cont ent or circulating serum luteinizing hormone. Taken together, these results indicate that seasonal reproduction in mallards is regulated not only by s easonal but also by social cues, and differences occur between the sexes, m onths in advance of actual breeding. (C) 2000 Academic Press.