ADRENAL WEIGHT IN WILD MALLARD AND DOMESTIC DUCKS AND SEASONAL ADRENAL WEIGHT CHANGES IN THE MALLARD

Abstract

Mallards and domestic ducks are conspecific. Relative adrenal weight is similar in newly hatched mallards and domestic ducks. Immature mallards have higher relative adrenal weights than domestic ducks of similar age. Adult female mallards also have higher relative adrenal weights and a higher proportion of cortex in the adrenal than adult female domestic ducks, but adult males of the two strains fail to show these differences.Adrenal weight is related to testicular weight in mallards and domestic ducks, but no correlation is evident between adrenal weight and weight of the ovary and oviduct in mallards. Mallards show no adrenal weight sex differences at any of the three ages sampled. A seasonal adrenal weight cycle is apparent in both sexes of the mallard with a weight increase related to the breeding season and another increase during the autumn and winter.The higher relative adrenal weights of (immature and adult female) mallards compared to those of domestic ducks are attributed to the mallards' greater exposure to stress. It is suggested that this effect operates also in adult male mallards but is obscured in the comparison with adult male domestic ducks because in the latter, which have much higher testicular weights, another factor responsible for the correlation between adrenal and testicular weight as noted above makes for increased adrenal weights.