New Insights into Female Humpback Whale (Megaptera novaeangliae) Reproductive Endocrinology Revealed by Steroid Hormone Profile Analysis

Abstract

The blubber steroid hormone profiles of 52 female humpback whales migrating along the east coast of Australia were investigated for possible seasonal endocrinological changes associated with reproduction. Individuals were randomly sampled in southeast Queensland during two stages of the annual migration, corresponding to two distinct phases of the breeding season, namely: before reaching the breeding grounds (northward migration; June/July), and after departing from the breeding grounds (southward migration; September/October). High concentrations of progesterone (>20.38 ng/g), which is recognised as an indicator of pregnancy in this species, were only detected in one sample. Further, the androgens testosterone and androstenedione were measured in unusually high concentrations (1.60 to 11.9 ng/g wet weight and 7.80 to 40.2 ng/g wet weight, respectively) in 35% of the females approaching the breeding grounds, and in moderately high concentrations in 23% of the female individuals spread throughout the whole migration. Based on season, single- hormone ranks and multi-variate analysis of the hormonal profiles, we offer an interpretation of the reproductive status of the sampled individuals. As seen in other cetacean species, we suggest that relatively high levels of testosterone and androstenedione could be markers of near-term pregnancy in the humpback whale. This is a significant finding, and implies that progesterone alone may not be a reliable marker for detection of pregnancy at the end of gestation. More research into the endocrinological role played by androgens in the reproductive cycle of the humpback whales is merited. Confirmation of these androgens as biomarkers of near-term pregnancy would facilitate more accurate monitoring of the annual fecundity of these populations via non- lethal and minimally invasive methods.