Abstract
The blubber steroid hormone profiles of 52 female humpback whales migrating along the east coast of Australia were investigated for seasonal endocrine changes associated with reproduction. Individuals were randomly sampled during two stages of the annual migration: before reaching the breeding grounds (northward migration; June/July), and after departing from the breeding grounds (southward migration; September/October). Assignment of reproductive status of the sampled individuals was based on season, single-hormone ranks and multi-variate analysis of the hormonal profiles. High concentrations of progesterone (>19 ng/g, wet weight), recognised as an indicator of pregnancy in this species, were only detected in one sample. However, the androgens, testosterone and androstenedione were measured in unusually high concentrations (1.6–12 and 7.8–40 ng/g wet weight, respectively) in 36% of the females approaching the breeding grounds. The absence of a strong accompanying progesterone signal in these animals raises the possibility of progesterone withdrawal prior to parturition. As seen with other cetacean species, testosterone and androstenedione could be markers of near-term pregnancy in humpback whales. Confirmation of these androgens as alternate biomarkers of near-term pregnancy would carry implications for improved monitoring of the annual fecundity of humpback whales via non-lethal and minimally invasive methods.
Highlights
Steroid hormones are the principal mediators of reproduction and stress response in mammals
This study showed that female humpback whales (HW) with blubber progesterone concentrations >54 ng/g were accompanied by a calf the following year
Near-term pregnant females of Southern Hemisphere HWs are found along the coasts of Australia between June and October during their movement from Antarctica to the breeding grounds, where parturition occurs[26,28]
Summary
Steroid hormones are the principal mediators of reproduction and stress response in mammals. Circulating progesterone in pregnant females is generally higher than in non-pregnant females by orders of magnitude[14] Such acute differences in concentration have enabled blubber progesterone to be validated as a biomarker of pregnancy in various cetacean species[2,15], including HWs12. By using pregnancy assignment through progesterone concentration, Clark et al.[9] measured pregnancy rates in HWs through two consecutive sampling periods within the same year: three to four and eight to nine months after the peak of conceptions, respectively Across this period, observed pregnancy rates dropped from 64% to 11%, with the lower pregnancy rates observed among females at a more advanced stage of gestation. By considering a suite of steroid hormones, interpretation and assignment of a given endocrine status is more thorough than measuring individual hormone signals[4,25]
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