A new insight into southern right whale reproduction via baleen endocrine and stable isotope analysis

Abstract

As capital breeders, southern right whales (Eubalaena australis; hereafter SRW) migrate from offshore summer feeding areas to coastal winter calving grounds. Although SRW population parameters are well studied in coastal regions, far less is known regarding their offshore activities. As a result, their foraging ecology and reproductive cycles are still poorly understood. Over the past decade, the South African population of SRWs has experienced a reproductive decline concurrent with a shift in their foraging strategy and an overall decrease in maternal body condition. These findings are hypothesised to be linked to climate change and the subsequent effects on prey availability. Considering that foraging success drives reproductive success, and ultimately population recovery in these capital breeders, the link between foraging ecology and reproduction needs to be better understood. Therefore, baleen plates of whales that stranded along the South African coast between 1987 and 2020 were sampled at regular intervals to create longitudinal stable isotope (δ15N and δ13C) and reproductive endocrine (oestrogen and progestagen) profiles. First, long-term foraging behaviour and preliminary insights into niche partitioning were evaluated using stable isotope analysis. The longitudinal isotope profiles showed north-south movement for most individuals, characteristic of migration between feeding- and calving grounds. There were, however, some individuals that appeared to not migrate at all. Results further suggested that demographic groups with higher energy requirements such as post-partum females and juveniles may be more restricted in their foraging compared to other groups but, due to the small sample size, these results are preliminary. The results of these analyses are pertinent for understanding how different groups may respond to climate-induced changes in prey availability, particularly with respect to reproduction. Then, using the same baleen plates of the adult females, endocrine analysis was implemented to investigate temporal aspects of reproductive biology in more detail. Profiles of baleen progesterone metabolite (bPM) concentrations showed pronounced, bimodal elevations at regular intervals. These were interpreted as individual pregnancies separated by inter-calving intervals. Sharp increases in baleen oestrogen metabolite (bEM) concentrations were hypothesised to be an indication of parturition. The length of successful gestation was estimated to last approximately 15 – 18 months, which is a few months longer than previous estimates. Some bPM profiles showed anomalies suggestive of loss of a pregnancy or a neonatal calf. Most interestingly, when endocrine and isotope profiles were combined, the initiation of elevations in bPM concentrations coincided with periods of decreasing carbon isotope values. This suggests that conception occurs offshore near the end of the feeding season as opposed to coastal waters as was previously believed. These new insights into SRW reproductive biology are critical to better understanding the causes of the reproductive failure currently observed among the South African population.