Progesterone concentrations and dietary stable isotope signatures in whiskers from female northern fur seals and Steller sea lions, 2007 to 2019

Keratinized tissues, including whiskers, are ideal for acquiring a record of physiological parameters. Unlike other tissues that provide a snapshot of physiological status, whiskers support longitudinal sampling for reproductive hormones and dietary stable isotopes. The objective of this project was to determine the reproductive and dietary history of two North Pacific otariids, Steller sea lions (Eumetopias jubatus) and northern fur seals (Callorhinus ursinus). Progesterone concentrations in whiskers were lower in Steller sea lions (35.4 ± 13.4 pg/mg) compared to northern fur seals (157.4 ± 115.8 pg/mg. Further, the progesterone concentrations in whiskers were greater and the peaks more distinct in northern fur seals compared to Steller sea lions. These findings made visually defining progesterone cycles challenging in Steller sea lions; however, the variation in stable isotope signatures were more distinct and easier to identify the peaks and troughs in Steller sea lions compared to northern fur seals. Cyclical patterns of δ15N were useful for identify periods of yearly growth within whiskers and measuring both stable isotopes and progesterone hormones allows for comparisons between progesterone concentrations between years within a whisker, recreating multi-year reproductive histories of female northern fur seals and Steller sea lions.