Baleen reveals sex-specific foraging, reproduction and pregnancy endocrinology in sei whales
Daniela Magalhaes Drummond de Mello, Alejandro A. Fernandez Ajó, Lilian Reiss, Kathleen Hunt, Matthew C. Rogers, Danielle Dillon, Faith M. Walker, Anna Riley, Daniel E. Sanchez, Christoph Mayr, Vreni Häussermann, Keri-Lee Pashuk, Matthew R. Coyle, Fernando Ribeiro Gomes, C. Loren BuckAbstract
The largest baleen whale mortality event on record, when more than 340 sei whales (Balaenoptera borealis) stranded in Chilean Patagonia during the extreme 2015 El Niño, provided a unique opportunity to reconstruct multi-year individual life histories from baleen plates. Baleens of seven females and four males were sampled to examine sex-specific foraging strategies, reproductive cycles and endocrine patterns through integrated molecular, isotopic and hormonal analyses. Sex was identified using a refined DNA amplification protocol. Patterns of δ13C and δ15N revealed significant annual oscillations, enabling estimates of baleen growth rates (15.7–20.5 cm yr−1) and retrospective timelines of up to 3.5 years. Males exhibited broader isotopic niche widths, suggesting sex-related foraging differences. Progesterone was detected in all samples, with sustained elevations in females indicating pregnancies of 7.7–10.7 months. One female exhibited two complete pregnancies recorded in baleen, enabling estimation of a 1.3-year intercalving interval. Concurrent elevations of cortisol and corticosterone were observed during pregnancy. Conceptions and births were distributed across multiple seasons, with no clear evidence of seasonal concentration. Together, these findings provide the first baleen-based reconstruction of sei whale reproductive history and endocrine variation, establishing critical baselines for assessing population resilience and informing conservation under rapid environmental change.