Multielement Stable Isotope Ratios in the Baleen of Maternal and Fetal Fin Whales Provide Evidence for the Most Recently Formed Segment of Balaenopterid Baleen

Baleen plates have been widely used to investigate the life cycle of mysticetes using a variety of chemical markers. Among these, stable isotope analysis (SIA) has been consistently applied to investigate their migratory movements and shifts in diet. However, the location of the baleen segment reflecting the most recently incorporated stable isotopes, corresponding to the whale's most contemporary habitat and diet composition, remains unclear and hampers the correct interpretation of the results. In this study, we analyzed δ 15 N, δ 13 C, and δ 34 S values along the baleen plate of a pregnant fin whale ( Balaenoptera physalus ) and its fetus to identify the baleen segment/s that were simultaneously deposited in both individuals. Our approach was based on the premise that the fetal baleen had been synthesized soon before sampling and therefore reflected contemporary habitat and diet signals of the pregnant whale. To allow comparison, the results were corrected for fetal–mother discrimination (F‐MD) factors (i.e., the isotopic differences between fetal and maternal tissues). Fetal δ 15 N and δ 13 C values aligned most closely with maternal baleen segments located 2–4 cm below the gingiva line, indicating that they had likely been deposited simultaneously with fetal baleen formation and therefore reflect the pregnant whale's most recent diet and habitat use. Conversely, fetal δ 34 S values did not consistently align with any maternal baleen segments, probably due to dissimilar incorporation rates and/or differences in tissue discrimination factors. Our results highlight the importance of extracting baleen from its base, below the gingival line, to correctly assign the timing of deposition of the various baleen segments and ensure accurate interpretation in chronological studies using intrinsic markers on baleen plates.