Baleen turnover and gut transit in mysticete whales and its environmental implications

Abstract

Baleen, a unique oral α-keratin, has flat cortical layers enclosing hollow horn tubules. Baleen grows continuously to replace erosive loss from feeding-related wear. This is essential for maintaining efficient filtration over a whale’s long life history. Baleen fragments are seen in stomach contents and feces. We focused on fin (Balaenoptera physalus) and bowhead whales (Balaena mysticetus), but examined eleven mysticete species. Histological features including variably sized tubules plus differential growth along plate axes (growing faster along the medial fringed edge) and between plates of a rack (central plates growing faster than others) relate to baleen strength and flexibility. Sheet-like cortical layers provide strength and probably hinder erosive shedding, whereas medullary (tubular/intertubular) keratin provides flexibility and likely promotes shedding. To calculate amounts of grown/lost baleen, we considered both erosive wear and basal growth, recognizing that each plate represents several years of growth. We estimate average annual loss of 70–100,000 cm2 (20–40,000 cm3) of baleen in balaenids and 25–50,000 cm2 (7–14,000 cm3) in balaenopterids. Baleen growth rates depend on age, size, and other life history parameters; the most reliable calculations come from morphometric data with plate dimensions measured along full racks. We argue that baleen turnover has been under-rated and baleen growth rates underestimated in the published literature. Baleen turnover maintains filter integrity/porosity, while gut passage possibly clears gastrointestinal tracts of endoparasites. Large volumes of shed keratin hold potential ecological consequences by providing food for microbial decomposers and detritivores throughout ocean ecosystems, especially in polar regions where most mysticete feeding occurs.