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Description
The four extant clades of mysticete whales (Balaenidae, Neobalaenidae, Eschrichtiidae, Balaenopteridae) employ a variety of filter-feeding methods to effectively consume large quantities of invertebrates and small fish. Mysticetes utilize a novel filter feeding apparatus, baleen, an epidermal tissue composed of keratin that occupies the place of teeth in the upper jaw. This is the first quantitative study to describe the morphological and structural variation of baleen across all four extant clades. Previous work by Williamson (1973) provided ecologically significant, broad scale measurements of baleen, including main plate length and width, laminae length and width, plate thickness, plate density, bristle diameter and bristle density. For the present study, morphometric data was collected from the baleen of sixty-five individuals representing 10 species, and converted into 12 baleen characters. Morphological comparisons and detailed descriptions were provided for all characters, including the full, intact baleen apparatus. Among characters of the full rack were baleen plate orientation, plate density, patterns in the shape/size of plates and patterns in bristle diameter and density. Scanning Electron Microscopy (SEM) was used to describe the ultrastructure of baleen bristles, which has been previously little explored (Pfeiffer, 1992). SEM examination demonstrates the similarity of baleen bristles to mammalian hair. Bristles comprised three epithelial layers: the cuticle, cortex and medulla. The cortex contained layers of diamond or triangular shaped cortical cells with microridges that extend parallel to the length of the bristle. There were no consistent differences in bristles between species. Finally, an extensive examination of baleen evolution using recent phylogenetic comparative methods was performed, using the total evidence phylogeny from Dem_r_ et al. (2008). Selective pressures (e.g. phylogenetic inertia and feeding ecology) that may be acting upon and driving this variation were examined, as well as phylogenetic signal and correlations between/within characters. Results indicate that each major mysticete lineage contains plate shapes and bristle diameters that are similar, suggesting characters with phylogenetic association (intra-clade similarity) or plesiomorphy. There are also characters, such as bristle density, which are not uniform within a clade, but instead suggest ecological specialization and convergence. Selective pressures investigated include prey preferences, feeding method and phylogenetic constraint, however, model tests indicate that none of these drive baleen morphology. Phylogenetic signal was also tested and found in two baleen characters, bristle diameter and plate shape. Bristle diameters tend to be correlated with one another and with the shape of baleen plates, emphasizing the phylogenetic signal for those characters. No direct correlations between baleen characters and size of prey were found, but results indicate that the inclusion of more data may provide significant results.
