Calcification provides mechanical reinforcement to whale baleen α-keratin

Hard α-keratins such as hair, nail, wool and horn are stiff epidermal appendages used by mammals in a variety of functions including thermoregulation, feeding and intraspecific competition. Hard α-keratins are fibre-reinforced structures consisting of cytoskeletal elements known as ‘intermediate filaments’ embedded in an amorphous protein matrix. Recent research has shown that intermediate filaments are soft and extensible in living keratinocytes but become far stiffer and less extensible in keratinized cells, and this stiffening may be mediated by air-drying. Baleen, the keratinous plates used by baleen whales during filter feeding, is an unusual mammalian keratin in that it never air dries, and in some species, it represents the most heavily calcified of all the hard α-keratins. We therefore tested the hypothesis that whale baleen is stiffened by calcification. Here, we provide, to our knowledge, the first comprehensive description of baleen material properties and show that calcification contributes to overcoming the shortcomings of stiffening this hard α-keratin without the benefit of air-drying. We also demonstrate striking interspecies differences in the calcification patterns among three species of baleen whales and provide novel insights into the function and evolution of this unusual biomaterial.