Flow-dependent porosity and other biomechanical properties of mysticete baleen

Abstract

SUMMARYDespite its vital function in a highly dynamic environment, baleen is typically assumed to be a static material. Its biomechanical and material properties have not previously been explored. Thus I tested sections of baleen from bowhead whales, Balaena mysticetus, and humpback whales, Megaptera novaeangliae, alone or in groups representing miniature ʻracksʼ, in a flow tank through which water and buoyant particles circulated with variable flow velocity. Kinematic sequences were recorded through an endoscopic camera or viewing window. One set of experiments investigated particle capture; another series analyzed biomechanical behavior, including fringe spacing, movement and interaction. Baleen fringe porosity directly correlates, in a mostly linear fashion, with velocity of incident water flow. However, undulation and interaction of fringes (especially of bowheads) at higher flow velocities can decrease porosity. Fringe porosity depends on distance from the baleen plate. Porosity also varies, with fringe length, by position along the length of an individual plate. Plate orientation, which varied from 0 to 90deg relative to water flow, is crucial in fringe spacing and particle capture. At all flow velocities, porosity is lowest with plates aligned parallel to water flow. Turbulence introduced when plates rotate perpendicular to flow (as in cross-flow filtration) increases fringe interaction, so that particles more easily strike fringes yet more readily dislodge. Baleen of bowhead whales, which feed by continuous ram filtration, differs biomechanically from that of humpbacks, which use intermittent lunge filtration. The longer, finer fringes of bowhead baleen readily form a mesh-like mat, especially at higher flow velocities, to trap tiny particles.

Decoupling Tooth Loss from the Evolution of Baleen in Whales

“…While fossil mysticetes do possess denticulate teeth (Figure 3), none exhibit the occlusion observed in filter feeding crabeater or leopard seals (Hocking et al, 2013). This hypothesis requires detailed biomechanical modeling, either using fluid dynamics or experimental physical models to test its viability in stem mysticetes, as has been done for baleen in extant mysticetes (Werth, 2004(Werth, , 2013. In particular, such work should focus on the distinction between dental filtration and bulk filtration, and whether the former can effectively lead to the latter.…”

Oral cavity hydrodynamics and drag production in Balaenid whale suspension feeding

“…In dorsal cutaway view with oral roof removed (bottom of figure), blue arrows indicate direction of water flow though and around baleen filtering apparatus in life as well as in experimental flow tank trials and computational modeling calculations (hypothetical but predicted from data of current study and previously published experiments [3, 5]). Water can flow anteroposteriorly (AP) within mouth along the tongue (APT channel) or the lip (APL channel).…”

“…With actual oral cavities measuring up to 3-5m in length (anteroposterior), 1-3m in height (dorsoventral), and 1-3m in width (mediolateral), the typical size of flumes (3m 3 or less) makes hydrodynamic analysis of a fully-sized baleen rack all but impossible. Thus only small sections of baleen can be studied experimentally, to yield information with regards to the local flows near and through baleen [5, 9]. However, and thanks to a rack’s long serial axial symmetry where pressure and (flow) celerity gradients vary little over length scales of 0.3m or less (Fig 1), and where the camber and triangular shape of individual plates also vary little along the length of the whole baleen rack, such data can be used in mathematical modeling of the global flows characterizing the entire oral cavity.…”

Hydration affects the physical and mechanical properties of baleen tissue

“…Limited studies of baleen's biomechanical properties have been published [13,19,31] and these have not reported the basic role of hydration in this tissue's natural state. This is an essential question and the rationale for this study, given baleen's highly dynamic role in a fully aqueous environment.…”