Suffused: baleen fringe mat porosity and hydrodynamics in balaenid and balaenopterid whales

Baleen plates of filter-feeding whales are longitudinally fibrous, separating where eroded medially into mats of fringes due to friction from water, prey, and the tongue. The fringes end up spreading-out, in other words suffusing, over the comb-like structure of the baleen assemblage. This study examined the relationships between mat morphology and the hydrodynamics it generates. Samples collected from nine rack locations on a bowhead whale (Balaena mysticetus) and fin whale (Balaenoptera physalus) were investigated with a new technique of mat porosity determination in a flume setting. Porosity was measured in the range of 5–20% and 8–37% in the bowhead and fin whale samples respectively. These were largest ventrally in both species, while remaining somewhat insensitive to the flume’s water speed. A new hydrodynamical model of the through-mat currents was used to estimate speeds of 0.15–3.0 cm/s and mat permeabilities of the order of 10−13 m2, depending on the applied pressure. Finally, and relative to samples collected near the entrance of the mouth, these trends were quantitatively similar in both species. With tongue- and flow-based erosion as the main mechanism for mat creation in all extant mysticetes, our analysis suggests baleen-generated filtration as having emerged early in their evolution.