Comparative morphology of rorqual whale baleen

Abstract

Rorqual whales are lunge filter-feeders that use baleen plates to process millions of liters of prey-laden water. Baleen is a keratinous oral tissue that hangs down from the roof of the mouth in bi-laterally symmetrical racks, with larger keratin plates positioned labially and smaller plates positioned lingually. The edges of the baleen plate fray into hairs, or fringes, that interlock, creating a dense mat. This mat allows the enormous amount of water engulfed to flow out of the oropharyngeal cavity and retains the prey captured. To define the morphological parameters that dictate water flow and prey capture, we used a multimodal approach to visualize and describe the hierarchical anatomy of baleen across five species of rorqual whales that span an order of magnitude in body length. We hypothesized that baleen morphology would vary by species and prey preference. We used computed tomography (CT) and scanning electron microscopy (SEM) to quantify and understand how variability in baleen morphology may affect filtration. CT showed cylindrical keratin hairs embedded in the baleen plates growing out of the gum tissue. SEM revealed that the keratin plates are worn away at the ventral side, peeling away the exterior layers and exposing the bristles. Although many morphological features of rorquals scale with body size, hair diameter does not. The differences in filter depth, hair diameter, and hair density that we found across the species may not be a proxy for prey size, but instead may reflect changes in resistance throughout the filter that affect fluid flow.