Whale Nasal Plug Anatomy Determines Blow Shape

Abstract Body : The upper respiratory tract of whales was modified in response to the need dive to variable depths to feed and to return to the surface to breathe. The blowholes of baleen whales (Mysticeti) migrated to the top of the skull (an adaptation for efficient breathing at the surface while swimming) and enlarged paired nasal plugs fill the nasal cavities (an adaptation that protects against water incursion). Blowholes are also adapted for evacuation of a large volume of air in a short period of time (a “blow”). Blowhole opening occurs when muscles contract to shift the nasal plugs forward (rostrally), while closure occurs when the muscles relax and the nasal plugs recoil back into the nasal cavity. Movement or shape changes of the plugs is hypothesized to enable pressure equilibration of the air-filled nasal cavities during diving/ascent to counter high ambient hydrostatic pressures faced at depth. The mechanisms that widen the nasal lumen or generate specific exhalation blow patterns are not understood. Anatomical comparisons of the upper respiratory tract of baleen whale species are needed to elucidate the functions related to species-specific shape variations of the nasal plugs of mysticetes. The nasal region (blowholes, upper nasal passages, nasal plugs) was dissected in six specimens from three species of rorquals: 1 fin (Balaenoptera physalus), 3 minke (B. acutorostrata), and 2 humpback (Megaptera novaeangliae) whales. Magnetic Resonance Images were used to generate high-resolution two- and three-dimensional reconstructions of the blowholes and nasal plugs in all six specimens. Results indicate nasal plug structure and composition are similar, but with species-specific size and shape variations. Fins have relatively small plugs with rounded tips. Minkes have the smallest relative sized plugs with pointy tips. Humpbacks have the largest relative sized plugs with rounded tips. All plugs were anchored medially along the cartilaginous nasal septum that limits plug excursions. Internal structure is comprised of fat interlaced with curved bands of skeletal muscle. Muscle fiber directions indicate a potential to curl the tip, but with limited shape-changing properties. Rather, medial anchoring likely contributes to deforming the plug shape. The plug is stretched when withdrawn rostrally from the anchor, causing flattening against the curvature of the bony nasal wall. This would allow a larger lumen for airflow, but the contours of the plug appear to direct air in a spiral path around the tissue. The spiraling geometry is species-specific, and likely contributes to the unique blow shapes observed in exhalation for different species.