The upper respiratory tract of rorquals, lunge-feeding baleen whales, must be protected against water incursion and the risk of barotrauma at depth, where air-filled spaces like the bony nasal cavities may experience high adverse pressure gradients. We hypothesize these two disparate tasks are accomplished by paired cylindrical nasal plugs that attach on the rostrum and deep inside the nasal cavity. Here, we present evidence that the large size and deep attachment of the plugs is a compromise, allowing them to block the nasal cavities to prevent water entry while also facilitating pressure equilibration between the nasal cavities and ambient hydrostatic pressure (Pamb) at depth. We investigated nasal plug behaviour using videos of rorquals surfacing, plug morphology from dissections, histology and MRI scans, and plug function by mathematically modelling nasal pressures at depth. We found each nasal plug has three structurally distinct regions: a muscular rostral region, a predominantly fatty mid-section and an elastic tendon that attaches the plug caudally. We propose muscle contraction while surfacing pulls the fatty sections rostrally, opening the nasal cavities to air, while the elastic tendons snap the plugs back into place, sealing the cavities after breathing. At depth, we propose Pamb pushes the fatty region deeper into the nasal cavities, decreasing air volume by about half and equilibrating nasal cavity to Pamb, preventing barotrauma. The nasal plugs are a unique innovation in rorquals, which demonstrate their importance and novelty during diving, where pressure becomes as important an issue as the danger of water entry.
Keywords: Baleen whale; Diving physiology; Hydrostatic pressure; Nasal cavities.
© 2020. Published by The Company of Biologists Ltd.