Werth Alexander J, Ito Haruka, Ueda Keiichi
Department of Biology, Hampden-Sydney College, Hampden-Sydney, Virginia, USA.
National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, Yokohama, Kanagawa, Japan.
J Morphol. 2020 Mar;281(3):402-412. doi: 10.1002/jmor.21107. Epub 2020 Jan 31.
Mandibular mobility accompanying gape change in Northern and Antarctic minke whales was investigated by manipulating jaws of carcasses, recording jaw movements via digital instruments (inclinometers, accelerometers, and goniometers), and examining osteological and soft tissue movements via computed tomography (CT)-scans. We investigated longitudinal (α) rotation of the mandible and mediolateral displacement at the symphysis (Ω ) and temporomandibular joint (Ω ) as the mouth opened (Δ). Results indicated three phases of jaw opening. In the first phase, as gape increased from zero to 8°, there was slight (<1°) α and Ω rotation. As gape increased between 20 and 30°, the mandibles rotated slightly laterally (Mean 3°), the posterior condyles were slightly medially displaced (Mean 4°), and the anterior ends at the symphysis were laterally displaced (Mean 3°). In the third phase of jaw opening, from 30° to full (≥90°) gape, these motions reversed: mandibles rotated medially (Mean 29°), condyles were laterally displaced (Mean 14°), and symphyseal ends were medially displaced (Mean 1°). Movements were observed during jaw manipulation and analyzed with CT-images that confirmed quantitative inclinometer/accelerometer data, including the unstable intermediate (Phase 2) position. Together these shifting movements maintain a constant distance for adductor muscles stretched between the skull's temporal fossa and mandible's coronoid process. Mandibular rotation enlarges the buccal cavity's volume as much as 36%, likely to improve prey capture in rorqual lunge feeding; it may strengthen and stabilize jaw opening or closure, perhaps via a simple locking or unlocking mechanism. Rotated lips may brace baleen racks during filtration. Mandibular movements may serve a proprioceptive mechanosensory function, perhaps via the symphyseal organ, to guide prey engulfment and water expulsion for filtration.
通过操纵小须鲸尸体的颌骨、使用数字仪器（倾角仪、加速度计和角度计）记录颌骨运动，并通过计算机断层扫描（CT）检查骨骼和软组织运动，对北大西洋和南极小须鲸张口时的下颌活动进行了研究。我们研究了张口（Δ）时下颌的纵向（α）旋转以及联合处（Ω）和颞下颌关节（Ω）的内外侧位移。结果表明张口有三个阶段。在第一阶段，当张口度从零增加到8°时，α和Ω旋转轻微（<1°）。当张口度在20°至30°之间增加时，下颌骨略微向外侧旋转（平均3°），后髁略微向内侧移位（平均4°），联合处前端向外侧移位（平均3°）。在张口的第三阶段，从30°到完全（≥90°）张口，这些运动发生逆转：下颌骨向内侧旋转（平均29°），髁向外侧移位（平均14°），联合处末端向内侧移位（平均1°）。在颌骨操纵过程中观察到了这些运动，并用CT图像进行了分析，这些图像证实了倾角仪/加速度计的定量数据，包括不稳定的中间（第二阶段）位置。这些移位运动共同保持了在颅骨颞窝和下颌骨冠状突之间拉伸的内收肌的恒定距离。下颌旋转可使颊腔体积增大多达36%，这可能有助于在须鲸的冲刺捕食中更好地捕获猎物；它可能通过简单的锁定或解锁机制加强和稳定张口或闭口。旋转的嘴唇可能在滤食过程中支撑鲸须架。下颌运动可能具有本体感觉机械感觉功能，也许是通过联合器官，以指导猎物的吞咽和滤食时水的排出。
