Multiaxial movements at the minke whale temporomandibular joint

Abstract

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 (Ω1) and temporomandibular joint (Ω2) 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.

Osteological correlates of evolutionary transitions in cetacean feeding and related oropharyngeal functions

“…Modern mysticetes lack a fused mandibular symphysis, apparently as an adaptation for multiple degrees of freedom of jaw rotation (Lambertsen et al, 1995;Werth et al, 2020) to improve gape and aid in ingesting and expelling larger volumes of water for bulk filtration. Pyenson et al (2012) described a novel sensory organ between the dentary bones of rorquals to coordinate lunge feeding.…”

“…This relates to a decrease in jaw adductor musculature (masseter, temporalis, and pterygoid muscles), which is notably reduced in cetaceans due to the lack of masticatory food processing, and to a lesser extent to the partial or total loss of teeth (and consequent raptorial grasping) that distinguishes numerous odontocete taxa (Werth, 2000). In some mysticetes, jaw rotation may play important roles in opening and closing the mouth during filter feeding (Lambertsen et al, 1995;El Adli and Demere, 2015;Werth et al, 2020), and slips of temporal musculature inserting on the mandible or on connective tissue "lips" projecting dorsally to the lower jaw, especially in balaenids, may aid in gape alteration (Lambertsen et al, 1995) and hence water ingestion and expulsion.…”

“…3389/fevo.2023.1179804 Frontiers in Ecology and Evolution 07 frontiersin.org shape of the mouth to acquire and especially to transport food items intraorally (Werth, 2006b). Like odontocetes, mysticetes also possess a clearly notched gape and lack obvious or conventional lips, but there are small bands of tissue, especially around the mandibles and projecting dorsally above them (Werth et al, 2020), that might function to constrain or "hold" baleen racks in place during filtration, to prevent keratinous baleen plates from bending or moving laterally as immense volumes of water are rapidly expelled through the baleen with consequent high forces being generated (Werth, 2013). Again, the presence and condition of bony surfaces adjacent to these jaw structures can be indicative of soft tissue features in extant cetaceans, just as changes in such features throughout the fossil record can reveal shifts in feeding.…”

Cetacean tongue mobility and function: A comparative review

“…Contraction of ventral groove musculature (Goldbogen et al, 2017; Shadwick et al, 2013), perhaps along with hydraulic rebound of ingested water (Potvin et al, 2009, 2020, 2021; Werth, Kosma, et al, 2019), forces engulfed water back out of the mouth, but now with narrowed gape, such that water flows through the curtain of paired baleen racks, trapping prey items along the eroded, fibrous medial surface (Table 1 #9; Werth, Straley, & Shadwick, 2016). The robust genioglossus muscle then presumably contracts, not to protrude the tongue outside the mouth as in many vertebrates, but instead simply to return the tongue to its resting position in the middle of the mouth (Werth & Ito, 2017), in preparation for gape closure (Werth, Ito, & Ueda, 2020; Werth, Rita, et al, 2018) and further lunges.…”