Whale jaw joint is a shock absorber

Rorquals (groove-throated whales, Balaenopteridae) include humpback, fin and blue whales, the largest animals that have ever lived. During ram-propelled lunge feeding on schooling fish and zooplankton, the jaws open wide to engulf massive volumes of prey-laden water, >70 m³ in blue whales (Balaenoptera musculus; Goldbogen et al., 2012) and 30–40 m³ in humpbacks (Megaptera novaeangliae; Simon et al., 2012). Even in the smallest rorqual species, northern and Antarctic minke whales (Balaenoptera acutorostrata, Balaenoptera bonaerensis), the engulfed water volume can be conservatively estimated at 3–6 m³ based on scaling of oral and total body size (Werth et al., 2018a). By powering whales' forward locomotion, the flukes and tail stock musculature develop the power necessary for lunge feeding (Goldbogen et al., 2017), but the mandibles and temporomandibular joint (TMJ) presumably bear the brunt of the enormous drag force generated when gape opens to accommodate water filling the greatly expanded oral pouch. Although no bite forces are encountered and the TMJ lies at the fulcrum of jaw movement, these mandibular and TMJ stresses likely pose a formidable challenge.

Whereas bowhead and right whales (Balaenidae) possess a typically mammalian fluid-filled synovial TMJ (Beauregard, 1882; Lambertsen et al., 1989), the rorqual TMJ has a large fibrocartilage pad but no joint capsule, synovial membrane or discrete articulating disc of hyaline cartilage (Brodie, 2001; Bouetel, 2005). The temporal glenoid cavity and head of the mandibular condyle are covered with smooth hyaline cartilage, but there is no cartilage between the fibrocartilage pad and the skull's mandibular fossa (Werth and Ito, 2017). The gray whale's (Eschrichtiidae) intermediate TMJ has a vestigial joint cavity yet also a rudimentary fibrocartilage pad (El Adli and Deméré, 2015). Rorqual TMJ anatomy has long attracted attention (Carte and MacAlister, 1868; Beauregard, 1882; Beneden, 1882; Schulte, 1916) but no functional investigation has been published to date.

However, several studies (Brodie, 1977; Pivorunas, 1977; Lambertsen, 1983; Lambertsen et al., 1995; Potvin et al., 2010; Goldbogen et al., 2017) concluded that the rorqual TMJ withstands intense forces as jaws are fully abducted during engulfment and to keep jaws closed (adducted) during rapid swimming. Further research has documented remarkable biomechanical properties, especially extreme elasticity or flexibility, of various rorqual oral tissues related to lunge feeding, including the ventral groove blubber or VGB (Orton and Brodie, 1987; Shadwick et al., 2013), nerves (Pyenson et al., 2012; Vogl et al., 2015; Lillie et al., 2017), blood vessels (Gosline and Shadwick, 1996; Lillie et al., 2013), sublingual fascia (Werth et al., 2019) and baleen (Werth et al., 2018b). This study focused on the biomechanical properties of the rorqual TMJ, in particular its response to compressive, tensile and shear stresses; histological study and investigation of joint motion are ongoing. We hypothesized, based on the irregular array of collagen and elastin fibers in the TMJ's fibrocartilage pad, that it might be mechanically anisotropic and show high levels of elasticity, particularly along specific orthogonal axes.

We examined the TMJ of seven deceased minke whale specimens, including four B. acutorostrata Lacépède 1804 (two adult: female 6.02 m body length, NEAq.MH.87.586.Ba stranded normal/fresh Code 2 at Truro, MA, USA; and female 4.6 m, 06-030.18.786 stranded normal/fresh Code 2 at Corolla, NC, USA; one juvenile: female 3.4 m stranded normal/fresh Code 2 at Virginia Beach, VA, USA; and one fetal: female 1.46 m, NEAq.MH,88.Bxx.Ba, mother stranded normal/fresh Code 2 at Cape Cod, MA, USA), and three fetal B. bonaerensis Burmeister 1867 (all ICR JARPA: male 1.45 m 03/04 318F, male 2.05 m 03/04 402F, female 2.09 m 05/06 486F). We examined/tested TMJs of three fin whales, Balaenoptera physalus (Linnaeus 1758) (two adult: male 17.68 m 24.7.F14.054, female 20.46 m 24.7.H9.F14.055; and one fetal: female 3.25 m 24.7.H8.F14.048F; all normal/fresh Code 2 at Hvalfjörður, Iceland). We dissected three additional rorqual TMJs (one sei whale, Balaenoptera borealis Lesson 1828; two humpback, Megaptera novaeangliae Borowski 1781) and performed field biomechanical tests on one of the humpback whales.

All specimens were dissected according to applicable statutes; no tissues were imported. For load cycle testing, larger cube-shaped tissue samples 10 cm on each side (1000 cm³) (Fig. 1) were excised from the left and right TMJ pad of each non-fetal northern minke whale, with a smaller cube 5 cm on each side (125 cm³) excised from the fetal northern minke whale’s left TMJ. For initial strength (failure) testing, cube-shaped tissue blocks 5 cm on each side (125 cm³) were excised from the left and right TMJ pad of the two adult northern minke whales, with smaller (64 cm³) cubes removed from the fetal northern minke whale's left TMJ. This yielded a total of seven blocks (six adult/subadult, one fetal) for load cycle testing and nine blocks (six adult, three fetal) for destructive testing to failure points along perpendicular orthogonal axes. All blocks were removed from the fibrocartilage pad's center, so that all six faces were of flat, sliced tissue. All blocks were cut with a large straight knife, with the TMJ pad removed from all bony attachments, placed on a stainless steel tray and pushed against a flat lucite sheet so that straight cuts could be made with no or minimal need for later trimming in the lab. All excised tissue blocks were immediately frozen on site (−20°C) for transport and storage. Because samples had no landmarks or other features, care was taken to mark axial orientations with permanent ink as blocks were excised and handled. Samples were also taken for future histological analysis. Blocks were thawed at room temperature 1–30 months after collection/freezing for lab testing of mechanical properties. The Antarctic minke whale specimens were scanned via CT imaging and dissected but not sampled for biomechanical testing. CT scans were conducted with a Siemens SOMATOM Spirit scanner (Munich, Germany) and analyzed with 3D medical imaging software (OsiriX MD 11.0 DICOM Viewer, Pixmeo, Geneva, Switzerland). The fin, humpback and sei whale TMJs were dissected and examined in the field, with field testing of compressive and tensile loading of one TMJ pad from each of the three fin whales and one of the humpback whales but not the sei whale, whose tissues were judged to be too deteriorated for proper biomechanical testing.

For laboratory material testing, thawed 125 cm³ tissue blocks of the northern minke whales were initially tested for uniaxial strength in three orthogonal axes (dorsoventral, mediolateral and anteroposterior) following the ink markings for orientation. Tissue samples were loaded in compression or tension with a Mark-10 ES30 universal testing machine with M4-200 force gauge running Mesur™Gauge recording software (Copiague, NY, USA). For compressive testing, samples were compressed between circular plates (Mark-10 G1009-2) affixed to the end of a mobile piston; for tensile testing, samples were clamped with wedge grips (Mark-10 G1061-3) with sharp 8 mm teeth that penetrated the tissue to hold it firmly in place. Sample cubes were not otherwise cut or punctured, and were tested to maximal compression and tension as judged by destruction (puncture or extrusion and tearing, respectively) of tissue fibers, with peak forces recorded in N and deformation in mm. Resulting deformation was recorded during loading/unloading cycles with the force gauge software as well as a data-linked Mitutoyo Digimatic micrometer (Kanagawa, Japan) to record tissue displacement (strain) due to applied tensing or compressing force.

Next, each of the seven larger (1000 cm³ adult or 125 cm³ fetal) tissue blocks was load tested uniaxially and sequentially in three dimensions (anteroposterior, dorsoventral and mediolateral, with N=20 loading/unloading cycles along each axis) to 95% of the same maximal compressive and tensile strengths determined during the earlier failure testing. As with the strength testing, both stress and strain were recorded with the Mesur™Gauge software and strain was additionally recorded with a digital micrometer physically attached and data-linked to the testing machine. With N=20 loading/unloading cycles for all seven tissue blocks, the combined total was N=140 for compressive and N=140 for tensile tests along each of the three orthogonal axes. However, because the fetal tissue sample was smaller in size and came from a much younger (prenatal) animal, its results were not included in the pooled load cycle data (Fig. 2), which included N=360 compressive and N=360 tensile tests (20 per sample×6 samples×3 axes), so that N=720 total load cycles.

For the other species, TMJ fibrocartilage pads were dissected and examined in the field, with simpler biomechanical testing of fresh tissue blocks from fin and humpback whales. Samples of these species were not removed and frozen for lab testing. Whole TMJ fibrocartilage pads and excised 10×10×10 cm blocks (1000 cm³, equivalent to those from minke whales, and also cut with a large straight knife as pads were held flat against lucite sheets) were tested in compression with a Mark-10 M4-200 force gauge (Copiague) pushing tissue between flat plates, and in tension with a Pesola Macroline spring scale (Schindellegi, Switzerland) displaying force in N, again testing N=20 along each orthogonal axis. Maximal axial compressive and tensile strength was not determined in these field tests, but the load cycling was performed to 95% of the maximal (failure) compression and tension as determined by the earlier testing on northern minke whale tissue samples. Additional field studies employed either slowly or rapidly applied forces precisely maintained at 50 N to determine whether the TMJ pad material might be viscoelastic or its recovery otherwise time limited such that it could aid in damping acceleration or absorbing and storing energy. Field and lab (maximal strength and load cycling) results were analyzed via t-tests to determine whether data (e.g. from different specimens, ages and species) were statistically significant.

In all specimens examined, the TMJ was a heterogeneous matrix of fibrocartilage including abundant white collagen and yellow elastin fibers (Fig. 1). However, no discernible differences in pad regions could be detected in situ or in excised samples. There were scattered fibroblasts, adipocytes and regions of what appeared to be highly hydrated chondrocytes. The entire fibrocartilage pad was approximately 0.007 m³ in volume in a 6 m minke whale (0.136 m³ in a 20 m fin whale). The pad was flexible to the touch and easily deformed 10–15 cm: when pushed, pulled or subjected to shear (≤70 N), the tissue resumed its original form within 1.5±0.3 s (mean±s.d.). Manipulation of fresh carcasses showed flexible jaws cannot easily be misaligned or disarticulated unless tissues are cut. When manually opened, jaws tended to close on their own (within 4 s in minke whales), although this could be due to many factors or non-TMJ tissues. Mandibular rotation (medial roll and lateral yaw) accompanied abduction; the pad twisted but easily resumed its original form.

Strength testing of the minke whale tissue revealed maximal loading of 29 N in compression (mean±s.d. 29.2±0.2 N dorsoventral, 29.4±0.2 N anteroposterior, 29.5±0.3 N mediolateral, all N=8) and 22–40 N in tension (22.3±0.3 N dorsoventral, 29.6±0.3 N anteroposterior, 39.7±0.3 N mediolateral). Loading profiles (Fig. 2) indicated a soft, highly elastic tissue, supporting the hypothesized rorqual TMJ flexibility. When compressed, each block shrank 70–90% along any axis with application of 24–26 MPa. When pulled under tensile forces of 19–35 MPa, the blocks increased by 176–224% of their original size (Fig. 2). Loading and unloading curves were similar, revealing little hysteresis (Fig. 2). There was close agreement of force gauge and micrometer displacement data (P=0.92). Field tests indicated no significant difference (P=0.81) between fresh TMJ tissue tested immediately postmortem or frozen and thawed for later laboratory testing. This elasticity range is comparable to other whale oral tissues noted above, and accords with field data of fresh fin whale TMJ pads, which also demonstrated compressive strains of 60–80% (mean±s.d. 68.4±1.3%, N=48) and tensile strains of 120–165% (129.7±4.1%, N=48); data from the single humpback TMJ pad showed even greater elasticity (mean±s.d. compressive strain 73.2±3.4%, tensile  strain 138.6±4.8%, N=25). Species differences between TMJ pads of minke versus fin (P=0.32) and humpback (P=0.56) whales were not significant. No differences were found between sex or size/age class, except that the fetal minke whale TMJ tissue was substantially more elastic than adult tissue (P=0.09), perhaps because mature collagen typically develops crosslinks or from other age-related histological change.

Our results suggest the fibrocartilage pad's mechanical anisotropy is limited (Fig. 2), as stress/strain vary little between axes (P=0.51). The pad was least elastic in the dorsoventral plane, stretching 166% under tension (with mean±s.d. 18.6±0.13 MPa, N=120 for all trials) and −68% under compression (with 24.5±0.19 MPa). It was most elastic in the mediolateral plane, where it stretched 224% under tension (with 35.2±0.09 MPa) and −88% under compression (24±0.07 MPa). Stretching was intermediate in the anteroposterior plane, at 202% (with 26±0.21 MPa) in tension and −78% (with 24±0.17 MPa) in compression.

Preliminary histology results did not indicate obvious differences in axial fiber arrangement, yet suggested slightly more elastin fibers (perhaps 15–20% more) running anteroposteriorly and dorsoventrally. We presume the strong collagen fibers mainly resist tension and the yellow elastin fibers aid in recovery from tensile and compressive loading, as in typical fibrocartilage (Benjamin and Ralphs, 1998) as well as other heavily loaded cetacean tissues that show high stiffness and elasticity (Gosline and Shadwick, 1996; Shadwick, 1999). Despite its close proximity to the ear, the fibrocartilage pad is unlikely to play a role in sound transmission, although its acoustic properties should be analyzed.

How these triaxial stress/strain results relate to jaw movements is uncertain. However, dissected TMJs enabled us to observe fibrocartilage pad alteration during jaw manipulation. Further, CT scans (Fig. 3) yielded internal views showing osteological relationships and dimensions plus soft tissue deformation during jaw abduction/adduction. The pad is not a wholly constant-volume structure but its deformation largely follows a simple scheme. As gape increases from zero (mouth closed) to full (opened), the pad shortens overall but appears to become compressed mainly in its ventral portion, whereas its dorsal portion is tensed. At the same time, the anteroposteriorly compressed pad extends mediolaterally (by about 40–60%) and to a lesser degree dorsoventrally (10–15%). Although uniaxial testing along individual axes yields interesting and potentially useful controlled data, it cannot replicate simultaneous loading along three axes, as almost certainly occurs in vivo.

Our results, including equivocal results of compressive/tensile tests conducted with varying speed, suggest the rorqual TMJ fibrocartilage pad could act like an articular shock-absorbing cushion whose stiffness damps acceleration via viscous friction and deformable elasticity absorbs shock during prey engulfment. The pad could allow for mandibular rotation and displacement during gape opening/closure to greatly expand buccal volume, and could aid in returning jaws to a closed position during filtration via elastic recoil with conversion of stored potential energy into kinetic energy. The role during rapid lunge feeding of rorquals' capacious oral pouch and elastic, accordion-like throat pleats (Pivorunas, 1977; Orton and Brodie, 1987; Goldbogen, 2010; Shadwick et al., 2013) depends on the jaw joint's ability to open widely and to close against huge loading forces generated by the massive volume of engulfed water and ensuing drag (Lambertsen, 1983; Arnold et al., 2005; Potvin et al., 2010). Previous studies (Lambertsen et al., 1995; Lambertsen and Hintz, 2004; Arnold et al., 2005; Goldbogen et al., 2011) concluded that the fully adducted (closed) jaw forms some sort of oral seal which, when combined with other oral adaptations, enables rorquals to stabilize jaws and control gape opening/closure while potentially minimizing energetic costs. The loose, flexible TMJ and mandibular symphysis and consequent wide gape (Fitzgerald, 2012; Pyenson et al., 2012) may have fueled rorquals' trophic success and consequent adaptive radiation (Kimura, 2002; Thewissen, 2014; Marx et al., 2016; Goldbogen et al., 2019).

For field access we are grateful to Kristjan Loftsson and staff of Hvalur in Iceland, Dr Y. Fujise at the Institute of Cetacean Research in Japan, and Erin Fougeres, Bill McLellan and participants/organizers of the NOAA/NMFS Marine Mammal Stranding Network of USA. For ideas and assistance, we thank colleagues including Bob Shadwick, Jeremy Goldbogen, Nick Pyenson, Jean Potvin, Margo Lillie, Wayne Vogl, Tom Ford, Pierre-Henry Fontaine, Keiichi Ueda, Seiji Otani, Tatsuya Isoda, Hiroshi Sawamura and Hiroto Ichishima.