Figure 1.
Unmodified digitized image of the anatomical model used for studying bite mechanics in S. fatalis.
(A) Lateral view. (B) Frontal view. The mandible has been fixed in a vertical position (simulating its vertical position against the neck of the prey) using a wooden base, metal brackets and wire. The cranium and vertebrae have been articulated using flexible vinyl tubing. Blue arrows: metal brackets; white arrow: vinyl tubing.
Figure 2.
Experimental (modified) digitized image of S. fatalis neck and skull.
Four hypothesized points of rotation (blue circles) are indicated: the caudal neck, the mid-neck, the atlantooccipital joint and the temporomandibular joint. For the purposes of the experiment, the original image (see Fig. 1A) was digitally manipulated to more fully extend the cranium at the atlantooccipital joint and to position the jaws at a maximum gape of 90°.
Figure 3.
Experimental image of S. fatalis neck and skull rotated 15° ventrally at the caudal neck.
(A) The mandible is held in a constant position relative to the cranium. (B) The mandible can rotate 15° dorsally to return to a vertical position. Blue circle: point of rotation; red dot: temporomandibular joint; arrow: mandible.
Figure 4.
Experimental image of S. fatalis neck and skull rotated 15° ventrally at four points.
The experimental image has been rotated 15° ventrally at the (A) caudal neck, (B) mid-neck, (C) atlantooccipital joint and (D) temporomandibular joint. The mandible can return to a vertical position. Note that the temporomandibular joint is the only point of rotation for the neck and skull that allows the mandible to remain stationary. Blue circle: point of rotation; red dot: temporomandibular joint. The blue arrow highlights the point of rotation.
Figure 5.
(A) Using the force of its body to press the buccal aspect of its abducted mandible into the side of the prey’s upturned throat, the cat restrains the prey by locking the prey’s head in a laterally rotated position. This technique is analogous to the rodeo technique of bulldogging a steer (B), in which the cowboy wrestles the steer to the ground by applying a rotational force to the steer’s head (via the muzzle). Note that the cat’s jaws are opened to the maximum gape of 90° and are in position for the strike. Black arrows: forces applied to the head/neck of the prey/steer; blue arrow: direction of rotation of the head of the prey/steer. Credits: (A) Copyright mari_art/Depositphotos, SimpleFoto/Depositphotos and Ralf Juergen Kraft/Shutterstock. (B) Copyright SimpleFoto/Depositphotos.
Figure 6.
Circular arc of the maxillary canine.
The long axis of the maxillary canines can be described by a circle that has its center located slightly anteroventral to the temporomandibular joint (after Wroe et al. [12]). In the Class 1 Lever Model, the use of the center of this circle as a virtual point of rotation would allow the maxillary canines to traverse the prey in the direction of their long axis (i.e., along a circular path).
Figure 7.
Canine Shear-Bite vs. Class 1 Lever Model.
(A) The canine shear-bite is a Class 3 lever with the fulcrum on one side (atlantooccipital joint), the force in the middle (ventral neck flexors) and the resistance on the other side (maxillary canine tips). (B) In contrast, the Class 1 Lever Model is a Class 1 lever: the fulcrum is in the middle (temporomandibular joint), the force is on one side (forelimb extensors), and the resistance is on the other side (maxillary canine tips). Although the canine shear-bite hypothesizes the anterior rotation of the cranium at the temporomandibular joint (enabling the jaws to close), this motion is not compatible with the other aspects of the model.
Figure 8.
(A) Bulldogging. (B) Strike. The cranium rotates anteriorly at the temporomandibular joint, and the temporomandibular joint rotates anteriorly at the virtual point, enabling the maxillary canines to follow their curvature into the prey. The virtual point in this location may result from a ventrally directed force through this point by the traction of the mandibular canines against the prey (vertical dotted line). Note that the virtual point maintains a constant relationship with the point of entry, whereas the mandible moves relative to the prey as the bite progresses.
Figure 9.
Class 1 Lever Model in action.
(A) An illustration of the internal anatomy of the modern horse shows the close proximity and superficial location of the trachea, esophagus and common carotid arteries along the ventral side of the horse’s neck. These structures are separated from the vertebral column by the left and right longus colli muscles. (B) Cross-sectional anatomy of the horse’s neck at the level of C2 with the ventral neck directed upward. The longus colli muscles can be seen positioned between the carotid arteries and the vertebral column. (C, D) Bulldogging and strike positions applied to the prey’s neck. The mandible is positioned on the side of the prey’s throat with the neck of the prey rotated upward from the ground at an angle. Piercing of the maxillary canines through the bilateral longus colli muscles results in circumferential enclosure of the ventral side of the prey’s neck. Credits: (A, D) Internal anatomy of horse. Artist: Friedrich Saurer/Science Source. (B, C) Cross-sectional anatomy of horse’s neck at C2. Copyright: J. Jones, Virginia Polytechnic Institute and State University, reprinted with permission. Accessed at: http://www.vetmed.vt.edu/education/curriculum/vm8644/equineneck/.
Figure 10.
The bite of the horse’s ventral neck is bounded on three sides by three rigid structures: the mandible, maxilla and maxillary canines. Closure of the jaws by action of the mandibular adductors (with or without augmentation) circumferentially compresses the tissue in a manner similar to a tourniquet, collapsing the carotid arteries and occluding cerebral blood flow. In the figure, the carotid arteries are not directly visible. Credits: Cross-sectional anatomy of horse’s neck at C2. Copyright: J. Jones, Virginia Polytechnic Institute and State University, reprinted with permission.
Figure 11.
Detailed view of the S. fatalis mandible.
(A) The crowns of the mandibular canines/incisors have a tapered, crescent-shaped appearance with posteromedial and posterolateral cutting edges. The edges extend to the cervical one-third of the crown where they often appear as small elevated cusps. This morphology suggests that the hide was pierced by the crown before coming to rest on the neck of the tooth, offering a mechanism for mandibular anchoring. Also note the long, thin, ventrolaterally positioned flange. (B) Verticalization of the mandibular symphysis.
Figure 12.
Evolution of the Class 1 Lever Model.
The Class 1 Lever Model and mandibular bite share the same fulcrum (the temporomandibular joint) and point of resistance (the canine tips or more posterior teeth) but differ in the position and orientation of the force. The Class 1 Lever Model may have evolved when cats began to use their mandibles to bulldog prey (and thereby restrain it) and then used their forelimbs to augment the bite. The greatly increased mechanical advantage of the Class 1 Lever Model compared to the mandibular bite is visually apparent in the much greater length of its in-lever (). In the above diagram, the mandibular bite has been modeled as a Class 3 lever. Because the cranium and mandible in the mandibular bite move towards one another, the mandibular bite can also be illustrated with the force directed dorsally at the coronoid process and the resistance directed ventrally at the mandibular canine tips. C1LM, Class 1 Lever Model; MB, mandibular bite.