Shoulder

Brachium

Antebrachium

Facial and Masticatory Muscles

Thorax and Heart

Brain

Body Wall

Hip and Thigh

Shank and Foot

Digestive System

Urinary and Reproductive Systems

Hip and Tail

Facial and Masticatory muscles of the Beaver (Castor canadensis)

Photos and text by Jason Lavender & Wei-Chung Chen

Overview of the facial and jaw musculature:

The diet of the beaver consists mainly of wood and other woody vegetation, making large muscles of mastication (temporalis and masseter) necessary [1]. Wood is very tough to chew, and the process of chewing is only the first step in a long process of digestion necessary to obtain nutrients from this relatively poor diet. The mechanics of chewing for the beaver reflect its herbivorous diet, and are common among many rodents. In carnivores, chewing is mostly accomplished by elevation and depression of the mandible. Meat is more easily broken down in the digestive system than wood, so this type of chewing is sufficient. In the beaver, however, chewing is accomplished mostly by grinding of the food. In the beaver, the temporomandibular fossa (the site where the mandible articulates with the cranium) is large and not well-defined, forming an anteriorly to posteriorly oriented groove. This allows the beaver to move its mandible anteriorly, in addition to normal elevation and depression. The teeth reflect this form of chewing as well. The occlusal surfaces of the teeth are uniquely divided into ribbon-like sections that run perpendicular to the axis of the grinding motion. Combined with ever-growing incisors, these features allow the beaver to subsist on a diet of very tough and woody materials.

In addition to using wood as a food source, beavers also gnaw and chew on wood in order to construct lodges and dams. The enlarged temporalis and masseter muscles and the ever-growing incisors are necessary to support this behavior.

One of the beaver’s most striking anatomical features is its massive salivary gland, which occupies a major portion of the neck region extending craniolaterally from an expansive area just below the clavicles and becoming smaller as it reaches the mandible. The salivary tissue is not clearly divisible into specific parts or individual glands, but rather is a large collection of fused salivary glands with multiple salivary ducts. This is a necessary adaptation to the woody diet on which beavers subsist.

Regarding the facial muscles of the beaver, there are several unusual adaptations that are useful both when burrowing and when swimming. First, the eyes have a nictitating membrane to keep water or dirt from getting into the eyes. Also, the mouth, ears, and nose are valvular, allowing the beaver to close off these openings. The beaver can close its mouth while holding something with its teeth, thus allowing it, for example, to carry a branch underwater without having its mouth open. 

Dissection:

We began the dissection of the right side facial and masticatory musculature by making a caudal to rostral cut along the midline of the dorsal surface of the cranium. We removed the skin with relative ease, but the deeper dissection proved more difficult. The underlying fascia in this area, specifically the fascia covering temporalis and masseter, is tightly bound to the muscle. In some areas, most especially near the sites of origin and insertion, the fascia could not be removed without risk of ripping the muscle fibers themselves. The first muscle that we reached was temporalis. This muscle, which is divided into superficial and deep portions, covers nearly the entire dorsal surface of the cranium, except in locations where muscles of the eye and ear are present. The eye and ear muscles attach to the cranium deep to the temporalis muscle, but pass through small gaps of the temporalis for the more superficial insertions. We also discovered a large lacrimal gland associated with eye. While it was not a surprise to find the gland in this location, it was much larger than expected. The lacrimal gland is just rostral to temporalis, and appears to overly a portion of its rostral insertion.

Once temporalis was exposed and the majority of its fascia removed, we made a dorsal to ventral cut along the lateral portion of the cranium approximately halfway between the ear and the eye. The platysma on the lateral portion of the cranium, especially near the neck, is very thick, measuring upwards of 1.5cm. We were able to observe the zygomatic arch, which is situated inferiorly to the temporalis muscle. The arch is very large and acts as the origin of the masseter. Inferior to the zygomatic arch is the large masseter. Similar to temporalis, masseter is tightly bound by a thin layer of fascia. The fascia could not be removed in its entirety, but we were able to expose the majority of the muscle. During the removal of this fascia, we encountered a clear division between masseter and buccinator. Buccinator is small compared to both masseter and temporalis, and is situated right along the cheek rostral to the masseter.

After completing the lateral dissections, we turned the beaver onto its back and dissected the tissues overlying the mandible. We reached the mental symphysis soon after, and then began a dissection of the muscles overlying the body of the mandible. We exposed both the right and left anterior bellies of the digastricus muscles. These muscles are tightly bound to the bone, and appear to cover the entire visible surface of the mandibular body.

After dissecting out these masticatory muscles, we next attempted to dissect the facial muscles. The beaver eye is very small, and though we were able to locate orbicularis oculi, we were unable to effectively separate it from the skin of the eyelid. Other dissections of the facial muscles proved equally difficult, and with the exception of orbicularis oculi, we were unable to expose these smaller muscles.

Conclusions and Comparisons:

Rodents have several unique chewing adaptations, including ever-growing incisors, large salivary glands, and relatively large masticatory muscles. When compared to the laboratory rat (Rattus norvegicus), the beaver’s adaptations appear even more extreme. The major differences between the rat and the beaver seem to result from the differences in diet between the two species. Beavers subsist mostly on wood, and on other vegetation that is difficult to break down by chewing. As a result, the masticatory muscles must be powerful (especially masseter and temporalis). While the rat’s masticatory muscles are also large compared to those of other mammals that do not rely so heavily upon vegetation in their diet, the beaver’s chewing muscles are proportionally even larger. Perhaps the most obvious difference is the size of the salivary glands. The rat has prominent salivary glands, but there is far less salivary tissue in the rat compared to the beaver. The rat salivary glands are definable, separate glands in the neck region, while the beaver salivary gland is essentially a large collection of salivary glandular tissue found in the neck region, the upper thoracic region (superficial to the deeper muscles), and in the tissue caudal to the mandible.

References:

1. Müller-Schwarze, D., & Sun, L. 2003. The Beaver: Natural History of a Wetlands Engineer. Comstock Publishing Associates, Ithaca. pp. 65-76.

2. Young, F.W. (1937). Studies of Osteology and Myology of the Beaver (Castor canadensis). Michigan State College, East Lansing. pp. 32-38.