beaver shoulder dissection 289L 2004

Some interesting observations concerning the surface anatomy of Castor canadensis:

Beavers are generally semi-aquatic, and thus spend a great deal of time in the water. Many of the unique anatomical and morphological characteristics of the beaver reflect this semi-aquatic lifestyle. The first and most unique feature to note is the tail. The large, flat, and scaly tail serves numerous biological and behavioral functions. The tail serves as a rudder during aquatic locomotion, and can also be slapped against the water surface to signal danger or anxiety. Biologically, the tail serves as an important site for fat storage and heat exchange. The hind limbs are large and webbed, serving as the main mode of propulsion in water, while the forelimbs are small and lack webbing, but are much more dexterous. While on land plantigrade locomotion is employed, and the small, robust forelimbs can be used for both carrying branches and burrowing. The shoulder area is characterized by large, robust musculature that is indicative of a burrowing rodent and reflects the beaver’s ancestry. The ribcage, which is robust, relatively wide, and compacted caudally-to-cranially, further adds to the support of the upper body necessary for burrowing. In comparison to the rodent, which is not solely specialized for a semi-aquatic or burrowing lifestyle, the musculature of the shoulder region is less developed and the size/shape of the ribcage is less compressed.

The eyes are quite small, indicating poor eyesight. They have both palpebrae, as well as nictitating membranes that provide extra eye protection for swimming underwater with their eyes open. The cranium is massive, supporting two enlarged and ever-growing incisors that can be used for both gnawing and digging. Sexual dimorphism in C. canadensis is nearly absent, making sex determination based on gross anatomical characteristics quite difficult. The main exception occurs during lactation, when the four pectoral mammae in females become enlarged. No external genitalia are present, since both sexes possess a cloaca. This single, lower-body opening serves the urinary and bowel tracts, the reproductive organs, and the castor glands. Both males and females have two pairs of these large glands, which produce a pungent secretion used to scent mark territories. A thick, dual-layered coat of fur covers the entire body, the outer layer consisting of course hairs and the inner layer consisting of softer fur.

We began by making a general, visual inspection of the specimen in an attempt to determine its sex. As mentioned previously, this is a difficult matter due to nearly absent sexual dimorphism in the species. Males possess a baculum, and females generally possess four pectoral mammae. Attempts to palpate a baculum failed. Two bilateral mammae were located, but despite efforts to locate more along the mammary lines, no others were seen. Given the identification of two relatively large mammae, however, we postulated that the specimen is female.

We began the dissection by making an incision down the midline of the back. Concentrating the primary dissections on the right side, we next made a lateral incision just caudal to the neck and a second lateral incision just cranial to the abdomen. The early dissection proved difficult, and attempts to separate the epidermis and dermis from the underlying muscle required many precise and minute cuts. This muscle was later identified as the panniculus carnosus, a surprising finding due to its unexpected thickness. The panniculus carnosus is a cutaneous muscle layer that in many mammals allows an animal to shake its skin, as a horse shakes its skin to remove insects. This was an unexpected finding given the semi-aquatic nature of C. canadensis. Our initial hypothesis was that the panniculus was enlarged due to shivering as a response to cold aquatic environments. This remains a plausible theory, though C. canadensis already possesses a thick, multi-layered coat of fur that provides more than adequate thermal insulation. Another theory suggests that the beaver uses the panniculus carnosus to change its body shape, and thus streamline itself during aquatic locomotion. The thickening of the panniculus as is approaches its insertion points around the neck and axillary regions further supports this theory, as this region is most important to the streamlining. After identifying this layer as the cutaneous musculature, we explored the superficial shoulder muscles by cutting and then reflecting the panniculus along the paths of the previous incisions. The fascia between this layer and the underlying muscles (e.g., trapezius and latissimus dorsi) was easily dissected.

While separating the panniculus and superficial muscles laterally, a large pocket of fat was discovered, temporarily impeding the dissection. There was some fat beneath the panniculus dorsally, but this layer became thicker as we dissected laterally and around to the ventral side. The largest amount of fat was encountered in the lateral areas of the thorax and upper abdomen, and ventrally in areas overlying the pectoral muscles. In these regions, the fat layer reached as much as 2cm thick.

After exposing the majority of the superficial muscles, we then proceeded to locate the scapular spine. Following the removal of the fascia surrounding this area, we were able to trace the origins/insertions of several muscles including infraspinatus, supraspinatus, and serratus ventralis. Overlying all these muscles were the rhomboids. The scapula is positioned tightly against the dorsal aspect of the ribcage, making dissection of the muscles attached beneath it and to its lower border difficult. After separating the fascia from the inferiolateral and areas around the scapula, as well as between the scapula and ribcage itself, subscapularis and teres major were exposed and we made a closer inspection of the insertion of serratus ventralis on the ribcage.

Muscle	Origin(s)	Insertion(s)	Action(s)
Cutaneous Muscles:
Platysma	Lateral surface of the body of the mandible and gonial angle	Anterolateral aspect of the neck	Twitches facial skin [1]
Panniculus Carnosus	Lumbosacral fascia, fascia of the thigh/leg musculature	Axillary regions, caudal regions of the neck	Moves skin on the thorax, abdomen, & pelvis, and may be used in streamlining the body [1, 2]

Branchiomeric Muscles:
Trapezius
a) Spinotrapezius	Spinous processes of upper thoracic vertebrae	Midline of scapular spine	Rotates scapula dorsocaudally [1]
b) Acromiotrapezius	Dorsal aspect of the ligamentum nuchae	Cranial edge of acromion and edge of scapular spine	Pulls scapula dorsally, and abducts the arm [1]
c) Clavotrapezius	Nuchal crest of the occipital bone	Cranio-dorsal surface of clavicle	Draws humerus cranially [1, 2]
Sternocleidomastoid	Sternum (manubrium)	Mastoid process of the temporal bone	Flexes head and neck, rotates head [1, 2]

Superficial Shoulder Muscles:
Pectoralis
a) Pectoralis Profundus Cephalus	Caudal half of the sternum, and from the costal cartilage between ribs 3 to 6	Greater tuberosity of the humerus	Rotates humerus, adducts forelimb [1, 2]
b) Pectoralis Profundus Caudalus	Cranial end of the linea alba	Ventral surface of the greater tuberosity of the humerus	Rotates humerus, adducts forelimb [1, 2]
c) Pectoralis Sublimis	Caudal clavicular surface, ventral surface of the sternum, and costal cartilage of ribs 1 to 7	Medial surface of the shaft of the humerus	Adducts humerus and forelimb, pulls humerus caudally [1, 2]
Latissimus Dorsi	Spinous processes of thoracic vertebrae and supraspinous ligament	Medial border and anterior surface of the humerus	Pulls forelimb dorsally [1, 2]
Levator Scapulae	Crest of the acromion	Wing of the atlas (C1)	Pulls scapula cranially [1]
Deltoids
a) Acromiodeltoid	Caudal edge of the acromion, and inferior border of the clavicle	Deltoid tuberosity of the humerus	Abducts the humerus [1, 2]
b) Spinodeltoid	Lower border of the scapular spine	Deltoid tuberosity of the humerus	Rotates and abducts the humerus [1, 2]

Deeper Shoulder Muscles:
Supraspinatus	Supraspinous fossa of the scapula, cranial and vertebral scapular borders	Anterior aspect of the greater tuberosity of the humerus	Extension of the shoulder joint [1, 2]
Infraspinatus	Infraspinous fossa of the scapula	Anterior aspect of the greater tuberosity of the humerus	Extension of the shoulder joint, rotates humerus [1, 2]
Teres Major	Dorsal aspect of the axillary border of the scapula	Teres tubercle and medial surface of the shaft of the humerus	Flexion of the shoulder, humeral rotation and flexion [1, 2]
Rhomboideus	Ligamentum nuchae and nuchal crest of the occipital bone	Dorsal border of the scapula	Draws base of the scapula forward, elevates the limbs [1, 2]
Subscapularis	Subscapular fossa	Medial surface of the lesser tuberosity of the humerus	Extension of the shoulder, adducts the forelimb [1, 2]
Subclavius	Lateral and medical clavicle	Rib 1, and first costal cartilage	Pulls clavicle toward the sternum [1, 2]
Serratus Ventralis
a) Serratus ventralis cervicis	Transverse processes of C4-C7, and rib 1	Costal scapular surface	Pulls scapula ventrally and caudally [1, 2]
b) Serratus ventralis thoracis	Lateral surfaces of ribs 3 to 8	Costal scapular surface	Pulls scapula ventrally and caudally [1, 2]

1. Young, F.W. (1937). Studies of Osteology and Myology of the Beaver (Castor canadensis). Michigan State College, East Lansing.

2. Hebel, R., & Stromberg, M. W. 1986. Anatomy and Embryology of the Laboratory Rat. BioMed Verlag, Federal Republic of Germany.