Dissection of beaver hip and tail, BAA 289L 2004

One of the most distinctive physical traits of a beaver is the tail. The tail is flat and scaly and functions in both behavioral and physiological roles. On land, the beaver uses its tail as a prop to balance and sit on when cutting trees. In the water, the beaver uses its tail as a rudder during diving and swimming. The beaver also uses its tail as a warning signal; when a beaver senses danger, it will slap its tail on the water’s surface as a warning to other beavers before it dives underwater. Beavers, notably the young, respond to an adult’s tail slap by diving immediately underwater water or by returning to the lodge [2].

Physiologically, the tail also serves as a site for fat storage. Additionally, it is involved in heat exchange through a countercurrent arrangement of blood vessels that run through the entire length of the tail. The base of the tail contains a rete mirabilis, which is an intertwined network of blood vessels. In this network of blood vessels, arteries with warm blood from the core of the body transfer heat to adjacent veins [1]. The veins take the heat back to the core, thus the blood at the surface of the tail is cooler. Overall, this mechanism protects the beaver from too much heat loss to the surrounding cold water [1].

We began the dissection by removing the scales from the beaver’s tail. However, this process proved much too time consuming. Thus, removed the scales and outer layers of skin simultaneously. We started this procedure on the ventral side of the tail. Although separation of the structures form the surrounding collagen and fat was difficult, we were able to see faint portions of the caudal vertebrae surrounding muscles/tendons. We exposed a bundle of long, thick tendons running just laterally on either side of the caudal vertebrae.

To compare the difference between the ventral and dorsal side of the tail, we next removed the skin and scales on the dorsal side of the tail. We immediately noticed differences between dorsal and ventral side, specifically the far fewer number of tendons on the dorsal aspect of the tail compared to the ventral side. We postulated that the number of tendons correlates with the power of tail action. Tendons located in the dorsal side of the tail are responsible for dorsal flexion (flexion of the tail upward) and tendons on the ventral side are responsible for ventral flexion (flexion of the tail downward). Since the beaver uses its tail to signal alarm, it needs to have a fast powerful downward flexing motion to hit the water surface, so a larger number of tendons are needed on the ventral side of its tail to make the motion stronger.

Muscle	Origin(s)	Insertion(s)	Action(s)
Coccygeus	pelvic surface of the pubis along the pubic rim of the obturator foramen	bodies of the last sacral segment and the first caudal vertebra	elevates the pelvis
Sacro-caudalis dorsalis
a) Medial part	spinous process of the lumbar vertebrae, the sacrum, and the first seven caudal vertebrae	articular processes of the caudal vertebrae	dorsal flexion of the tail (upward flexion of the tail)
b) Deep part	dorsum of the sacrum	the caudal margin of the caudal vertebrae	dorsal flexion of the tail (upward flexion of the tail)
c) Lateral part	lateral margin of the fused transverse processes of the sacrum and from the transverse processes of the first six caudal vertebrae	sides of the caudal spinous processes	dorsal flexion of the tail (upward flexion of the tail)
Sacro-caudalis ventralis
a) Medial superficial
- Pelvic head	anterior margin of the obturator foramen, from the pelvic surface of the anterior extremity of the anterior ramus of the ischium	ventrolateral surface of the more distal caudal vertebrae	ventral flexion of the tail (downward flexion of the tail)
- Vertebral head	bodies and interarticular fibrocartilage of the last three sacral and first three caudal vertebrae	ventrolateral surface of the more distal caudal vertebrae	ventral flexion of the tail (downward flexion of the tail)
b) Medial deep	bodies of the last two lumbar vertebrae, the bodies and transverse processes of the sacral and the first nine caudal vertebrae	lateral aspect of the bodies of the caudal vertebrae	ventral flexion of the tail (downward flexion of the tail)
c) Lateral superficial	lateral border of the obturator foramen	transverse processes of the second to seventh caudal vertebrae	ventral flexion of the tail (downward flexion of the tail)
d) Lateral deep	ventral surface of the transverse processes of the first seven caudal vertebrae	transverse processes of the seventh to twelfth caudal vertebrae	ventral flexion of the tail (downward flexion of the tail)

The beaver’s tail is one of its most unique features. It serves many more functions than a typical mammalian tail does. The shape of the beaver tail is unique. It is large and flat, with increased surface area that aids in heat exchanges and in aquatic locomotion. As a result of this morphology, the caudal vertebrae are very flat and short.

Using the microscope, we compared scales from the tails of the beaver and the rat. Compared to the rat, the beaver tail contains much more fat, and has relatively larger scales. Although the scales are shaped differently, the rat tail is similar to the beaver tail in that each scale has 2-4 hairs emerging from underneath it that run distoproximally. Compared to the beaver, the hairs on the rat tail are proportionally longer.

Because of the multiple functions that the beaver’s tail serves, it must maintain a stiff form, and thus the beaver cannot curl its tail. The beaver tail has a tri-layer of collagen fibers that provide greater support to the tail than just the fat would. Given the relatively small width of the caudal vertebrae in comparison to the size of the tail as a whole, the presence of the collagen fibers adds support, and also prevents the possibility of the beaver curling its tail.

Another interesting feature of the beaver tail is the presence of haemal sesamoids on the ventral surfaces of the caudal vertebrae. While haemal arches are a common feature of many terrestrial vertebrates, they are not typically seen in mammals. Haemal sesamoids, which are not actually sesamoids, but provide the same function (greater mechanical leverage), are often found in the caudal vertebrae of mammals with muscular and/or prehensile tails. In comparison of the caudal vertebrae of the beaver, which has a relatively strong, muscular tail, to the caudal vertebrae of the opossum, which has a prehensile tail, haemal sesamoids were seen ventrally in the caudal vertebrae of both animals.

1. Marchand, P.J. 1996. Life in the cold. Lebanon, New Hampshire: University Press of New England.

2. Morgan, L.H. 1868. The American Beaver and His Works. J. B. Lippincott & Company, Philadelphia.

3. Young, F.W. (1937). Studies of Osteology and Myology of the Beaver (Castor canadensis). Michigan State College, East Lansing. pp. 73-75.