Dissection of Beaver thigh, BAA289L 2004

The beaver leg is highly specialized, and is characterized by numerous features related to the main function of the hindlimb: aquatic locomotion. Beavers propel themselves with their hind legs while swimming. The most obvious and unusual feature of the hindlimb, which is visible upon even the most superficial inspection, is the webbing of the feet. Each foot is webbed in between the digits, and when the digits are abducted, the webbing spreads [1]. This increases the surface area of the feet greatly, and they act as efficient paddles during aquatic locomotion.

Another unusual aspect of the hind limb is that it is tightly bound to the pelvic and gluteal regions. The leg is permanently flexed at the knee, and very little flexion/extension is possible at the knee and hip due to the tightly bound anatomy of the leg. The major muscles involved in the binding of leg to the caudal portion of the body are biceps femoris, gluteus maximus, and semitendinosus. Biceps femoris originates at the ischial tuberosity, and attaches all along the shaft of the tibia and in the lateral crural region. Gluteus maximus is also implicated, originating from the transverse processes of the first three caudal vertebrate and ischial tuberosity, and attaching to the patella. Thus, gluteus maximus can also serve as a leg extensor. Semitendinosus originates from the spinous processes of the last three sacral and first two caudal vertebrae, and converges on a tendon that travels with the calcaneal tendon to insert on the calcaneus. Adductor longus is also implicated in this anatomy, originating from pubis and inserting on the femur, the medial femoral epicondyle, and more distally on the medial femoro-tibial ligament.

The quadriceps muscles are proportionally smaller than the other leg and thigh muscles, given that there is little extension possible at the knee. The paddling action performed by the leg is accomplished by alternating plantarflexion and dorsiflexion of the foot [1]. As a result, the leg muscles and extrinsic foot muscles responsible for these actions are relatively large.

The proportions of the forelimb and hindlimb are different, and reflect their differing functions. The beaver forelimb is characteristic of a digging rodent, and thus the musculature of the proximal brachium and of the shoulder is large and robust. In the hind limb, the major function is locomotion, and the most important movement occurs at the ankle joint rather than at the hip (which would be considered the hind limb equivalent of the shoulder joint). Whereas the upper limb is characterized by more robust musculature that acts at the shoulder, the most powerful musculature of the hind limb works across the ankle joint.

We began the dissection on the ventral side, making a cranial to caudal incision down the midline, beginning just caudal to the linea alba. We removed the skin and fascia overlying the muscles of the thigh, and then began a dorsal dissection. We exposed the dorsal thigh muscles by making an incision down the midline of the back, followed by two horizontal incisions (one cranial and one caudal). We focused on the ventral aspect of the thigh musculature to begin the deeper dissections. While removing the fascia on the lateral portion of the thigh, we encountered a thick, tendinous sheet that had to be removed. This tendinous sheet, which originates dorsally and wraps around to insert on the anterolateral aspect of the thigh, is the fascia lata.

We next encountered a large vein, which originates cranially, overlies the adductors, and continues down into the leg. This vein is the greater saphenous vein, and we were able to work around the vein without severing it. Once all of the muscles were exposed, we began separating out the muscles, which proved difficult due to tight grouping of the medial muscles. What originally appeared to be the largest, and most cranial muscle was later identified as the most caudal portion of the abdominal pouch. The internal abdominal obliques, and to a smaller extent transversus abdominis, are visible and slightly overlie the most cranial thigh muscles.

Once we identified the exposed musculature, we were surprised to see that the adductor muscles are very large and well-defined. These muscles are in fact part of the reason that the thigh and leg are so tightly bound to the caudal regions of the body, and their various attachments run the entire length of the femur.The dorsal dissection proved more difficult, as there were large pockets of fat. Additionally, the connective tissues between the hind limb and caudal portion of the body are incredibly tight in this area. At several points, it was impossible to separate the skin from the underlying fascia and muscle without damaging the muscle itself. We were able to identify the gluteals, which are surprisingly small. However, as mentioned above, the leg is naturally constrained in a flexed position, and there would be no need for large gluteals to extend the hindlimb.

Muscle	Origin(s)	Insertion(s)	Action(s)
Superficial hip & thigh muscles
Tensor fasciae latae	lateral aspect of the iliac crest and lumbo-sacral aponeurosis	converges distally onto a tendon that inserts into the distal aspect of the trochanter tertius of the femur	abducts the thigh
Gluteus maximus	sacral aponeurosis and intermuscular septa, tendinous fibers from ischial tuberosity	converges distally onto a tendon that inserts onto the patella	abducts and extends the thigh
Gluteus medius	gluteal surface of the ilium and lumbo-sacral fascia	medial portion of the trochanter major of the femur	extends and abducts the hip
Quadriceps
a) Vastus femoris	trochanter major and anterior surface of the femoral shaft	converges distally and inserts into the upper half of the patella and into the medial tibio-patellar ligament	extends the leg at the knee joint
b) Rectus femoris	ilium (cranial to the margin of the acetabulum)	fuses distally with the vastus femoris, and inserts into the proximomedial portion of the patella	extends the leg at the knee joint
Gracilis	pubic margin along the symphysis	passes over adductor magnus, adductor longus, and semimembranosus to insert on the tibial crest	adducts the thigh and extends the leg
Adductor longus	ventral surface of the pubis, lateral to the symphysis	posterior surface of the femur, medial femoral epicondyle, and medial femoro-tibial ligament	adducts the thigh
Adductor magnus	pubis, medial to the origin of semimembranosus	posterior surface of the femoral shaft, trochanter tertius, and medial femoral epicondyle	adducts the thigh
Adductor brevis	lateral surface of the ischial tuberosity, and tendon of origin of biceps femoris	medial femoral epicondyle, and medial femoro-tibial ligament	adducts the thigh
Pectineus
a) Larger portion	pectineal crest of the pubis	lower part of the trochanter minor and medial epicondyle of the femur	adducts the thigh
b) Smaller portion	anterior margin of the superior pubic ramus	converges onto a tendon that insets into the shaft of the femur	adducts the thigh
Hamstrings
a) Semitendinosus	spinous processes of the last three sacral and first two caudal vertebrae	converges on a tendon that travels with the calcaneal tendon to insert on the calcaneus	plantarflexes the foot
b) Semimembranosus	posterior margin of the inferior pubic ramus and posterolateral portion of the ventral suface of the inferior ischial ramus	converges on two tendons: tendon 1 inserts on the medial femoral epicondyle, the medial patello-tibial ligament, and proximal tibia; tendon 2 inserts on the upper portion of the tibial crest	extends the thigh and flexes the leg
c) Biceps femoris	lateral aspect of the ischial tuberosity	aponeurosis of the lateral crural region and tibial shaft	extends the thigh and flexes the leg

Deep hip flexors and extensors
Piriformis	pelvic surface of the transverse processes of the third and fourth sacral vertebrae	medial aspect of the border of the trochanter major dorsal to the insertion of obturator externus	adducts the thigh
Obturator internus	outer margin of the obturator foramen, except in the most anterior part	trochanteric fossa of the femur	abducts the thigh
Obturator externus	lateral surface of the pubis adjacent to the posterior and medial margins of the obturator foramen	distal border of the trochanter tertius of the femur	adducts and laterally rotates thigh
Quadratus femoris	lateral surface of the body of the ischium	posterior surface of the trochanter major of the femur	extends the thigh
Gemellus anterior	lateral surface of the anterior end of the superior pubic ramus	converges onto a tendon with gemellus posterior, which then inserts into the trochanteric fossa of the femur	laterally rotates and abducts the thigh
Gemellus posterior	dorsal border of the superior ischial ramus	converges onto a tendon with gemellus anterior, which then inserts into the trochanteric fossa of the femur	laterally rotates and abducts the thigh

Conclusions and Comparisons:

The beaver leg is unusual because it is permanently flexed at the knee, and tightly bound by muscles and other connective tissues to the caudal regions of the body. The distal thigh and proximal leg are tightly bound by muscles including adductor longus, gluteus maximus, and two hamstrings (semitendinosus and bicep femoris). Although the webbing of the feet, and the large size of the dorsal leg muscles that plantarflex the foot are adaptations to the beaver’s semi-aquatic lifestyle (and its need to propel itself in the water), the beaver must still be able to move quadrupedally on land. Due to the limited extension at the knee, however, the beaver is restricted to a relatively slow and short gait.

Many rodents, including the laboratory rat, walk with their femurs nearly parallel to the ground. While this position of the femur is similar to beaver’s anatomy, the rat is not limited by a tightly bound muscle anatomy like the beaver is. This position in locomotion is not as difficult to most of the smaller rodents, however, because the hindlimbs are supporting far less weight. For the beaver, the large, and tightly bound muscles may aid in its ability to support its larger size while still maintaining the parallel position of the femur.

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

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