Dissection of Beaver shank and foot, BAA 289L 2004

The beaver leg and foot are highly specialized for aquatic locomotion. The feet are webbed, and the legs are bound by muscles to the caudal part of the body, resulting in very little movement at the hip and knee joint as compared to the ankle joint. The feet provide the main propulsive force when beaver swim. In fact, the hindlimb is so tightly bound to the pelvic and gluteal regions that very little movement occurs at the knee joint. The leg is permanently flexed at the knee, thus restricting flexion and extension at the joint. It is the repeating plantarflexion and dorsiflexion motion at the ankle joint that propels the beaver in water. The leg muscles and extrinsic foot muscles that cross the ankle joint are relatively large compared to the proximal leg and hip muscles.

The intrinsic muscles of the foot are much more developed than these of the hands. The interossei are visibly striated in the foot, whereas they are only tendinous slips in the hands. The interossei are important in abduction and adduction of the digits. Because abduction of the digits spreads the webbing and increases the surface area with which to push against the water, it is important for the beaver to be able to maintain strong abduction of the pedal digits over long periods of time. For this reason, the extensor tendons are larger than the flexor tendons.

We began the dissection of the leg by making a straight incision down the midline of the ventral aspect of the leg. We removed the skin and fascia on the ventral anterior surface, before making a similar incision down the dorsal posterior midline of the leg. Removal of the skin and connective tissues in the more distal parts of the leg was difficult, because in this region, the leg is tightly bound to the caudal portion of the trunk. Once the muscles were exposed, we separated and identified them, and also dissected out the deeper muscles.

We began dissection of the foot on the dorsal side. The skin and fascia is thin (approximately 0.25 cm), but is tightly bound to the muscles. During dissection and separation of the extensor tendons, we encountered a large, flat ossification that was embedded in the dorsomedial extensor retinaculum that wraps around from the dorsomedial aspect of the foot to the ventral side. Another retinaculum (not ossified) was encountered on the proximolateral aspect of the foot. The tendons on the dorsal aspect are relatively large, and have a complex branching pattern.

Before dissecting the plantar surface, we first removed the fat and fascial sheath around the calcaneal tendon, which comprises tendons from plantaris, soleus, gastrocnemius, and semitendinosus. The plantar surface dissection proved much more difficult, as the pad was especially thick. The proximal pad is very thick and dense, while the distal pad is thinner and overlies a softer, deeper fat pad. After removing the plantar pads, we exposed the flexor tendons, and also identified the four pair of interossei.

Muscle	Origin(s)	Insertion(s)	Action(s)
Shank and foot musculature
Tibialis anterior	lateral margin of the patella, lateral tibio-patellar ligament, and anterior border and lateral surface of the tibia	medial cuneiform and base of metatarsals 1 and 2	dorsiflexes and inverts the foot
Extensor digitorum longus	lateral epicondyle of the femur	converges onto four tendons that insert into the terminal phalanges of digits 2-5	extends digits 2-5
Extensor digitorum brevis	upper and lateral surface of the calcaneus	fuses with a single tendon of extensor digitorum longus that goes to the fifth digit	extends the fifth digit
Extensor hallucis longus	fibular head and shaft, interosseus ligament, and intermuscular septum between EHL and EDL	terminal phalanx of digit 1	extends the first digit
Fibularis muscles
a) Fibularis longus	inferior surface of the head of the femur	base of the first metatarsal	everts the foot and extends the knee
b) Fibularis brevis	femoral head: lateral epicondyle of the femur fibular head: posterior surface of the head of the fibula	lateral aspect of the base of the fifth digit and corresponding metatarso-phalangeal sheath	dorsiflexes and everts the foot
c) Fibularis tertius	posterior surface of the middle third of the fibula	base of the fifth metatarsal	everts the foot
Abductor digiti quinti	antero-lateral surface of the upper three-fourths of the fibula, interosseus ligament, and the fibular head	lateral surface of the tuberosity on the proximal extremity of the fifth metatarsal	abducts the fifth digit
Gastrocnemius	medial epicondyle of the femur	lateral aspect of the proximal portion of the tuber calcis	plantarflexes and everts the foot
Plantaris	lateral epicondyle of the femur	plantar aspect of the tuber calcis and through the plantar aponeurosis	plantarflexes the foot
Soleus	femoral head: lateral epicondyle of the femur distal to the origin of plantaris and the patella fibular head: upper 1/3 of the fibular shaft	femoral head: proximal extremity of the tuber calcis medial and lateral to the insertion of gastrocnemius fibular head: anterior surface of the tendon of the femoral head	plantarflexes the foot
Flexor digitorum longus	posterior surface of the upper four-fifths of the fibula, posterior surface of the upper two-thirds of the tibia, and the interosseus ligament	combines with the quadratus plantae and converges on five tendons, which insert onto the terminal phalanges of digits 1-5	flexes the digits
Flexor hallucis longus	upper portion of the posterior surface of the tibial shaft	medial surface of the first cuneiform	flexes the first digit
Flexor digitorum brevis	os calcis through the plantar aponeurosis	medial and lateral aspects of the base of the second phalanx	flexes the second digit
Tibialis posterior	upper half of the medial border of the tibia	fascia covering the medial surface of the tarsus	plantarflexes and inverts the foot
Popliteus	lateral epicondyle of the femur	medial surface of the upper 1/4 of the shaft of the tibia	flexes the knee
Quadratus plantae	latero-posterior surface of the distal end of the os calcis	tendon of the flexor digitorum longus	flexes the digits
Interossei	four pairs, each originating from the plantar surface of the base of metatarsals 2-5	medial and lateral sides of the base of the proximal phalanges of digits 2-5	abduct and adduct digits 2-5

Conclusions and Comparisons:

The most obvious and unusual trait of the beaver’s feet compared those of many other mammals is the webbing. This is an adaptation to the beaver’s semiaquatic lifestyle, and is necessary for the beaver to be able to propel itself in the water. The large dorsal leg muscles are also related to this necessity, as the plantarflexing motion of the feet requires a great deal of force to propel the beaver. Additionally, the interossei of the beaver feet are surprisingly large. These intrinsic muscles of the hands/feet are usually small and tendinous, but they are large and striated in the beaver feet. This is necessary for the abduction of the digits to spread the webbing of its feet during swimming.

The leg and even the foot, to some extent, are tightly bound to the caudal portion of the body. The limb is permanently flexed at the hip and knee, at which little movement is possible. As a result, many of the thigh muscles function differently in the beaver than in other mammals. For example, semitendinosus, a hamstring, acts as a plantarflexor (its tendon crosses the ankle joint with the calcaneal tendon, and inserts on the calcaneus). A somewhat similar situation was seen in the cat, where another hamstring, the biceps femoris, inserted distally on the calcaneus by crossing the ankle joint with the calcaneal tendon. The gluteus maximus muscle also inserts more distally in the beaver than it does in many other mammals, its fibers converging on the patella.

The fibularis muscles of the beaver are also surprisingly long. The fibularis longus originates at the head of the femur, and crosses the knee joint and ankle joint to insert on the base of the first metatarsal. Thus, it has leverage to both evert the foot, and to extend the knee Even the fibularis brevis muscle originates more proximally than in many mammals, the beaver having femoral and fibular heads that originate on the lateral epicondyle of the femur and the head of the fibula, respectively.

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