Dissection of deep hip and tail of the opossum, BAA 289L 2004

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Hip and Tail

Deep Hip and Tail of the Opossum (Monodelphis domestica)

Text and photos by Sarah Ogburn and Linda Brogdon

Introduction

All didelphids have a prehensile tail. Although Monodelphis domestica is less arboreal than many didelphids, it still uses its tail for safety while walking on branches and for hanging upside down. Its tail is considerably shorter than those of other genera within Didelphidae, however. This feature lends it its common name—the grey short-tailed opossum.

Dissection

The deep muscles of the hip were dissected during the pelvis and thigh lab. The psoas major and minor and iliacus muscles become visible when the epipubic bone is pushed out of the way toward the midline. They are long, strap-like muscles that insert between the vastus internus muscle and the adductor muscles. The quadratus lumborum muscle is visible deep to the psoas muscles.

We removed the skin from the tail using the probe and small scissors. It was necessary to cut along the length of the tail and then remove the skin rather than simply peel the skin off. The muscles are covered with a very thick fascia that makes dissection and identification of the extremely tendinous tail difficult. The muscles were easier to identify nearer the base of the tail, as they are larger in this area and not covered by as much tendon or fascia.

The medial multifidus lumborum muscle of the posterior back continues onto the tail as the extensor caudae medialis muscle. It is the most medial of the tail muscles, lying on either side of the spinous processes of the vertebrae. It is segmented but continues down the length of the tail; each segment crosses a few vertebrae at a time. The longissimus lumborum muscle of the back is just lateral to the multifidus lumborum muscle. It continues as the extensor caudae lateralis muscle in the tail. The extensor caudae lateralis is just lateral to the extensor caudae medialis muscle. It is somewhat thicker than its medial counterpart, and is also segmented. It continues down the length of the dorsal aspect of the tail. Both of these muscles are very well developed and recognizable near the base of the tail, but become more tendinous and difficult to identify as they continue posteriorly. The abductor caudae externus is most visible and largest near the base of the tail. It is ventral to extensor caudae lateralis and is the most lateral of the dorsal tail muscles. It is also segmented and runs most of the length of the tail.

Just ventral to the abductor caudae externus muscle is the abductor internus muscle. It is the most lateral of the ventral tail muscles. It is larger than the abductor caudae externus muscle at the base of the tail, but is mostly obscured by tendons and fascia along the length of the tail and its size is more difficult to judge. It is segmented and runs the length of the tail. The flexor caudae lateralis muscle is just medial to the abductor caudae internus muscle. It is a very thick, well developed muscle, but is obscured by the thick fascia and tendons that cover the tail. Medial to it is the smaller flexor caudae medialis muscle. It is not covered by as much fascia and tendon as the other ventral tail muscles and can be seen without removing any connective tissue. The muscles on the ventral side of the tail are all larger than those on the dorsal side of the tail.

Discussion

In general, animals with prehensile tails exhibit a specific suite of specializations of the tail. The posterior caudal vertebrae are short so that the tail can curl tightly around supports. The muscles on the ventral side of the tip of the tail are larger than those on the dorsal side—allowing for a strong grip. The caudal flexor musculature is large as are the intertransversarii muscles. Musculature at the base of the tail is well developed and helps to protect the intervertebral joints from the strain of hanging by the tail. The tendons of the tail musculature span fewer vertebrae than those of non-prehensile tails. This allows the tail to form tighter curves around branches [1, 2]. This feature was apparent when we compared the opossum tail to the rat tail. The tendons of the rat’s tail muscles cross more vertebrae and the tail is smoother, less tendinous, and less covered in fascia than the opossum’s tail. The muscles looked more continuous than those in the opossum’s tail. The opossum’s tail muscles segments were much shorter than the rat’s and the muscles were larger and thicker.

Many animals with strong tails (not necessarily prehensile, however) exhibit small sesamoid bones forming partial haemal arches on the ventral side of the caudal vertebrae. These are muscle attachment sites for the well developed ventral tail musculature. Both the opossum and beaver skeleton have these sesamoid bones in the skeletons that we examined.

Most didelphids have tails that are as long or longer than the head and body combined; usually their tails have 30 caudal vertebrae. Monodelphis, however, has a tail shorter than its body and head, with only 20 caudal vertebrae [1]. This is a derived feature among the normally long-tailed didelphids. Despite the shortening of the tail and more terrestrial lifestyle, Monodelphis is still capable of supporting itself upside down by the tail [1].

Aside from hanging upside-down, the prehensile tail is used for safety while walking on branches. An opossum will curl the tail loosely around the branch it is walking on so that it can quickly grip the branch if it loses its footing or if the branch is shaken [3].

Juvenile opossums tend to hang by their tails more often than do adult individuals.During normal terrestrial walking, the tail is curled ventrally [3]. Hanging upside down by the tail leaves the animal fairly vulnerable and it is important that the animal be able to resume a normal quadrupedal stance quickly. The crurococcygeus and femorococcygeus muscles can flex the tail laterally and move the hind-limbs posteriorly when the animal is hanging by the tail. The ability to move the hind-limbs posteriorly while suspended by the tail allows the animal to grasp the branch it is suspended from and quickly hoist itself back onto that branch [4].

Muscles’ Origin, Insertion and Action (from our observations and sources [4] and [5])

Muscle		Origin	Insertion	Action

iliacus		ventral face of the ilium	lesser trochanter of femur	hip flexion and thigh eversion
psoas major		ventral surfaces of lumbar vertebrae	lesser trochanter of femur	hip flexion, thigh eversion, back flexion
psoas minor		ventral surfaces of lumbar vertebrae	ischium	back flexion
quadratus lumborum		posterior thoracic and lumbar vertebrae	ilium	lateral flexion of trunk
gemellus superior		lateral edge of of ischium	tendon of obturator internus	internal rotation of hip, thigh abduction, thigh eversion
gemellus inferior		lateral edge of ischium	tendon of obturator internus	internal rotation of hip, abduction of the thigh, thigh eversion
obturator internus		inner rim of obturator foramen	intertrochanteric line	abducts and everts the thigh
obturator externus		inner rim of obturator foramen	intertrochanteric line	thigh adduction, rotation, protraction
quadratus femoris		ischium	posterior surface of femur distal to greater trochanter	retracts and rotates thigh
femorococcygeus	third and fourth caudal vertebrae		proximal half of posterior face of femur	hip extension, pulling hindlimbs toward the tail while hanging by the tail, lateral flexion of the tail
piriformis	S2 and first caudal vertebrae		greater trochanter of femur	internal rotation of hip
crurococcygeus		third caudal vertebra	medial border of tibia	flexion of the knee, some hip extension, pulling hind limbs back when hanging by the tail, lateral flexion of the tail
multifidus spinae (in segments along the length of the back)		mammalary processes of thoracic vertebrae	spinous processes of successive thoracic and lumbar vertebrae	extension of back
extensor caudae medialis (in segments along length of tail)		spinous processes of lumbar and caudal vertebrae	caudal articular processes of successive caudal vertebrae	extension of the tail, slight lateral bending of the tail
longissimus lumborum (in segements along the length of the back)		transverse processes of thoracic and lumbar vertebrae	transverse processes of thoracic and lumbar vertebrae	extension of back
extensor caudae lateralis (in segments along the length of the tail)		transverse and articular processes of sacral and caudal vertebrae	ventral side of successive caudal vertebrae	dorsolateral extension of the tail
abductor caudae medialis		medial border of ilium, transerverse processes of sacral and caudal vertebrae	dorsolateral surfaces of successive caudal vertebrae	lateral and slight dorsal bending of the tail
abductor caudae lateralis		transverse processes of caudal vertebrae	lateral edge of transverse processes of successive caudal vertebrae	lateral and slight ventral bending of the tail
flexor caudae medialis		ventrolateral border of sacral and caudal vertebrae, haemal arches of caudal vertebrae	haemal arches of caudal vertebrae	ventral and some lateral bending of the tail
flexor caudae lateralis		ventrolateral surfaces of lumbar, sacral and caudal vertebrae, haemal arches of caudal vertebrae, transverse processes of caudal vertebrae	haemal arches of caudal vertebrae	ventral and some lateral bending of the tail

We used figures from sources [4] and [5] during our dissection.

References:

1. Argot, C. 2003. Functional-adaptive anatomy of the axial skeleton of some extant marsupials and the paleobiology of the Paleocene marsupials Mayulestes ferox and Pucadelphys andinus. Journal of Morphology 255: 279-300.

2. Youlatos, D. 2003. Osteological correlates of tail prehensility in carnivorans. Journal of Zoology, London 259: 423-430.

3. Hartman, C. 1952. Possums. University of Texas Press, Austin.

4. Elftman, H.O. 1929. Functional adaptations of the pelvis in marsupials. Bulletin American Museum of Natural History 58: 189-232.

5. Stein, B. 1981. Comparative limb myology of two opossums, Didelphis and Chironectes. Journal of Morphology 169: 113-140.

Artwork: Weil, from Stubbs' 1776 "Anatomy of the Horse."
Background free from Eos Development, with slight color modification.