Tree shrew deep hip and tail muscles
Photos and Text: Terence Mitchell
To understand the muscles of the tail it is first necessary to have a clear view of the deep back muscles. See the body wall and back musculature page if you have not dissected this region. Find an existing free edge of skin around the tail and using a pair of rat-tooth forceps pull caudally to remove the fur in one large piece. A continuation of the thoracodorsal fascia forms a tough layer around the tail. Insert a sharp probe beneath this fascia near the base of the tail. Run the probe caudally between the fascial and muscle layers, and then tear the fascia along the length of the probe. Repeat this procedure as necessary to reveal the muscle layers beneath. Use a pair of rat-tooth forceps to pull off as much fascia as possible.
The medial two deep back muscles, longissimus and multifidus, contribute significantly to the tail musculature. In Figure 1, note that the medial most muscle, multifidus, runs inferiorly into the tail. By the 4th or 5th caudal vertebrae it breaks up into several smaller intervertebral muscles. Lateral to multifidus, longissimus, renamed extensor caudae lateralis in the tail, sends several strong tendons inferiorly, which attach to the transverse processes of the caudal vertebrae. Both muscles extend the tail. From a lateral view of the tail, Figure 2, you can see a relatively large muscle extending from the thoracodorsal fascia and inserting on the transverse processes of the caudal vertebrae. This muscle, the abductor caudae externus, is responsible for abducting the tail. On the ventral side of the tail, Figure 3, observe the strong tendinous fibers running down the length of the tail on either side of the midline. The sacrococcygeus arises from the lumbar vertebrae and gives rise to these tendons, which insert on the ventral aspect of the caudal vertebrae. These medial fibers of sacrococcygeus, known as infracoccygeus, have a lateral counterpart, the lateral head of sacrococcygeus. These fibers form a muscular band in the tail just lateral to infracoccygeus. Both muscles flex the tail.
Tails are used by a wide variety of animals for diverse purposes such as countercurrent heat exchange, grasping, propulsion, communication, and balance. While I do not know of any papers examining any of these uses in tree shrews, it seems most likely that tupaiids are using their tails for balance. Balance is particularly important in mammals that move fast or change direction rapidly and mammals that are scansorial or arboreal. Tails are useful in an animal like a tree shrew, because they allow it to change its center of mass to keep it directly over a shifting and narrow substrate, such asbranches. In a moderately arboreal tree shrew like Tupaia belangeri the tail is long and its musculature relatively differentiated and powerful. The highly arboreal Ptilocercus lowii has 31-33 caudal vertebrae compared to the 24 caudal vertebrae of most Tupaia species [1]. The 7-9 extra vertebrae in the tail probably account for the longer external tail noted by observers of wild Ptilocercus [2]. This additional length presumably augments the balancing ability of Ptilocercus.
1 Sargis, E.J. 2001. A preliminary qualitative analysis of the axial skeleton
of tupaiids (Mammalia, Scandentia): functional morphology and phylogenetic implications.
J. Zool. Lond. 253: 473-483.
2 Martin, R.D. 1990. Primate origins and evolution. Princeton: Princeton University Press.
Le Gros Clark, W.E.1924. The myology of the tree shrew (Tupaia minor). Proceedings of the Zoological Society of London 1924: 559-567.
Le Gros Clark, W.E.1926. On the anatomy of the pen-tailed tree shrew (Ptilocercus lowii). Proceedings of the Zoological Society of London 1926: 559-567.