Tree shrew facial and masticatory muscles
Photo and text: Terence Mitchell
Removing the skin - Starting from an existing edge of loose skin, use the rat-tooth forceps to gently pull the skin off of the head. Use scissors to cut the skin into manageable pieces. If you are very careful some of2 the superficial facial muscles will remain after the skinning process is complete. The jaw musculature and one of the facial muscles, the auricular, can be seen in Figure 1.
Temporalis - Arises from the temporal fossa and surrounding ridges and the zygomatic arch, and inserts onto the coronoid process of the mandible. The temporalis extends dorsally on the right and left sides to the temporal ridges. Each side has an anterior and posterior temporal ridge, which meet at oblique angles. At the point where the anterior and posterior ridges intersect, they contact the intersection of the ridges from the other side of the skull. At the most corresponding anterior and posterior ridges from opposing sides- and therefore the temporalis muscle arising from them, are separate by 4 mm. The temporalis is the primary muscle responsible in elevating the mandible.
Masseter - Arises from the zygomatic arch and inserts into the angle and ramus of the mandible. The masseter is responsible for elevating the mandible.
Mastication
The temporalis and masseter muscles along with the pterygoids and tongue muscles are primarily responsible for chewing. These muscles often show differences in cross-sectional area corresponding to dietary specializations. In carnivores and other mammals relying on a strong shearing motion, the temporalis muscle comprises about 64% of the total muscle weight and the masseter accounts for approximately 28% [1]. Herbivores emphasize a grinding mastication, and have a correspondingly large masseter, 50% of the total muscle weight. While I could not find published results of tree shrew cross-sectional area or muscle weights, the relative size of these muscles indicate that tree shrews fall in with the carnivore-shear model. Studies of tree shrews in the wild indicate that they do eat a very high percentage of insects, thereby necessitating shearing mastication and a large temporalis muscle.
Tree shrews are cited as a good model system for studies of early eutherian
mastication because of their simple, tribosphenic, hypsodont molars [2]. Early
hypotheses suggested that a simple scissor-like movement of the mandible dominated
masticatory performance in tupaiids [3]. However, behavioral studies predating
these hypotheses demonstrated that something more complicated was occuring [4].
Instead, tree shrews seemed to fall in with primates having a puncture-crush
phase followed by a chewing phase. Later studies showed that chewing in these
animals was too variable to describe a typical chewing method for any individual
[5]. Instead, chewing motions were grouped according to type of food eaten or
averaged for a species. These composites did not appear to be similar to other
data presented for tree shrews in the literature or to the puncture-crush/true
chew model. While many aspects of tree shrew mastication are variable, chewing
always occurs on only one side at a time. Further evidence for the complex nature
of tree shrew mastication is found in the examination of the gross morphology
of several structures associated with chewing [6]. A shallow mandibular fossa,
small mandibular condyle, and loose joint ligaments suggest a wide range of
motion at the temporomandibular joint.
Tree shrews also possess a mobile mandibular symphysis, an incomplete fusion
of the two halves of the mandible, which may prevent dangerous contact between
the sharply cusped teeth during rapid chewing. In addition, this high degree
of flexibility may allow fine-tuning of chewing patterns thereby yielding more
efficient food breakdown [7]. Behavioral evidence also suggests complexity such
as lateral jaw movements [8]. If omnivorous tree shrews are assumed to be an
intermediate between either a reptilian or an earlier insectivorous mammalian
form of mastication and a later mammalian condition involving differentiated
molars, then this structural and behavioral complexity has some interesting
implications [5]. For instance, cheek tooth morphology would not necessarily
be a good indicator of diet. Instead, a tribosphenic form coupled with a wide
range of jaw movements would allow new species to colonize unfilled niches rapidly.
Therefore, dental microwear studies may play an important role in determining
the diets of early mammals. Tree shrews are a good model for testing correlations
between specific food types and diagnostic patterns of microwear.
Movement of sense organs
Active tree shrews will often stop and rotate their ears for extended periods of time [9]. They are very skittish and will often scurry away after such an episode, even when the observer detects no sounds. Tree shrews have acute hearing, accompanied by large auricular muscles. The auriuclar mm. are responsible for directing the orientation of the pinnae (Figure 1). Tree shrews also have strong erector vibrissae muscles attached to their whiskers [10]. Such muscles may be particularly useful for moving the vibrissae during rooting behavior, where vision and hearing are of diminished utility.
1 Turnbull, W.D. 1970. Mammalian Masticatory Apparatus. Fieldiana: Geology.
18(2): 149-356.
2 Butler, P.M. 1980. The tupaiid dentition. In: W.P. Luckett, ed. Comparative
Biology and Evolutionary Relationships of Tree Shrews. New York: Plenum Press.
3 Noble. 1979. Comparative functional anatomy of the temporomandbiular joint.
In G.A. Zarb, ed: Temporomandibular Joint Function and Dysfunction. St. Louis:
C.V. Mosby, pp. 15-41.
4 Hiiemae, K.M. and R.F. Kay. 1973. Evolutionary trends in the dynamics of
primate mastication. Symp. IVth Int. Congr. Primatol. 3:28-64.
5 Fish, D.R. and F.C. Mendel. 1982. Mandibular movement patterns relative to
food types in common tree shrews, Tupaia glis. Am. J. Phys. Anthropol.
58(3):1-15.
6 Fish, D.R. 1983. Aspects of Masticatory Form and Function in Common Tree
Shrews, Tupaia glis. J. Morphol. 176:15-29.
7 Scapino, R.P. 1965. The third joint of the canine jaw. J. Morphol. 116:23-50.
8 Kay, R.F. and K.M. Hiiemae. 1974. Jaw movement and tooth use in recent and
fossil primates. Am. J. Phys. Anthropol. 40:227-256.
9 Emmons, L.H. 2000. Tupai: A Field Study of Bornean Treeshrews.
10 Le Gros Clark, W.E.1924. The myology of the tree shrew (Tupaia minor). Proceedings of the Zoological Society of London 1924:559-567.
Davis, D.D. 1938. Notes on the anatomy of the tree shrew Dendrogale.
Field Mus. Publ. Chicago Zool. 20:383-405.
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.
to Comparative Mammalian Anatomy home
to mammalian facial and masticatory muscles