Bat facial and masticatory muscles
Photo and text: Kathy Coulombe and Sarah Winstanley
The external ear (pinna) has been removed here; the bar between the temporalis and masseter muscles is the zygomatic arch.
Most of the facial muscles extend into the fascial layer of the epidermis. This made the removal of this superficial layer complicated as it was difficult to separate it from the deep structures, especially around the mouth.
Tadarida braziliensis is insectivorous. Given the need to pierce and slice its prey, we expected that the temporalis muscle would be enlarged to create a powerful jaw closing action, compared with a smaller masseter muscle (which is usually enlarged for herbivores).
The platysma runs just deep to the epidermal layer from the pectoralis muscles, over the whole neck region up to the mouth and cheek area. This muscle looks just like another layer of the skin, and given that it is so closely connected to the epidermis, it is difficult to differentiate the two.
Tadarida braziliensis has protrusive lips that extend far beyond the teeth. Perhaps this is related to the hairs on the lips. These hairs may be used for tactile or scent sensation to aid with the capture of prey. This species has been known to use facial expressions for communication.[1] This use of the facial muscles may increase the size of the orbicularis oris.
It is interesting to note that the stapedius muscle in echolocating bats, such as Tadarida braziliensis, is greatly enlarged. This muscle acts to dampen the vibrations from the stapes to the auricular vestibule. Since these bats produce such a high frequency of sound for navigation, it is important to be able to reduce the noise they hear when the sound is emitted, so that it does not interfere with the echo of that sound when it returns.[2]
The buccinator muscle is a tiny muscle lateral to the mouth. This muscle expels air from the oral cavity and acts to keep the food in the mouth during chewing.
While it was difficult to determine the number of teeth in our specimen due to the rigidity of the mandibular muscles, we noticed that the teeth of the specimen demonstrate the sharp shearing crests of the molars and premolars, and pointed incisors and canines. These teeth are adaptive for a diet of insects, as they are specialized to pierce and slice the insect prey.
The temporalis muscle is relatively very large, as expected from the power needed to chew the insects. The temporalis body originates from the occipital bone of the neurocranium. At the origin, the muscle body is a large bulb and covers the entire braincase, which then spreads laterally to insert on the coronoid processes of the mandible.
The masseter muscle is small and has a round body at the insertion point on the ramus of the mandible. The size of this muscle is as expected, since it is primarily useful for pressure in the molar teeth for grinding grass, leaves and fruit. Since the diet of Tadarida braziliensis does not include flora, it is logical that the masseter is relatively reduced.
| Muscle | Origin | Insertion | Action |
| platysma p. 535 [3] | fascia superficial to deltoid and pectoralis | mandible, cheek, corner of mouth, and orbicularis oris | depression of mandible, and flexion of neck and lower face |
| buccinator p. 535 [3] | mandibular process | corner of mouth | adduction of the cheek against the molars |
| orbicularis oculi p. 535 [3] | medial portion of the orbit, and the lacrimal bone | skin around orbit and tarsal plate | closes eyelids |
| orbicularis oris p. 535 [3] | maxilla, mandible, and deep surface of skin | lips | compresses and protrudes lips |
| temporalis p. 586 [3] | medial neuro-cranium | medial tip of coronoid process of mandible, and ramus of mandible | closure of the jaw, and elevation of the mandible |
| Remarks: muscle of mastication | |||
| masseter p. 532, 586 [3] | inferior border of the zygomatic arch | lateral portion of the ramus of the mandible, and coronoid process of mandible | closure of the jaw, and elevation and protrusion of the mandible |
| Remarks: muscle of mastication | |||
1. Tuttle, M. 1994. The Lives of Mexican free-tailed bats. Bats. 12:6-14
2. Vaughan, T.A. 1970. The Muscular System. In: Biology of Bats,Vol.I, ed.
W.A. Wimsatt. New York: Academic Press.
3. Agur, A.M.R., & Lee, M.J. 1999. Grant's Atlas of Anatomy, tenth ed. Philadelphia, Lippincott Williams & Wilkins.