Electromyography of the anterior temporalis and masseter muscles of owl monkeys (Aotus trivirgatus) and the function of the postorbital septum

Anthropoids and tarsiers are distinguished from all other vertebrates by the possession of a postorbital septum, which is formed by the frontal, alisphenoid, and zygomatic bones. Cartmill (1980) In: Evolutionary Biology of the New World Monkeys and Continental Drift. New York: Plenum, p 243-274] suggested that the postorbital septum evolved in the stem lineage of tarsiers and anthropoids to insulate the eye from movements arising in the temporal fossa. Ross (1996) Am J Phys Anthropol 91:305-324] suggested that the septum insulates the orbital contents from incursions by the line of action of the anterior temporal muscles caused by the unique combination of high degrees of orbital frontation and convergence. Both of these hypotheses must explain why insulation of the orbital contents could not be achieved by decreasing the size of the anterior temporal musculature with a corresponding increase in size of the remaining jaw adductors, rather than evolving a postorbital septum. One possibility is that the anterior temporalis is an important contributor to vertically directed bite forces during all biting and chewing activities. Another possibility is that reduction in anterior temporal musculature would compromise the ability to produce powerful bite forces, either at the incisors or along the postcanine toothrow. To evaluate these hypotheses, electromyographic (EMG) recordings were made from the masseter muscle and the anterior and posterior portions of the temporalis muscles of two owl monkeys, Aotus trivirgatus. The EMG data indicate that anterior temporalis activity relative to that of the superficial masseter is lower during incision than mastication. In addition, activity of the anterior temporalis is not consistently higher than the posterior temporalis during incision. The data indicate relatively greater activity of anterior temporalis compared to other muscles during isometric biting on the postcanine toothrow. This may be due to decreased activity in superficial masseter and posterior temporalis, rather than elevated anterior temporalis activity. The anterior temporalis is not consistently less variable in activity than the superficial masseter and posterior temporalis. The EMG data gathered here indicate no reason for suggesting that the anterior temporal muscles in anthropoids are utilized especially for incisal preparation of hard fruits. Maintenance of relatively high EMG activity in anterior temporalis across a wide range of biting behaviors is to be expected in a vertically oriented and rostrally positioned muscle such as this because, compared to the posterior temporalis, superficial masseter and medial pterygoid, it can contribute relatively larger vertical components of force to bites along the postcanine toothrow. The in vivo data do not support this hypothesis, possibly because of effects of bite point and bite force orientation.