Jaw-muscle electromyography during chewing in Belanger's treeshrews (Tupaia belangeri)
Article first published online: 14 OCT 2004
Copyright © 2004 Wiley-Liss, Inc.
American Journal of Physical Anthropology
Volume 127, Issue 1, pages 26–45, May 2005
How to Cite
Vinyard, C. J., Williams, S. H., Wall, C. E., Johnson, K. R. and Hylander, W. L. (2005), Jaw-muscle electromyography during chewing in Belanger's treeshrews (Tupaia belangeri). Am. J. Phys. Anthropol., 127: 26–45. doi: 10.1002/ajpa.20176
- Issue published online: 1 APR 2005
- Article first published online: 14 OCT 2004
- Manuscript Accepted: 21 NOV 2003
- Manuscript Received: 17 JAN 2003
- NSF. Grant Numbers: SBR-9420764, BCS-01-38565, BCS-0094666
- jaw adductors;
- primate origins
We examined masseter and temporalis recruitment and firing patterns during chewing in five male Belanger's treeshrews (Tupaia belangeri), using electromyography (EMG). During chewing, the working-side masseters tend to show almost three times more scaled EMG activity than the balancing-side masseters. Similarly, the working-side temporalis muscles have more than twice the scaled EMG activity of the balancing-side temporalis. The relatively higher activity in the working-side muscles suggests that treeshrews recruit less force from their balancing-side muscles during chewing. Most of the jaw-closing muscles in treeshrews can be sorted into an early-firing or late-firing group, based on occurrence of peak activity during the chewing cycle. Specifically, the first group of jaw-closing muscles to reach peak activity consists of the working-side anterior and posterior temporalis and the balancing-side superficial masseter. The balancing-side anterior and posterior temporalis and the working-side superficial masseter peak later in the power stroke. The working-side deep masseter peaks, on average, slightly before the working-side superficial masseter. The balancing-side deep masseter typically peaks early, at about the same time as the balancing-side superficial masseter. Thus, treeshrews are unlike nonhuman anthropoids that peak their working-side deep masseters early and their balancing-side deep masseters late in the power stroke. Because in anthropoids the late firing of the balancing-side deep masseter contributes to wishboning of the symphysis, the treeshrew EMG data suggest that treeshrews do not routinely wishbone their symphyses during chewing. Based on the treeshrew EMG data, we speculate that during chewing, primitive euprimates 1) recruited more force from the working-side jaw-closing muscles as compared to the balancing-side muscles, 2) fired an early group of jaw-closing muscles followed by a second group of muscles that peaked later in the power stroke, 3) did not fire their working-side deep masseter significantly earlier than their working-side superficial masseter, and 4) did not routinely fire their balancing-side deep masseter after the working-side superficial masseter. Am J Phys Anthropol, 2004. © 2004 Wiley-Liss, Inc.