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Masseter electromyography during chewing in ring-tailed lemurs (Lemur catta)
Christopher J Vinyard1, Christine E Wall, Susan H Williams
1Department of Anatomy, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio 44272, USA. cvinyard@neoucom.edu
American Journal of Physical Anthropology
|December 14, 2005
Summary
Ring-tailed lemurs show distinct masseter muscle activity patterns during chewing, with higher working-side activation. These findings support the hypothesis that symphyseal fusion in primates strengthens jaws against chewing forces.
Area of Science:
- Primate Anatomy and Biomechanics
- Mammalian Mastication
- Evolutionary Biology
Background:
- The structure of the mandibular symphysis, the joint connecting the two halves of the lower jaw, varies significantly across primate species.
- Symphyseal fusion is common in anthropoid primates, suggesting a functional advantage related to jaw mechanics during feeding.
- Understanding masseter muscle recruitment patterns during chewing can provide insights into the functional significance of symphyseal morphology.
Purpose of the Study:
- To investigate electromyography (EMG) activity and firing patterns of the masseter muscles in ring-tailed lemurs (Lemur catta) during mastication.
- To compare these patterns with those observed in anthropoid primates and other prosimians (e.g., galagos).
- To test the hypothesis that symphyseal fusion in anthropoids is linked to specific muscle recruitment strategies that resist jaw forces.
Main Methods:
- Electromyography (EMG) was used to record masseter muscle activity in four adult ring-tailed lemurs.
- Muscle recruitment and firing patterns were analyzed during the chewing of various food consistencies.
- Working-side to balancing-side (W/B) ratios of scaled EMG activity were calculated for superficial and deep masseter muscles.
Main Results:
- Ring-tailed lemurs exhibited significantly higher EMG activity in the working-side deep masseter (2.4x) and superficial masseter (1.7x) compared to the balancing side.
- The deep masseter W/B ratio in lemurs resembled that of galagos, being higher than in anthropoids, consistent with unfused symphyses.
- Lemurs displayed an early peak in working-side deep masseter activity, differing from the late peak observed in anthropoids, which is linked to mandibular symphysis bending.
Conclusions:
- The observed masseter recruitment patterns in ring-tailed lemurs, characterized by greater working-side activation and an early deep masseter peak, support the symphyseal fusion-muscle recruitment hypothesis.
- The absence of the anthropoid-like late balancing-side deep masseter firing pattern in lemurs correlates with their unfused, weaker mandibular symphysis.
- Deep masseter firing patterns are not conserved across strepsirrhine primates, suggesting independent evolution of masticatory strategies.