Masticatory motor patterns in ungulates: a quantitative assessment of jaw-muscle coordination in goats, alpacas and horses.

The research investigates how jaw-muscle coordination during chewing varies among three species of herbivorous hoofed animals – goats, alpacas, and horses. The differences arise mainly in the synchronization of certain distinct muscle groups, affecting the timing of jaw movements and the generation of bite force.

Methodology:

• The researchers studied patterns of jaw-muscle coordination during rhythmic mastication in goats, alpacas, and horses. These species demonstrate the broad, horizontal jaw movements typical of many large herbivorous mammals.
• They utilised electromyograms, or electrical recordings of muscle activity, to document the action of several key jaw muscles: the superficial masseter, deep masseter, posterior temporalis, and medial pterygoid muscles.
• They assessed timing differences between muscle pairs using an evolutionary model of jaw-muscle function that identifies two distinct muscle groups responsible for the closing and food reduction phases of mastication.

Findings:

• The closing and food reduction phases of mastication are primarily controlled by two distinct muscle groups, termed triplet I and II. The muscles involved in these triplet groups are the balancing-side superficial masseter and medial pterygoid, the working-side posterior temporalis (triplet I), the working-side superficial masseter and medial pterygoid, and the balancing-side posterior temporalis (triplet II), along with the asynchronous action of the deep masseters on the working and balancing sides of the jaw.
• There were significant differences between the species in terms of the coordination of these muscle triplets. Alpacas, and to a lesser extent, goats, exhibited the triplet coordination pattern, while horses did not.
• All three species demonstrated asynchrony or lack of simultaneous action in the deep masseter muscles on the working and balancing sides of the jaw. The process of jaw closing was initiated by the working-side muscle, while the balancing-side muscle activated significantly later during closing.
• However, goats differed from the other two species concerning the timing of the balancing-side deep masseter relative to the triplet II muscles.

Implications:

• The research provides valuable insights into variations in the coordination of jaw muscles across different species of herbivorous mammals. These differences can influence the transverse movements of the jaw as well as the production of bite force during mastication.
• These variations can potentially be linked to evolutionary factors, diet, and other ecological and functional influences on the masticatory process.