WILLIAMS, S.H.*; WALL, C.E.; VINYARD, C.J.; HYLANDER, W.L.; Duke University, Durham, NC; Duke University, Durham, NC; Duke University, Durham, NC; Duke University, Durham, NC: Jaw-muscle motor patterns in ungulates: is there a transverse pattern?

Similarities in masticatory apparatus form and jaw movements in artiodactyls and perissodactyls have led to the idea that ungulate grinding is linked to a derived jaw-muscle recruitment pattern, known as the “transverse pattern” (Weijs, 1994). In the transverse pattern, large transverse jaw movements are said to be associated with a separation in jaw-muscle peak activity. The working-side temporalis (WT) and balancing-side superficial masseter (BSM) and medial pterygoid (BMP) (Triplet I) are thought to peak much earlier than the working-side superficial masseter (WSM) and medial pterygoid (WMP) and balancing-side temporalis (BT) (Triplet II) as compared to mammals with less transverse jaw movement. Here, we test if there is uniform jaw-muscle recruitment indicative of a transverse pattern. We recorded EMG during chewing from the left and right temporalis, medial pterygoid and superficial and deep masseters in goats, alpacas and horses. Results show substantial variation in the timing of jaw-muscle peak activity among these species. In alpacas and goats, Triplet I muscles peak almost simultaneously followed by a prolonged delay and near simultaneous peak activity of Triplet II muscles. Horses do not show this pattern. Peak activity of the WT and BMP is delayed by nearly 100ms relative to the BSM. The WSM is similarly offset relative to other Triplet II muscles. Moreover, the goat balancing-side deep masseter peaks prior to the WSM whereas in horses and alpacas it peaks almost 80ms after the WSM. By lumping all ungulates into a single transverse pattern, we likely are obscuring important variation in ungulate jaw-muscle recruitment patterns that have a significant impact on ungulate masticatory biomechanics.