Biomechanical Constraints on Molar Emergence in Primates (ASUF - 30006669)

Biomechanical Constraints on Molar Emergence in Primates (ASUF - 30006669) Biomechanical Constraints on Molar Emergence in Primates A species life history is the schedule of its allocation of energetic resources to growth, maintenance, and reproduction (Stearns, 1992; Charnov, 1993). Life history is described by a series of variables, such as gestation length, weaning age, and age at first reproduction. The age at which the permanent first molar (M1) emerges into the oral cavity is strongly correlated with life-history variables across primates (Smith, 1989). The mechanism underlying the relationship between M1 emergence age and life history remains unknown, however. Two unanswered questions stand in the way of a more complete understanding of the timing of molar emergence in primates: (1) Why is M1 emergence age so closely associated with a species life history? and (2) What factors influence variation in M1 emergence age among primates? Previous attempts to describe the factors that influence molar emergence schedules focused on the effects of body size, brain size, diet, and spatial/design constraints on dental development (Godfrey et al., 2001; Boughner and Dean, 2004; Dirks and Bowman, 2007). Collectively these studies have edged us closer to understanding the mechanisms that regulate the timing of molar emergence, but to date no single study has provided an explanatory model that accounts for all of the variation in molar emergence schedules among primates. The masticatory system is configured to avoid catastrophic distraction of the temporomandibular joint (TMJ) (i.e., when the mandibular condyle is pulled away from its articulation with the temporal bone) during chewing (Greaves, 1978; Spencer, 1999). When the jaw adductors work together they produce a combined muscle force vector (i.e., the muscle resultant), the position of which depends on the muscles lines of action, positions relative to one another, and sizes (Fig. 1). One key aspect of masticatory system configuration is the position of molars relative to this muscle resultant: the distalmost molar is always anterior to the muscle resultants intersection with the occlusal plane (Greaves, 1978; Spencer, 1999). Biting posterior to this point leads to TMJ distraction. Molars are, therefore, always positioned such that catastrophic TMJ damage is mitigated during mastication. It is reasonable to assume that the same constraints on molar position exist throughout ontogeny.