An ontogenetic perspective on symphyseal fusion, occlusion and mandibular loading in alpacas (Vicugna pacos)

A primary hypothesis for the evolution of mandibular symphyseal fusion in some mammals is that it functions to resist loads incurred during routine mastication. Anecdotal support for this hypothesis is based on the fact that when the symphysis fuses, it typically does so early during postnatal ontogeny prior to or around the time of weaning. However, little is known about the process of fusion, particularly relative to feeding behaviors and the dynamics of mastication, including occlusion and masticatory loading. In the present study, we investigate the timing and process of symphyseal fusion in alpacas (Vicugna pacos) in the context of maturation of the oral apparatus and oral behavior. We also report on in vivo strains from the symphysis and corpus in young alpacas prior to and following full fusion and M₁ occlusion. Results show that fusion begins rostrally by 1 month and is complete by 6-7 months whereas all deciduous premolars and M₁ come into occlusion by 6 months. Although symphyseal loading patterns are maintained throughout ontogeny, in young alpacas symphyseal strain magnitudes are low compared with adults but corpus strain magnitudes are comparable to those found in adults. Reduced symphyseal loading in young individuals is contrary to what might be predicted given that the symphysis is still fusing. When considered in light of the development of occlusion and rumination, strain magnitudes may be necessarily low and reflect an overall delay in the maturation of masticatory dynamics.