Abstract
Rodents are characterised by a distinctive masticatory apparatus which includes a single pair of enlarged and continually growing incisors. This morphology, termed diprotodonty, has also independently evolved in a number of other mammals, including the aye-aye. This study examined the functional significance of the internal “root” of the elongated rodent-like incisor. The mandibles of four rodents and an aye-aye were modelled to exhibit incrementally shorter incisor roots. Finite element analysis was used to predict stress and strain patterns across the jaw to determine whether the length of the incisor root contributes to the resistance of mechanical forces encountered in the mandible during incision. It was found that von Mises stresses increase in the region of the mandible local to where the incisor is removed, but that the stress distribution across the wider mandible is only minimally affected. Thus, the long internal incisor appears to play a small role in resisting bending forces close to the incisor alveolus, and may act with the arch-like mandibular shape to strengthen the mandible in this region. However, the impact across the whole mandible is relatively limited, suggesting the highly elongate incisor in diprotodont mammals may be principally driven by other factors such as rapid incisor wear.
Highlights
Rodents are characterised by a distinctive masticatory apparatus which includes a single pair of enlarged and continually growing incisors
The core aim of this study is to determine whether the length of the incisor root is functionally significant for the resistance of mechanical forces encountered during incision
Stress distributions are very similar between variant models of the same taxon, with differences being mostly restricted to the ventral margin of the mandible, local to the incisor alveolus
Summary
Rodents are characterised by a distinctive masticatory apparatus which includes a single pair of enlarged and continually growing incisors. Rodentia is the most speciose order of mammals, containing over 2500 extant species[1], all possessing a highly specialised and distinctive masticatory apparatus including a pair of elongated, curved and continually growing incisors This condition, known as diprotodonty, is not, unique to rodents, having evolved independently in a number of other mammalian groups including lagomorphs, hyraxes, marsupials, and p rimates[2,3]. The aye-aye (Primates) in particular has a masticatory apparatus very similar to that of rodents, with a pair of continually growing incisors that lack enamel on their lingual side and form a sharp chisel-like groove with wear[4]. These are used to gnaw through bark to obtain xylophagous insect larvae[5,6]. Other biological curved structures such as long bones have been proposed to make the location and orientation of strains more predictable[23,24]
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