SUMMARY Excessive, repetitive and altered loading have been implicated in the initiation of a series of soft- and hard-tissue responses or `functional adaptations' of masticatory and locomotor elements. Such adaptive plasticity in tissue types appears designed to maintain a sufficient safety factor, and thus the integrity of given element or system, for a predominant loading environment(s). Employing a mammalian species for which considerable in vivo data on masticatory behaviors are available, genetically similar domestic white rabbits were raised on diets of different mechanical properties so as to develop an experimental model of joint function in a normal range of physiological loads. These integrative experiments are used to unravel the dynamic inter-relationships among mechanical loading, tissue adaptive plasticity, norms of reaction and performance in two cranial joint systems:the mandibular symphysis and temporomandibular joint (TMJ). Here, we argue that a critical component of current and future research on adaptive plasticity in the skull, and especially cranial joints, should employ a multifaceted characterization of a functional system, one that incorporates data on myriad tissues so as to evaluate the role of altered load versus differential tissue response on the anatomical, cellular and molecular processes that contribute to the strength of such composite structures. Our study also suggests that the short-term duration of earlier analyses of cranial joint tissues may offer a limited notion of the complex process of developmental plasticity, especially as it relates to the effects of long-term variation in mechanical loads, when a joint is increasingly characterized by adaptive and degradative changes in tissue structure and composition. Indeed, it is likely that a component of the adaptive increases in rabbit TMJ and symphyseal proportions and biomineralization represent a compensatory mechanism to cartilage degradation that serves to maintain the overall functional integrity of each joint system. Therefore, while variation in cranial joint anatomy and performance among sister taxa is, in part, an epiphenomenon of interspecific differences in diet-induced masticatory stresses characterizing the individual ontogenies of the members of a species,this behavioral signal may be increasingly mitigated in over-loaded and perhaps older organisms by the interplay between adaptive and degradative tissue responses.

Dietary consistency has been shown to influence cross-sectional area and fiber type composition of the masticatory muscles. However, little is known about the effects of dietary consistency on masticatory muscle fiber architecture. In this study, we explore the effects of dietary consistency on the internal architecture of rabbit masseter muscle. Because activity patterns of the rabbit chewing muscles show inter-and intramuscular heterogeneity, we evaluate if alterations in fiber architecture are homogeneous across various portions of the superficial masseter muscle. We compared masseter muscle fiber architecture between two groups of weanling rabbits raised on different diets for 105 days. One group was raised on a diet of ground rabbit pellets to model underuse of the masticatory complex, while the other group was fed a diet of intact pellets and hay blocks to model an overuse diet. In all portions of the superficial masseter, physiological cross-sectional areas (PCSAs) are greater in the overuse compared to underuse diet rabbits. Thus, the mechanical demands for larger muscle and bite forces associated with early and prolonged exposure to a tough diet are met by an increase in PCSA of the superficial masseter. The larger PCSA is due entirely to increased muscle mass, as the two rabbit groups show no differences in either fiber length or angle of pinnation. Thus, increasing pinnation angle is not a necessary biomechanical solution to improving muscle and bite force during growth. The change in PCSA but not fiber length suggests that variation in dietary consistency has an impact on maximum force production but not necessarily on excursion or contraction velocity.

Previous analyses of symphyseal fusion in the extinct Malagasy lemurArchaeolemur identified several functional characteristics of joint morphology that vary postnatally (Ravosa and Simons in American Journal of Physical Anthropology 95: [63][64][65][66][67][68][69][70][71][72][73][74][75][76] 1994). To complement that study, we used an imaging technique (microCT) that provides novel data on ontogenetic and local variation in biomineralization along the mandibular symphysis before complete ossification among adult Archaeolemur. Our sample of unfused symphyses comprised juveniles from the 2 earliest postnatal dental ages examined previously. We imaged each specimen (ca.18 μm volume elements) with slices parallel to the coronal plane, i.e., orthogonal to the joint articular surface. In ≤5 labiolingually equidistant joint sites, we collected 40 contiguous slices (18-μm intervals). Each of the 5 joint sites is represented by 1 slice, with biomineralization values sampled at 5 equidistant points along the articular surface and at 3 external cortical bone points. Our analysis of Archaeolemur indicates the presence of ontogenetic increases in bone mineral density accompanying increases in joint size and the number and distribution of symphyseal rugosities. Such postnatal changes are particularly marked for the middle of the joint presumed to lie adjacent to a degrading fibrocartilage pad. In Archaeolemur, labial regions of the symphysis Int J Primatol (ossify earlier and are likewise more biomineralized. Ontogenetic increases in symphyseal biomineralization, overall size, and fusion are consistent with elevated masticatory stresses owing to the postweaning shift to adult-like feeding behaviors. However, the labiolingual pattern of fusion and biomineralization in Archaeolemur appears related more to constraints on synostosis owing to the lingually located vascular supply characteristic of mammalian symphyses.

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