Abstract
BackgroundPlasticity, i.e. non-heritable morphological variation, enables organisms to modify the shape of their skeletal tissues in response to varying environmental stimuli. Plastic variation may also allow individuals to survive in the face of new environmental conditions, enabling the evolution of heritable adaptive traits. However, it is uncertain whether such a plastic response of morphology constitutes an evolutionary adaption itself. Here we investigate whether shape differences due to plastic bone remodelling have functionally advantageous biomechanical consequences in mouse mandibles. Shape characteristics of mandibles from two groups of inbred laboratory mice fed either rodent pellets or ground pellets mixed with jelly were assessed using geometric morphometrics and mechanical advantage measurements of jaw adductor musculature.ResultsMandibles raised on diets with differing food consistency showed significant differences in shape, which in turn altered their biomechanical profile. Mice raised on a soft food diet show a reduction in mechanical advantage relative to mice of the same inbred strain raised on a typical hard food diet. Further, the soft food eaters showed lower levels of integration between jaw regions, particularly between the molar and angular region relative to hard food eaters.ConclusionsBone remodelling in mouse mandibles allows for significant shifts in biomechanical ability. Food consistency significantly influences this process in an adaptive direction, as mice raised on hard food develop jaws better suited to handle hard foods. This remodelling also affects the organisation of the mandible, as mice raised on soft food appear to be released from developmental constraints showing less overall integration than those raised on hard foods, but with a shift of integration towards the most solicited regions of the mandible facing such a food, namely the incisors. Our results illustrate how environmentally driven plasticity can lead to adaptive functional changes that increase biomechanical efficiency of food processing in the face of an increased solicitation. In contrast, decreased demand in terms of food processing seems to release developmental interactions between jaw parts involved in mastication, and may generate new patterns of co-variation, possibly opening new directions to subsequent selection. Overall, our results emphasize that mandible shape and integration evolved as parts of a complex system including mechanical loading food resource utilization and possibly foraging behaviour.
Highlights
Plasticity, i.e. non-heritable morphological variation, enables organisms to modify the shape of their skeletal tissues in response to varying environmental stimuli
If the remodelling occurring in this context has an evolutionary adaptive value, i.e. mice fed on hard food display mandibles that are functionally more efficient for food processing than mice fed on soft food
Shape differences between mice fed hard vs. soft food Based on principal component analyses on the residuals after Procrustes superimposition, between 3 and 6 axes were sufficient to summarize more than 95% of the total morphological variance (Table 1)
Summary
Plasticity, i.e. non-heritable morphological variation, enables organisms to modify the shape of their skeletal tissues in response to varying environmental stimuli. Plastic variation may allow individuals to survive in the face of new environmental conditions, enabling the evolution of heritable adaptive traits It is uncertain whether such a plastic response of morphology constitutes an evolutionary adaption itself. That remodelling significantly affects mandible shape has been demonstrated by several experiments, most involving feeding rats or mice food of different consistency, ending with mandibles of different morphology [12,13,14,15,16] It is unclear, if the remodelling occurring in this context has an evolutionary adaptive value, i.e. mice fed on hard food display mandibles that are functionally more efficient for food processing than mice fed on soft food. Bone remodelling and muscle activity, acting between parts that interact to perform a single function, may be a factor promoting integration of the mandible beyond genetic and developmental modularity [19,20]
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