Adaptive plasticity in the mouse mandible

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.

Rapid Morphological Change in the Masticatory Structures of an Important Ecosystem Service Provider

“…The plastic changes seen in food hardness studies of mice are typically an overall broadening of muscle attachment points with exposure to hard food types [39,26]. For example, Myers et al [26] found that Peromyscus maniculatus raised on a hard diet in the laboratory had a broader zygomatic plate, similar to our study (Fig 6).…”

What shapes the oral jaws? Accommodation of complex dentition correlates with premaxillary but not mandibular shape

“…These two species occupy different morphospaces along PC2, particularly at mid stages of growth, indicating that differences in the medial part of the occluding edge emerge as the fish grows. stronger, and which have less variation in mandibular shape than those fed soft food (Anderson et al, 2014). Diet also influences the structure of cichlid pharyngeal jaws, with hard food increasing pharyngeal jaw size and molar-like tooth development (Gunter et al, 2013;Gunter and Meyer, 2014;Schneider et al, 2014).…”

Geographic Cline in the Shape of the Moose Mandible: Indications of an Adaptive Trend

“…More particularly, foraging and diet consistency have been shown to have a significant impact on the growth and shape remodelling of the mandible as bones respond to their functional environment (Renaud et al 2010;Young and Badyaev 2010). Furthermore, the shift in the shape caused by hard food, requiring a higher bite force, was recently proven to enhance the biomechanical abilities of the mouse jaw and increase its adaptation to its function, i.e., more powerful mastication (Anderson et al 2014). In this sense, it could be that the latitudinal shape trend observed here in the moose reflects a plastic response to a gradient in diet consistency.…”

“…Barboza and Bowyer (2000) hypothesized that the sexual segregation in dimorphic ungulates is a consequence of the differing life-histories and energetic needs among males (body and antler growth) and females (gestation and lactation). As the differences in the diet can induce phenotypic changes in the mandible through biomechanical function (Anderson et al 2014), it is possible that the clear differences in the mandible shape between the two sexes reflect their different dietary optimum and foraging behavior (Barboza and Bowyer 2000). An alternative (or additional) process is brought about by the seasonal growth and presence of antlers in males, plausibly affecting the structure of the cranium (Bartosiewicz 1987;Nygrén et al 1992aNygrén et al , 1992b.…”

“…In addition, the relationship between the shape of the mandible, its biomechanical function and foraging ecology has recently been gaining increasing interest in evolutionary morphology (e.g., Monteiro et al 2010;Young and Badyaev 2010;Prevosti et al 2011;Anderson et al 2014). In ungulates, the research has mostly concentrated on the inter-specific differences in the mandible shape in relation to the various feeding categories, such as browsers, grazers, or mixed feeders (Hofmann and Stewart 1972;Hofmann 1989;Pérez-Barbería and Gordon 1999;Raia et al 2010;Fraser and Theodor 2011).…”