A Geometric Morphometric Analysis of Geographic Mandibular Variation in the Dwarf Gerbil Gerbillus nanus (Gerbillinae, R
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ORIGINAL PAPER
A Geometric Morphometric Analysis of Geographic Mandibular Variation in the Dwarf Gerbil Gerbillus nanus (Gerbillinae, Rodentia) Bader H. Alhajeri 1 Accepted: 28 October 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Previous studies have demonstrated the pliability and adaptability of mammalian mandibles in response to different ecological demands such as food availability. The dwarf gerbil Gerbillus nanus has a wide distribution in Asia in which it encounters varied environments. These varied environments can house different food resources and it would be adaptively advantageous for the opportunistic rodents to have mandibles best suited for the most prevalent food resource. Geometric morphometric analysis was used to test this hypothesis by examining mandibular variation in 436 G. nanus specimens sampled from 44 localities. Based on this sample, mandibular morphology seems to be highly conserved in this species, with both size and shape being nonsignificantly different between pairs of geographic groups (i.e., locality clusters) and ecoregions. Mandibular size and shape are both not strongly predicted by 48 examined environmental variables. Despite this, the most variable aspect of mandibular shape was associated with its caudal regions, spanning from the coronoid process to the ventral mandibular border. These are regions of temporalis and masseter muscle attachments and thus mandibular variation could be associated with differential (potentially plastic) masticatory adaptations. Keywords Balochistan gerbil (Gerbillus nanus) . Climate . Diet . Ecoregion . Geometric morphometrics . Mandible
Introduction Mammalian mandibles are highly diverse in structure, but they share basic characteristics and serve similar functions (Greaves 2012). In rodents, jaws are operated by two main muscles: (1) occurring along the coronoid process is the temporalis, which chiefly facilitates incisor gnawing and (2) attaching to both the angular process and the ramus is the masseter, which is used in molar mastication (Anderson et al. 2014; West and King 2018). The jawbone is pliable, and its shape can physically change (i.e., via phenotypic plasticity) in response to the forces applied to it by masticatory muscles (West and King 2018; McIntosh and Cox 2019). Various experiments showed that food firmness physically
Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s10914-02009530-9. * Bader H. Alhajeri [email protected] 1
Department of Biological Sciences, Kuwait University, 13060 Safat, Kuwait
modifies mandible shape in a plastic manner; laboratory mice raised on soft food significantly differ in mandible shape from those raised on hard food, with the latter generally being more robust, dense, and with more developed processes, and thus more suitable to process such food by exerting greater force (e.g., Mavropoulos et al. 2005; Renaud and Auffray 2010; Anderson et al. 2014). These results suggest that food av
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