Department of Evolution, Ecology and Organismal Biology, The Ohio State University, Columbus, OH, United States of America.
The Ohio State University at Marion, Marion, OH, United States of America.
PLoS One. 2023 Apr 6;18(4):e0284094. doi: 10.1371/journal.pone.0284094. eCollection 2023.
Bite force can be a limiting factor in foraging and can significantly affect the competitive ability and lifetime fitness of mammals. Tamiasciurus squirrels feed primarily on conifer seeds and have a strong bite force to mechanically extract seeds from conifer cones with their mouths. In the North Cascades region, Douglas squirrels (Tamiasciurus douglasii) and red squirrels (T. hudsonicus) occupy ecologically different forests with different hardnesses in conifer cones. The ranges of these species overlap in a narrow hybrid zone where these forests meet near the crest of the North Cascades. We examined interspecific divergence in dietary ecomorphology in allopatry, in sympatry within the hybrid zone, and between hybrids and each parental species. We focused on three craniodental traits, including the incisor-strength index as a proxy measure for maximal bite force, cranial-suture complexity, and mandible shape. We find that these sister squirrel species differ in bite force and suture complexity in allopatry and sympatry and that mandible shape changes with the expected hardness of accessed food items, but is not significantly different between species. Furthermore, we find that hybrids display morphologies that overlap with hybrid zone red squirrels but not with hybrid zone Douglas squirrels. This work shows how important ecological processes at shallow evolutionary timescales can impact the divergence of morphological traits in taxa with extreme conservation of craniomandibular shape.
咬合力可能是觅食的限制因素，并会显著影响哺乳动物的竞争能力和终生适应能力。花鼠主要以针叶树种子为食，它们具有很强的咬合力，可以用嘴从针叶树球果中机械地提取种子。在北喀斯喀特山脉地区，道格拉斯花鼠（Tamiasciurus douglasii）和红松鼠（T. hudsonicus）占据着生态环境不同、球果硬度不同的森林。这些物种的分布范围在北喀斯喀特山脉山顶附近的狭窄混合区重叠。我们在异地、混合区的同域和杂种与每个亲种之间，检查了饮食生态形态的种间差异。我们专注于三种颅面特征，包括作为最大咬合力代理测量的门牙强度指数、颅缝复杂性和下颌骨形状。我们发现，这些姐妹花鼠物种在异地和同域中具有不同的咬合力和颅缝复杂性，下颌骨形状随所摄入食物的预期硬度而变化，但在物种之间没有显著差异。此外，我们发现杂种的形态与混合区红松鼠重叠，但与混合区道格拉斯花鼠不重叠。这项工作表明，在进化时间尺度较浅的情况下，生态过程是如何影响具有颅面形态极端保守性的分类群的形态特征分化的。
