Getting a head in hard soils: convergent skull evolution and divergent allometric patterns explain shape variation in a highly diverse genus of pocket gophers (Thomomys)

Background: High morphological diversity can occur in closely related animals when selection favors morphologies that are subject to intrinsic biological constraints. A good example is subterranean rodents of the genus Thomomys, one of the most taxonomically and morphologically diverse mammalian genera. Highly procumbent, tooth-digging rodent skull shapes are often geometric consequences of increased body size. Indeed, larger-bodied Thomomys species tend to inhabit harder soils. We used geometric morphometric analyses to investigate the interplay between soil hardness (the main extrinsic selection pressure on fossorial mammals) and allometry (i.e. shape change due to size change; generally considered the main intrinsic factor) on crania and humeri in this fast-evolving mammalian clade. Results: Larger Thomomys species/subspecies tend to have more procumbent cranial shapes with some exceptions, including a small-bodied species inhabiting hard soils. Counter to earlier suggestions, crania...

研究背景：当选择压力青睐受内在生物学约束影响的形态特征时，近缘动物类群中可能出现极高的形态多样性。囊鼠属（Thomomys）的地下啮齿类便是极佳范例——该属是哺乳动物中分类学与形态学多样性最高的类群之一。高度前倾的掘齿型啮齿动物颅骨形态，往往是体型增大所带来的几何形态学效应。事实上，体型更大的囊鼠物种多栖息于硬度更高的土壤中。本研究借助几何形态测量学分析（geometric morphometric analyses），探究了在这一快速演化的哺乳动物支系中，土壤硬度——穴居哺乳类（fossorial mammals）所面临的主要外在选择压力——与异速生长（allometry，即体型变化引发的形态改变，通常被视为核心内在因素）对颅骨及肱骨的共同作用。研究结果：体型更大的囊鼠物种/亚种往往具备更前倾的颅骨形态，但存在部分例外，例如一类栖息于硬质土壤的小型物种。与此前的研究推测相悖，颅骨……