Lessa E P
Museum of Vertebrate Zoology, University of California, Berkeley 94720.
Prog Clin Biol Res. 1990;335:211-30.
The widely recognized convergence of subterranean mammals offers unique opportunities for the study of patterns, causes and consequences of morphological integration. For example, behavioral and biomechanical observations have revealed a diversity of digging modes among subterranean mammals. In rodents, scratch-digging and chisel-tooth digging, alone or combined, are widespread. This dual nature of the digging apparatus must be understood to avoid erroneous assessments of specializations based on forelimb modifications only. Additionally, bulldozing and disposal of loose soil is performed by head-lifting, by means of the forelimbs, or with the hindlimbs. Insectivores are known to resort to scratch-digging, sand-swimming, or to a unique system known as humeral rotation. Semi-fossorial species among rodents and insectivores are scratch-diggers. In contrast, fully subterranean taxa show further specializations for scratch-digging and/or resort to some of the other digging modes. The ecological correlates of these morphological trends are still poorly known; work in geomyids suggests that scratch-digging specializations are efficient in friable soils, whereas tooth-digging allows the utilization of a much broader spectrum of soil types. This indicates that alternative pathways of morphological specialization are not equivalent in their ecological potentialities. Important morphological features, such as incisor procumbency among tooth-diggers, are shown to be constrained by structural, allometric, and mechanical factors. In geomyids, the same procumbent morphologies may be acquired as a byproduct of size increases, by means of adaptive shifts independent from size, or by a combination of both. Thus, geometric similarity may not be indicative of functional similarity. Further constraints result from the integration of different functions. For instance, it is suggested that mastication imposes limitations upon potential modifications of jaw morphology for tooth-digging. Scratch-digging appears to be less constrained by locomotion, but the effects of integration of various functions in fore- and hind-limbs are largely unexplored. Multiple approaches, combining behavioral, functional, ecological, structural, and phylogenetic data, are necessary for the study of morphological evolution among subterranean mammals.
地下哺乳动物广泛存在的趋同现象为研究形态整合的模式、原因及后果提供了独特的机会。例如，行为学和生物力学观察揭示了地下哺乳动物挖掘方式的多样性。在啮齿动物中，单独或组合使用的抓挠式挖掘和凿齿式挖掘很普遍。必须理解挖掘器官的这种双重性质，以避免仅基于前肢形态变化对特化进行错误评估。此外，松散土壤的推土和处理是通过前肢或后肢抬举头部来完成的。食虫动物已知会采用抓挠式挖掘、沙泳或一种称为肱骨旋转的独特方式。啮齿动物和食虫动物中的半穴居物种是抓挠式挖掘者。相比之下，完全地下生活的类群在抓挠式挖掘方面表现出进一步的特化，和/或采用一些其他挖掘方式。这些形态学趋势的生态关联仍知之甚少；对囊鼠科动物的研究表明，抓挠式挖掘特化在易碎土壤中效率较高，而齿式挖掘则能利用更广泛的土壤类型。这表明形态特化的不同途径在生态潜力上并不等同。重要的形态特征，如齿式挖掘者的门齿前倾，被证明受到结构、异速生长和机械因素的限制。在囊鼠科动物中，相同的前倾形态可能是体型增大的副产品，通过独立于体型的适应性转变获得，或者是两者的结合。因此，几何相似性可能并不意味着功能相似。不同功能的整合会产生进一步的限制。例如，有人认为咀嚼对用于齿式挖掘的颌骨形态的潜在改变施加了限制。抓挠式挖掘似乎受运动的限制较小，但前肢和后肢各种功能整合的影响在很大程度上尚未得到探索。研究地下哺乳动物的形态进化需要结合行为、功能、生态、结构和系统发育数据的多种方法。
