English Lauren T
Department of Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, U.S.A.
J Morphol. 2018 Feb;279(2):154-162. doi: 10.1002/jmor.20760. Epub 2017 Oct 17.
Dermal ossifications, including osteoderms, are present in many vertebrates and are frequently interpreted as a defense against predators. Nevertheless, osteoderms remain ubiquitous in adult crocodilians while being absent in hatchlings, even though adults rarely experience predation. In other biological systems, increased variation, particularly fluctuating asymmetry, have proven useful for identifying biological structures likely to have evolved under relaxed selection, which in turn may inform their function. Therefore, using the keratinous scutes as proxies for the underlying osteoderm morphology, I investigated the average intraspecific variability of geometry and fluctuating asymmetry in dorsal scutes in five species of crocodilians. I first tested for differences in variability of scute length and width, then for differences in bilateral fluctuating asymmetry of scute number, before finally investigating scute distribution patterns for each species compared to hypothetical rectangular and hexagonal scute arrangements. The American crocodile, Crocodylus acutus, shows significantly more asymmetry than other species, which is consistent with relaxed selection on osteoderms in this species. A suspected decrease in intraspecific aggression within Crocodylus acutus, in conjunction with the inferred relaxed selection, suggests that, in general, crocodilian osteoderms function primarily as defensive armor in aggressive encounters with conspecifics. The smooth-fronted caiman, Paleosuchus trigonatus, exhibits increased variation in scute dimensions linked to the mediolateral offset of osteoderms in adjacent rows, possibly resulting in a more rigid carapace. Unfortunately, comparative data on crocodilian behavior, physiology, and development is extremely limited and restricts the ability to explore other potential explanations for the patterns observed, highlighting the need for more research on rare and cryptic crocodylians.
包括骨板在内的真皮骨化现象在许多脊椎动物中都存在，并且常常被解释为一种抵御捕食者的防御机制。然而，骨板在成年鳄鱼中仍然普遍存在，而在幼鳄中却不存在，尽管成年鳄鱼很少遭受捕食。在其他生物系统中，增加的变异性，特别是波动不对称性，已被证明有助于识别可能在放松选择下进化的生物结构，这反过来可能为其功能提供线索。因此，我以角质鳞片作为潜在骨板形态的替代物，研究了五种鳄鱼背部鳞片的几何形状平均种内变异性和波动不对称性。我首先测试了鳞片长度和宽度变异性的差异，然后测试了鳞片数量的双侧波动不对称性差异，最后将每个物种的鳞片分布模式与假设的矩形和六边形鳞片排列进行比较。美洲鳄（Crocodylus acutus）表现出比其他物种明显更多的不对称性，这与该物种骨板的放松选择一致。美洲鳄种内攻击性的疑似降低，加上推断出的放松选择，表明一般来说，鳄鱼的骨板在与同种个体的攻击性遭遇中主要起防御盔甲的作用。钝吻古鳄（Paleosuchus trigonatus）的鳞片尺寸变异性增加，这与相邻排骨板的内外侧偏移有关，可能导致更坚硬的背甲。不幸的是，关于鳄鱼行为、生理和发育的比较数据极其有限，限制了探索观察到的模式的其他潜在解释的能力，这突出了对珍稀和隐秘鳄鱼进行更多研究的必要性。
