Why do frog and toad forelimbs suddenly (but asynchronously) appear every time metamorphosis is near?

1. The internal development and the emergence of the forelimbs at metamorphosis is a defining feature of anuran amphibians (frogs and toads). However, although forelimb emergence is considered sudden, it is rarely synchronous. Any asynchrony may or may not exacerbate the increased drag that is predicted to occur with the emergence of the forelimbs at metamorphic climax. 2. Despite the impact forelimb emergence is hypothesized to have on individual survival and life-history evolution, the degree of asynchrony between forelimb emergence, and any consequences of such asymmetry, has not been investigated. The asynchrony in forelimb emergence also provides an opportunity to test the currently held evolutionary basis for the internal development and sudden emergence of the forelimbs in anurans. 3. Using a diverse range of anuran taxonomic groups, we measured the time between, and pattern of, the emergence of the forelimbs across a range of species. To examine the evolutionary impacts of forel...

1. 无尾两栖动物（anuran amphibians，即蛙与蟾蜍）的标志性特征之一，是其前肢的体内发育以及变态发育（metamorphosis）阶段的伸出。尽管学界普遍认为前肢伸出具有突发性，但该过程极少呈现完全同步性。这种不同步性可能会，也可能不会加剧变态高峰期（metamorphic climax）前肢伸出时理论预测会出现的阻力增幅。
2. 尽管有假说提出前肢伸出会对个体生存及生活史演化产生影响，但目前尚未有研究探讨前肢伸出的不同步程度，以及这类不对称性所带来的各类后果。此外，前肢伸出的不同步性，也为验证学界目前关于无尾两栖动物前肢体内发育与突发性伸出的进化理论基础，提供了研究契机。
3. 本研究选取了多个多样化的无尾两栖动物分类类群，测量了不同物种前肢伸出的时间间隔与模式，以探究前肢伸出不同步性的进化影响……