Data from: 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 forelimb emergence, we assessed locomotory performance when individuals had zero, one or two forelimbs emerged. 4. The duration of time between the emergence of the two forelimbs was longer and more variable than predicted. Furthermore, no species suffered impaired burst speeds nor was their angle of escape affected as the forelimbs emerged asynchronously. In fact, burst swimming speed was faster after the emergence of one and two forelimbs than prior to their emergence. 5. Fundamentally, our results call into question the proposition, long accepted, that internal forelimb development is associated with locomotion and reducing drag during metamorphosis. This does not appear to be the case, and we suggest that anatomical or developmental constraints or advantages may be responsible.

1. 无尾两栖类（anuran amphibians，即蛙和蟾蜍）的标志性特征之一，是其体内发育以及变态阶段前肢的萌出。尽管学界普遍认为前肢萌出过程具有突发性，但该过程极少同步完成。前肢萌出的不同步性，既可能加剧也可能不会加剧变态高峰期前肢萌出时理论上会出现的阻力升高现象。
2. 尽管学界假设前肢萌出会对个体存活及生活史演化产生影响，但前肢萌出的不同步程度，以及该不对称性可能带来的任何后果，目前尚未得到研究。此外，前肢萌出的不同步性，也为检验学界目前关于无尾类前肢体内发育与突发性萌出的演化基础假说，提供了研究契机。
3. 本研究选取了多个不同的无尾两栖类类群，测量了不同物种前肢萌出的时间间隔与模式。为探究前肢萌出的演化影响，我们分别评估了个体在0个、1个或2个前肢萌出时的运动表现。
4. 两个前肢萌出的时间间隔，比理论预测更长且变异度更高。此外，当前肢不同步萌出时，未观测到任何物种出现爆发速度受损或逃逸角度受影响的情况。事实上，在萌出1个或2个前肢后，个体的爆发游泳速度反而高于前肢萌出前的水平。
5. 从根本上来说，我们的研究结果对长期以来被广泛接受的假说提出了质疑——该假说认为无尾类的体内前肢发育，与变态阶段的运动能力及阻力降低相关。而我们的研究结果似乎并不支持这一观点，我们推测这可能是受解剖学或发育约束，或是发育优势所导致的。