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. 变态过程中前肢的内部发育与萌出，是无尾两栖动物（anuran amphibians，即蛙与蟾蜍）的标志性特征。尽管学界普遍认为前肢萌出过程具有突发性，但该过程极少呈现同步性。在变态高峰期（metamorphic climax）前肢萌出时，理论上会伴随阻力升高，而这种不同步性可能加剧也可能不会加剧该阻力升高效应。
2. 尽管学界假设前肢萌出会对个体存活与生活史演化产生影响，但目前尚未有研究探讨前肢萌出的不同步程度，以及该不对称性所带来的任何后果。此外，前肢萌出的不同步性，也为检验学界当前所持的“无尾类前肢内部发育与突发性萌出的演化基础”这一观点提供了契机。
3. 本研究选取了多个分类类群多样的无尾两栖动物，对多个物种的前肢萌出间隔时长与萌出模式进行了测量。为探究前肢萌出的演化影响，我们评估了个体分别处于0只、1只及2只前肢萌出状态时的运动表现（locomotory performance）。
4. 两只前肢的萌出间隔时长，不仅长于理论预测值，且变异程度也更高。此外，当前肢萌出呈现不同步性时，没有任何一个物种出现爆发速度受损的情况，其逃逸角度也未受影响。事实上，在萌出1只或2只前肢后，个体的爆发游泳速度（burst swimming speed）要快于前肢萌出前的水平。
5. 从根本上来说，本研究结果对学界长期以来广泛认可的“前肢内部发育与变态过程中的运动能力提升及阻力降低相关”这一命题提出了质疑。现有结果似乎并不支持该命题，我们推测，这或许是由解剖学或发育层面的限制因素或优势条件所导致的。