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

Summary 
 
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. 
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. 
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. 
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. 
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.