Morphological correlates of sprint swimming speed in five species of spadefoot toad tadpoles: comparison of morphometric methods.

The relationship between vertebrate morphology and swimming performance has long interested biologists. Recent work on predator-induced morphological plasticity of anuran tadpoles has increased this interest. Here, I use data on five species of spadefoot toad tadpoles (Scaphiopodidae) to compare linear and geometric morphometrics. Linear measures explain only 7-26% of the variation in swimming speed, depending on species, whereas geometric morphometrics could explain 24-46% of the same variation. I also compare two methods for examining how similar the morphology-swimming speed relationship is among species. A canonical variate derived from a MANCOVA approach successfully detected species differences in these relationships, whether using linear or geometric methods, but a canonical correlation approach failed in both cases. Overall, tadpoles with smaller bodies, larger tails, and larger tail muscles are faster swimmers but the details of how these shape changes are achieved differed among species. For example, in some species a smaller body was achieved primarily by reducing abdomen size, whereas in others both the head and abdomen are smaller.Faster swimmers also had deeper tails, especially in the posterior half of the tail. This pattern would have been missed in standard linear morphometrics which usually only measures maximum tail depth.