Skull allometry in the marine toad, Bufo marinus

Scaling predictions pioneered by A.V. Hill state that isometric changes in kinematics result from isometric changes in size. These predictions have been difficult to support because few animals display truly isometric growth. An exception to this rule is said to be the toads in the genus Bufo, which can grow over three orders of magnitude. To determine whether skull shape increases isometrically, I used linear measurements and geometric morphometrics to quantify shape variation in a size series of 69 skulls from the marine toad, B. marinus. Toads ranged in body mass from 1.8 gm to a calculated 1, 558.9 gm. Of all linear measurements (S/V length, skull width, skull length, levator mass, depressor mass, adductor foramen area), only the area of the adductor foramen increased faster than body mass; the remaining variables increased more slowly. In addition, modeling the lower jaw as a lever-arm system showed that the lengths of the closing in- and out-levers scaled isometrically with body mass despite the fact that the skull itself is changing allometrically. Geometric morphometrics discerned areas of greatest variability with increasing body mass at the rear of the skull in the area of the squamosal bone and the adductor foramen. This increase in area of the adductor foramen may allow more muscle to move the relatively greater mass of the lower jaw in larger toads, although adductor mass scales with body mass. If B. marinus feeds in a similar manner to other Bufo, these results imply that morphological allometry may still result in kinematic isometry.