Skull allometry in the marine toad, Bufo marinus.

Area of Science:

• Comparative anatomy
• Biomechanics
• Evolutionary biology

Background:

• A.V. Hill's scaling predictions link isometric kinematics to isometric size changes.
• True isometric growth is rare in animals, making it difficult to test these predictions.
• Toads (genus Bufo) are known for significant growth, offering a model for studying scaling.

Purpose of the Study:

• To investigate whether skull shape in the marine toad (B. marinus) increases isometrically with body size.
• To quantify skull shape variation across a wide size range of B. marinus.
• To determine if morphological allometry in toad skulls can still result in kinematic isometry.

Main Methods:

• Utilized linear measurements and geometric morphometrics on 69 B. marinus skulls.
• Analyzed skull measurements (S/V length, skull width, skull length, levator mass, depressor mass, adductor foramen area) relative to body mass.
• Modeled the lower jaw as a lever-arm system to assess scaling of in- and out-levers.

Main Results:

• Most linear skull measurements scaled isometrically or allometrically slower than body mass.
• The adductor foramen area was the only linear measurement to scale faster than body mass.
• Jaw lever arm lengths scaled isometrically with body mass, despite overall allometric skull changes.
• Geometric morphometrics identified the squamosal bone and adductor foramen as areas of greatest variability with increasing body mass.

Conclusions:

• Despite allometric skull shape changes in B. marinus, jaw lever systems maintain isometric scaling with body mass.
• Increased adductor foramen area in larger toads may accommodate larger jaw muscles.
• Morphological allometry in toad skulls does not preclude kinematic isometry, supporting A.V. Hill's predictions in this context.