Abstract
Functional characteristics of the jaw apparatus, for example bite force, in vertebrates is a combination of the skeleton and the musculature. In birds, bite force has been measured directly or calculated using various methods including summation of forces generated by the different elements of the jaw musculature. However, there have been no reports of the relationships between body size with the mass of the different muscle groups in a closely related group of birds. This study explored allometry in the different jaw muscle masses from parrot (Psittaciformes) species differing in body mass by 40-fold. It was hypothesised that the different muscle masses would exhibit isometry with body mass and skull size. Parrot heads were dissected and the masses of the individual muscle complexes were recorded. Data were subjected to phylogenetically-controlled regression analysis to document scaling effects with body mass and skull size. Most, but not all muscles, exhibited positive allometry with body mass but most were isometric with skull size. Consequently, as parrots get bigger, their skulls get proportionally longer, but that the muscles within the head isometrically scaled relative to the size of these proportionally larger skulls. The large muscles imply greater bite forces in parrots than have been reported to date, which seems to be associated with an increase in skull size to accommodate more muscles. It is unknown whether this pattern is applicable to other birds within specific orders or even across birds as a whole. There needs to be further investigation into the allometry of the morphological and functional properties of the avian jaw musculature.