Scaling of jaw muscle size and maximal bite force in finches

Fringillids and estrildids differ in their husking performance on hard closed-shelled seeds, which are cracked before they are eaten. The time required to husk a seed is directly related to seed hardness, and husking time is therefore expected to be related to bite force as well. We investigated whether there is a significant difference in jaw muscle mass and maximal bite force between fringillids and estrildids. The analysis shows that fringillids have relatively larger jaw muscles than estrildids and are able to produce higher bite forces than estrildids of the same body size. This difference in jaw muscle mass mainly results from a difference in jaw closing muscles. Compared with other birds, the jaw muscles of both fringillids and estrildids scale strongly positively allometric with body size. Muscle fibre length scales negatively allometric with body size, which results in relatively high muscle and bite forces. Comparison with the scarce data available for other trophic groups suggests that the scaling of jaw muscle size depends on diet and that jaw muscle size in finches is an adaptation to their feeding behaviour.