The relationship between shape of the skull and bite force in finches

In finches husking time is non-linearly related to the ratio of seed hardness to maximal bite force. Fringillids produce larger bite force and husk relatively hard seeds faster than estrildids of similar size. This is at least partly explained by their relatively larger jaw muscle mass and a difference in husking technique. However, the effect of differences in skull geometry on bite force is unclear. In this study differences in skull morphology that may contribute to the difference in bite force between fringillids and estrildids are analyzed. The shape of the skull was described by the 3D coordinates of a set of landmarks and, after eliminating size, the effect of differences in the shape of the skull on bite force was determined using a static force model. EMG recordings of jaw muscles during seed cracking were used to validate assumptions about the muscle activation patterns used for the static bite force model. The analysis shows that most of the variation in skull geometry is related to differences in size. Although the shape of the skull is highly convergent between fringillids and estrildids, the shape of the skull differs significantly between the two groups. A principal component analysis of the landmark coordinates shows several patterns of allometric shape changes, one of which is expressed more strongly in estrildids than in fringillids. Three characters dominate the effect of shape changes on bite force. Bite force increases with a more caudal position of the quadrate, a more downward inclined beak and a relatively short jugal and palatine. A more downward inclined beak is typically found in estrildids. The height of the upper bill and a number of other changes in skull shape have little effect on bite force. An estimate of the relative contributions of jaw muscle size and skull geometry to the difference in bite force between fringillids and estrildids suggests that the contribution of muscle size is much larger than the contribution of skull geometry.