Fig. 2. Phylogenetic hypothesis of the taxa used in this study compiled from Messenger and McGuire (1998) and Irwin and Arnason (1994). Species in which head-butting has been observed during male—male aggression are marked with a filled circle. See text for references. The numbers at the terminal branches represent the percentage sexual dimorphism in body length (top) and the percentage of the lateral projected body area made up by the melon (bottom). Numbers at nodes represent the contrast values for the node. The contrast value for a given node of the tree was calculated by taking the average of the descendant nodes along one bifurcation and subtracting it from the average along the other bifurcation and dividing this difference by a measure of the variance weighted by the lengths of the subsequent branches (Felsenstein, 1985).