SEXUAL DIMORPHISM OF HEAD SIZE IN PODARCIS-HISPANICA ATRATA - TESTINGTHE DIETARY DIVERGENCE HYPOTHESIS BY BITE FORCE ANALYSIS

Sexual dimorphism of relative head size is a widespread phenomenon in lizards, males having larger head/trunk ratios than females. In an att empt to explain this sexual dimorphism several hypotheses have been fo rmulated. The two most frequently cited ones are: 1) sexual selection acting on those structures important in intrasexual competition and 2) natural selection for reduction of food competition between the sexes . In the insular subspecies of Podarcis hispanica (P. h. atrata) males tend to have significantly larger heads than similarly sized females. We here test an implicit assumption of the dietary divergence hypothe sis, namely that an increase in head size results in an increase in ga pe width and/or bite force, thereby allowing the larger headed sex to exploit larger prey classes. Using a static bite force model, we calcu lated the magnitude of bite forces for given directions at given posit ions on the jaws and for different head sizes. We experimentally deter mined the hardness of three different prey items and compared the data to the maximal bite force produced by both sexes. Our results suggest an important difference in male and female bite capacity, which may b ear significant ecological relevance, and are in agreement with the im plicit assumption of the dietary divergence theory.