2.4 The change of skull size from 1953 to 2004
We first analysed the changes in the overall size of S. araneususing the full dataset from 1953-2004, which was only available for juveniles in July and adults. We tested the significance of the slopes of the linear regressions of all three dimensions over time: skull height, width, and length. We added a quadratic term to the linear model to test for linearity of the relationships and determine whether it improved the model according to an F test with a type I sum of squares. However, inspection of the residuals from the linear regression suggested a more subtle nonlinearity of the skull size over time. Thus, we additionally used locally weighed regression smoothing (LOESS; Cleveland 1979). We selected the smoothing parameter by minimizing Akaike’s information criterion, which strikes a balance between the residual sum of squares and the complexity of the fit.