The effects of latitude and body length on the degree of melanism. The figure shows the relationship between a) latitude and the degree of melanism and b) snout-vent length and the degree of melanism in adult common frogs caught from the wild. The degree of melanism is defined as the proportion of dorsal surface area covered by pigmentation spots large than 1 mm2. In figure (a) the circles represent means and vertical bars standard errors of the means. In figure (b) the circles represent partial residuals after correcting for population effects and latitude (fixed at the mean of the data, 61°16'N). The curves in both figures describe the predicted degree of melanism. The prediction curves and partial residuals are based on a linear mixed model incorporating sex, latitude, snout-vent length, the interaction of sex and latitude, and the interaction of sex and snout-vent length as fixed effects, and population as a random effect. The model was fitted to arcsine transformed data. The curves are back-transformed predictions and hence not linear.