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Which line to follow? The utility of different line-fitting methods to capture the mechanism of morphological scaling

Bivariate morphological scaling relationships describe how the size of two traits co-varies among adults in a population. In as much as body shape is reflected by the relative size of various traits within the body, morphological scaling relationships capture how body shape varies with size, and therefore have been used widely as descriptors of morphological variation within and among species. Despite their extensive use, there is continuing discussion over which line-fitting method should be used to describe linear morphological scaling relationships. Here I argue that the ‘best’ line-fitting method is the one that most accurately captures the proximate developmental mechanisms that generate scaling relationships. Using mathematical modeling, I show that the ‘best’ line-fitting method depends on the pattern of variation among individuals in the developmental mechanisms that regulate trait size, and the morphological variation this pattern of developmental variation produces. For Drosophila traits, this pattern of variation indicates that major axis regression is the best line-fitting method. For morphological traits in other animals, however, other line-fitting methods may be more accurate. I provide a simple web-based application for researchers to explore how different line-fitting methods perform on their own morphological data.