In an investigation of the postnatal growth of the vertebral column of the Norway rat (Rattus norvegicus), we recorded hind-foot length as a standard metric, along with skull, tail, femur, and tibia lengths, against which to compare the growth of axial components. We confirmed a nonlinear relationship of head–body length against hind-foot length, tail length, and tibia length across the time course from neonate to adult and also discovered a nonlinear relationship between both skull and femur length to head–body length. Differences in growth rate are directly related to preweaning and postweaning periods. The pattern of differential growth was distinctly least pronounced for femoral length. We therefore advocate the latter as the most appropriate to use as an easily measured proxy for growth across the entire neonate to adult growth period. This study reveals implications for the choice of optimal variables used as size proxies and also suggests functional implications of shifts in form–function relationships from unweaned to weaned individuals. However, variation in body form across mammals and altricial versus precocial modes of natal expression will continue to complicate the search for appropriate comparative metrics in the study of the development and evolution of body form.
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