Body mass is an important organism-level variable in mammalian biology, correlated with physiology, life history, and ecology. To analyze the dynamics of body size evolution, increases and decreases in body mass were tallied for ancestor-descendant (AD) species pairs for 519 terrestrial caniform taxa. To account for uncertainty phylogeny, a bootstrapping routine shuffled hypothesized AD pairs, and average proportions of increases were binned as a function of ancestral body mass. A set of models relating the rate of body size increase were evaluated with the Akaike Information Criterion (AIC). AIC selected three models of the candidate set as equivalent in support by the observed body mass data. These three models propose body size increase for small AD pairs and body size decrease for large AD pairs, although they differ in their treatment of taxa at intermediate sizes.

These results demonstrate the presence of constraints bounding the caniform distribution at large and small body sizes, stabilizing the distribution through time, which stands in contrast to a broader mammalian pattern. At a finer phylogenetic scale, subclades within intermediate size classes display proportions that are significantly different from unbiased, with several clades previously cited as examples of “Cope's Rule” showing biased increases in size, and basal mustelids (badgers, and allied genera), Mephitidae (skunks), and Vulpini (“foxes”) exhibiting biased decreases. The caniform pattern is therefore the result of superimposed, clade-specific trajectories, demonstrating that the inferred dynamics of body size evolution and even the direction of trends in body size evolution within the Caniformia, and for mammals in general, depend on the hierarchical scale of the analysis.

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