Differing limb functions and their potential influence upon the diversification of the mustelid hindlimb skeleton

Though form-function relationships of the mammalian locomotor system have
been investigated for over a century, recent models of trait evolution
have hitherto been seldom used to identify likely evolutionary processes
underlying the locomotor system’s morphological diversity. Using
mustelids, an ecologically diverse carnivoran lineage, I investigated
whether variation in hindlimb skeletal morphology functionally coincides
with climbing, digging, swimming, and generalized locomotor habits by
using 15 linear traits of the femur, tibia, fibula, calcaneum, and
metatarsal III across 44 species in a principal components analysis. I
subsequently fit different models of Brownian motion and adaptive trait
diversification individually to each trait. Climbing, digging, and
swimming mustelids occupy distinct regions of phenotypic space
characterized by differences in bone robustness. Models of adaptive and
neutral evolution are, respectively, the best fits for long bone lengths
and muscle in-levers, suggesting that different kinds of traits may be
associated with different evolutionary processes. However, simulations
based upon models of best fit reveal low statistical power to rank the
models. Though differences in mustelid hindlimb skeletal morphology appear
to coincide with locomotor habits, further study, with sampling expanded
beyond Mustelidae, is necessary to better understand to what degree
adaptive evolution shapes morphological diversity of the locomotor system.