Muscle architecture in mustelid mammals: anatomical proxies for muscle function and their relation to locomotor habit

Though skeletal adaptations for locomotion in mammals are well documented, possible limb adaptations in terms of muscle architecture have received far less study. Examining mustelid mammals and sampling climbing, digging, and swimming specialists, as well as generalists, we here investigate whether differences in the architecture of limb muscles coincide with differing locomotor habits as is the case for limb skeletal morphology. For 49 individuals across 19 mustelid species, we measured fascicle length, belly mass, and physiological cross-sectional area (PCSA) for six forelimb and six hindlimb muscles. For fore- and hindlimbs separately, we performed a size-standardized principal components analysis for each of the three architectural traits to determine whether locomotor habits coincide with differences in muscle architecture. Digging mustelids are typified by shoulder flexors and elbow extensors that are long-fascicled and both strong and massive. Climbing mustelids are characterized by short-fascicled muscles, with shoulder and elbow flexors and knee and ankle extensors being relatively massive among the sampled muscles. Swimming mustelids tend to have relatively long-fascicled muscles, and their flexors and extensors of both the wrist and ankle are both strong and powerful; however, sea otters appear to have overall more massive muscles than other otters, likely due to their mode of swimming. In comparison to otters, the muscles of mink are also relatively long-fascicled, and like otters, mink tend to have massive and strong distal limb muscles, though to proportionally lesser degree among the limb’s muscles in comparison to otters. Among generalists, the strength of forelimb muscles relative to one another is akin to the relative strengths among the forelimb muscles in digging mustelids.Our results show that the relative masses of muscle bellies within a limb largely distinguish mustelid locomotor habits, but whether this is the result of selection should be the subject of future study.. These results indicate that muscle architecture likely reflects limb usage in mustelids, though future studies must sample muscles and species more extensively. Studies investigating the evolution of the locomotion and locomotor adaptations should consider specializations in muscle architectural traits such as belly mass, fascicle length, and PCSA in addition to specializations in skeletal morphological traits.