Dietary ecology and its relationship with both muscle architecture and bite force potential has been studied in many mammalian (and non‐mammalian) taxa. However, despite the diversity of dietary niches that characterizes the superfamily Musteloidea, the masticatory muscle fiber architecture of its members has yet to be investigated anatomically. In this study, we present myological data from the jaw adductors in combination with biomechanical data derived from craniomandibular measurements for 17 species representing all four families (Ailuridae, Mephitidae, Mustelidae, and Procyonidae) of Musteloid. These data are combined to calculate bite force potential at each of three bite points along the dental row. Across our sample as a whole, masticatory muscle mass scaled with isometry or slight positive allometry against both body mass and skull size (measured via a cranial geometric mean). Total jaw adductor physiological cross‐sectional area scaled with positive allometry against both body mass and skull size, while weighted fiber length scaled with negative allometry. From a dietary perspective, fiber length is strongly correlated with dietary size such that taxa that exploit larger foods demonstrated myological adaptations toward gape maximization. However, no consistent relationship between bite force potential and dietary mechanical resistance was observed. These trends confirm previous findings observed within the carnivoran family Felidae (as well as within primates), suggesting that the mechanisms by which masticatory anatomy adapts to dietary ecology may be more universally consistent than previously recognized. Anat Rec, 302:2287–2299, 2019. © 2019 American Association for Anatomy

The fascicular architecture of skeletal muscle dictates functional parameters such as force production and contractile velocity. Muscle microarchitecture is typically determined by means of manual dissection, a technique that is inherently destructive to specimens. Furthermore, fascicle lengths and pennation angles are commonly assessed at only a limited number of sampling sites per muscle. We present the results of a digital technique to non-destructively assess muscle architectural variables for three jaw-adductor muscles within a specimen of the cercopithecine primate Macaca fascicularis (crab-eating macaque). The specimen is first subjected to a contrast-enhanced staining protocol to increase the density of internal soft tissues. High-resolution µCT scans are then collected and segmented to isolate individual muscles. A textural orientation algorithm is then applied to each muscle volume to reconstruct constituent muscle fascicles in three dimensions. Using this technique, we report muscle volume, fascicle length, angle of pennation, and physiological cross-sectional area (PCSA) for each muscle. These data are compared to results collected using traditional dissection of the contralateral muscles. Reconstructions of muscle volume and pennation angle closely correspond to the dissection results. The degree of similarity between measurements of fascicle length and PCSA varies between muscles, with temporalis demonstrating the greatest disparity between techniques; likely reflecting the complex geometry and fascicular arrangement of this muscle. The described technique samples a much larger number of fascicles than had previously been possible and non-destructively investigates the internal architecture of preserved specimens. We conclude that this approach demonstrates great potential for quantifying muscle internal architecture. Anat Rec, 301:363-377, 2018. © 2018 Wiley Periodicals, Inc.

No vertebrate, living or extinct, is known to have possessed an odd number of limbs. Despite this ‘forbidden phenotype’, gaits that use odd numbers of limbs (e.g. tripedalism or pentapedalism) have evolved in both avian and mammalian lineages. Tripedal locomotion is commonly employed by parrots during climbing, who use their beaks as an additional support. However, it is unclear whether the beak functions simply as a stabilizing hook, or as a propulsive limb. Here, we present data on kinetics of tripedal climbing in six rosy-faced lovebirds ( Agapornis roseicollis ). Our findings demonstrate that parrots use cyclical tripedal gaits when climbing and the beak and hindlimbs generate comparable propulsive and tangential substrate reaction forces and power. Propulsive and tangential forces generated by the beak are of magnitudes equal to or greater than those forces generated by the forelimbs of humans and non-human primates during vertical climbing. We conclude that the feeding apparatus and neck flexors of parrots have been co-opted to function biomechanically as a propulsive third limb during vertical climbing. We hypothesize that this exaptation required substantive alterations to the neuromuscular system including enhanced force-generating capabilities of the neck flexors and modifications to locomotor central pattern generators.

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