The Jaw Adductors of Strepsirrhines in Relation to Body Size, Diet, and Ingested Food Size

Abstract

Body size and food properties account for much of the variation in the hard tissue morphology of the masticatory system whereas their influence on the soft tissue anatomy remains relatively understudied. Data on jaw adductor fiber architecture and experimentally determined ingested food size in a broad sample of 24 species of extant strepsirrhines allows us to evaluate several hypotheses about the influence of body size and diet on the masticatory muscles. Jaw adductor mass scales isometrically with body mass (β = 0.99, r = 0.95), skull size (β = 1.04, r = 0.97), and jaw length cubed (β = 1.02, r = 0.95). Fiber length also scales isometrically with body mass (β = 0.28, r = 0.85), skull size (β = 0.33, r = 0.84), and jaw length cubed (β = 0.29, r = 0.88). Physiological cross-sectional area (PCSA) scales with isometry or slight positive allometry with body mass (β = 0.76, r = 0.92), skull size (β = 0.78, r = 0.94), and jaw length cubed (β = 0.78, r = 0.91). Whereas PCSA is isometric to body size estimates in frugivores, it is positively allometric in folivores. Independent of body size, fiber length is correlated with maximum ingested food size, suggesting that ingestive gape is related to fiber excursion. Comparisons of temporalis, masseter, and medial pterygoid PCSA in strepsirrhines of different diets suggest that there may be functional partitioning between these muscle groups.