The Anatomy of Mastication in Extant Strepsirrhines and Eocene Adapines

Abstract

The jaw adductor muscles in strepsirrhines were dissected and their fiber architecture was quantified. Bite force and leverage were estimated using values for physiological cross-sectional area (PCSA) of the jaw adductors and lateral photographs of skulls. Jaw adductor mass, PCSA, fiber length, and bite force scale isometrically to body size. An experiment carried out at the Duke Lemur Center demonstrated that ingested food size also scales isometrically to body size.

Folivorous strepsirrhines are characterized by short jaw adductor fibers, uniformly small ingested food size, large masseter and medial pterygoid muscles (in PCSA and mass), and large estimated bite force for their jaw length. Large-bodied folivores have especially large jaw adductors. Small-bodied folivores have especially short jaws, but do not have especially large jaw adductors. Folivores probably can generate large bite forces; they possess short jaws (short bite load arms) and/or large jaw adductor cross-sectional areas.

Frugivorous strepsirrhines are characterized by long jaws, large (but variable) ingested food size, large temporalis muscles, and small estimated bite force for their jaw length. Frugivores have long jaw adductor fibers that likely maintain tension during the ingestion of large objects (e.g., fruits). The temporalis is large in frugivores, not because it has superior leverage during incision, but because its fibers likely do not stretch as much at wide gapes as those of the other adductors.

Correlations between osteological landmarks and jaw adductor dimensions in strepsirrhines were used to infer jaw adductor dimensions in <em>Adapis parisiensis</em> and <em>Leptadapis magnus</em> (Adapinae) from the Eocene of Europe. Inferred PCSA and lateral photographs were used to estimate bite force and leverage in these adapines. An analysis of shearing quotients was also performed. Inferred jaw adductor mass, PCSA, bite force, and shearing quotients are great in adapines relative to extant strepsirrhines. All anatomical signals suggest a diet rich in tough leaves and other structural plant parts, perhaps with some small fruits. <em>Adapis</em> was likely more folivorous than <em>Leptadapis</em>.