Ontogeny of the Jaw Adductor Resultant: Using Strepsirrhine Primates as a Case Study
Authors: Prufrock Kristen Alexandra , Perry Jonathan Marcus Glen 
Published: April 2020

The masticatory system is involved in a key period in mammalian development – the transition from suckling to independent feeding. A number of biomechanically important anatomical changes occur in the masticatory system through ontogeny. Changes to the morphology of the cranium and bony jaw apparatus can modify the regions where masticatory muscles originate and insert, and can therefore alter the orientation of the masticatory muscles and how forces are directed during biting. Changes to the orientation of the masticatory muscles through ontogeny have been noted in several mammalian taxa, suggested to achieve more vertical occlusal forces (perpendicular to the bite plane) for independent feeding. In this study, we investigate if and how the orientation of the jaw adductor resultant (OJAR; the summed lines of action of the masticatory muscles) changes through ontogeny in one group of mammals, strepsirrhine primates. Given that the magnitude of a muscle vector is proportional to muscle force, we also investigate if and how the relative physiological cross‐sectional areas (PCSAs) of masticatory muscles (e.g., temporalis PCSA/total masticatory muscle PCSA) change through ontogeny.

The masticatory muscles (temporalis, masseter, and medial pterygoid) of perinates, infants, juveniles, and adults from six strepsirrhine species (Eulemur macaco, Lemur catta, Lepilemur leucopus, Otolemur crassicaudatus, Propithecus coquereli, and Varecia rubra) were dissected. Muscles mass and fiber length data were then collected to estimate muscle PCSA. The OJAR (measured in degrees relative to the bite plane) was estimated using lateral photographs of dissected specimens. The relationship between the OJAR and age within species was assessed using reduced major axis regressions after natural log transforming the data. Differences in relative PCSAs of masticatory muscles (within species, through ontogeny) were evaluated using Kruskal‐Wallis tests and pairwise differences between age categories (perinate, infant, juvenile, adult) were evaluated using Conover‐Iman tests.

For most species, there is a weak negative relationship (Pearson’s r = −0.10 to −0.73) between the OJAR and age. However, Otolemur crassicaudatus was found to have a weak positive relationship (Pearson’s r = 0.30). In addition, age poorly explains most of the variation in the OJAR (R2 = 0.01 to 0.53). No significant differences in relative PCSAs of masticatory muscles were found among age categories within species, except in the case of Otolemur crassicauduatus. In O. crassicaudatus, relative temporalis PCSA was found to be significantly lower in perinates than in adults (p = 0.02).

These data suggest that orientation of the OJAR does not change substantially through ontogeny in strepsirrhines, and therefore does not strongly affect how forces are directed during biting in the anterior‐posterior plane. This is also supported by the lack of variation in relative PCSA of the masticatory muscles through ontogeny. This may be evidence of strong stabilizing factors in ontogeny that favor a specific direction of overall jaw adductor force at all life stages.

Support or Funding Information

Duke Lemur Center Director’s Fund Award