Strepsirrhine Diets and The Pattern of Masticatory Muscle Development

Diet has been suggested to play an important role in developmental pacing. Somatic growth and dental development are expected to be slow in frugivorous primates because of seasonal food shortages and greater competition for food with adults, whereas folivorous primates are expected to grow more rapidly because food resources are less limited and/or because folivores need to be better equipped than frugivorous weanlings to break down fibrous foods. Here, we present the first dataset on the mass and fiber architecture of jaw adductors in newborn strepsirrhines to assess differences in the developmental maturity of the chewing muscles. We predicted that chewing muscle maturity at birth would be more advanced in folivores, which tend to be more dentally precocious at birth, than in the more dentally‐delayed frugivores. The study sample includes four strepsirrhine species: Varecia rubra (frugivore) , Eulemur macaco (frugivore/folivore) , Lemur catta (frugivore/folivore), and Propithecus coquereli (folivore). The masticatory muscles of infant strepsirrhines were dissected to collect muscle mass and fiber length (FL) data, and to calculate physiological cross‐sectional area (PCSA). PCSA provides an estimate of muscle force. Muscle data were compared to a published dataset on adult strepsirrhines. Muscle mass, FL, and PCSA maturity were operationalized as a ratio (i.e., infant FL/adult FL). At birth, there is already considerable variation among species in the masticatory muscles. No muscle stands out as being more developmentally advanced than the others; notably, contrary to published hypotheses, temporalis is not always the most mature muscle at birth. However, in all cases FL was found to be closer to the adult condition in newborns than muscle mass or PCSA. This is likely because FL growth is so strongly tied to bony growth. Propithecus coquereli was found to have the most mature temporalis and masseter muscles (in terms of mass, FL, and PCSA), but not the most mature medial pterygoid muscles. Furthermore, Varecia rubra , the most frugivorous taxon in the sample, generally has more mature jaw adductors at birth than the more folivorous Lemur catta . In addition, jaw adductor muscle maturity at birth does not appear to be influenced by adult muscle mass, FL, or PCSA – for example, the taxon with the largest temporalis in adulthood does not necessarily have the largest temporalis at birth. These results suggest that chewing muscle maturity is not necessarily more advanced in folivores than in frugivores. Although Propithecus coquereli is more dentally advanced at birth than the other lemur taxa in this study, the jaw adductors do not all follow this pattern. Chewing muscles might be following different developmental and functional cues than are the teeth; for example, they are subject to geometric rearrangements of the cranium and mandible during development. Future work will explore these patterns in additional strepsirrhine species and match changes in muscle geometry to changes in skull geometry. Support or Funding Information Duke Lemur Center Director's Fund Award to KA Prufrock This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .