Morphology and relationships of the orangutan fatty cheek pads.

The present paper examines the anatomical relationships as well as possible developmental and functional relationships of the fatty cheek pad characteristic of the adult male orangutan. The research involves the dissection of 11 orangutans of both sexes in a variety of age categories. All specimens possess either a fat pad or a subcutaneous connective tissue fascial compartment in the lateral face extending superiorly from slightly above the inferior border of the mandible to the temporal region. In immature specimens as well as adult females, fat deposits within the connective tissue compartment are scarce or nonexistent, whereas pubescent male specimens or older possess accumulations of fibro-fatty tissues in this region. The extensive fat accumulations of the adult male cheek flange are related to age and sex and to modifications in related facial musculature, especially mm. platysma, zygomaticus, orbicularis oculi, and orbitotemporalis and frontalis. These muscles are clearly related to the cheek pad structures in both sexes but appear to assume a supportive role in the males. The cheek pad has no direct bony attachments, but its mass may have a significant effect on facial morphology. The development of the cheek pad is temporally variable and its function remains speculative although the presence of a fully developed flange may be related to behavioral modifications.