You may have previously noticed that some of the primates in the lab may have ridges or crests on the top and/or back of their skulls. These crests are important and illustrate the size of the muscle attachment areas in an animal. The size of the crest is also a good predictor for the social grouping of an animal. The crest on the top of the skull is called the sagittal crest, as it runs down the sagittal suture line. The crest on the posterior portion of the skull is called the nuchal crest. They provide attachment sites for some of the primary muscles related to biting and chewing, such as the temporalis and masseter muscle. These muscles also pass through the zygomatic arch.