Tuataras may be grey, olive, or brickish red in color. They range in adult length from about 40 cm (female) to 60 cm (large male), with the male generally reaching larger proportions. They lack external ears, have a diapsid skull (two openings on either side), and posess a "parietal eye" on the top of their head. Other lizards also have this "third-eye," which contains a retina and is functionally similar to a normal eye, though the function has not been clearly recognized and a scale grows over it in adult tuataras. The male tuatara displays a striking crest down the back of the neck, and another down the middle of the back. The female has a less developed version of this. Unlike all other living toothed reptiles, the tuatara's teeth are fused to the jaw bone (acrodont tooth structure). The tuatara has a very slow metabolism and is a very long-lived species. It's not uncommon for an individual to live for over 100 years.