Tuataras display sexual dimorphism, as the males are larger, weighing up to 1 kilogram (2.2 pounds), almost twice the weight of the females. The spiny crest on their back, made of triangular soft folds of skin, is bigger in males than in females and can be stiffened for display. The male abdomen is narrower than the females.

As their teeth wear down, older tuataras have to switch to softer prey such as earthworms, larvae and slugs and eventually have to chew their food between smooth jaw bones.

In tuataras, both eyes can focus independently and are specialized with a ‘duplex retina’ that contains two types of visual cells for vision by both day and night and a tapetum lucidum which reflects on to the retina to enhance vision at night. There is also a third eyelid on each eye, the nictitating membrane.

The tuatara has a third eye on the top of its head called the ‘parietal eye’. It has its own lens, cornea, retina with rod-like structures and degenerated nerve connection to the brain, suggesting it evolved from a real eye. The parietal eye is only visible in hatchlings, which have a translucent patch at the top centre of the skull.

After four to six months it becomes covered with opaque scales and pigment. Its purpose is unknown, but it may be useful in absorbing ultraviolet rays to manufacture vitamin D, as well as to determine light/dark cycles and help with thermoregulation.

Adult tuataras are terrestrial and nocturnal reptiles, though they will often bask in the sun to warm their bodies. Hatchlings hide under logs and stones and are diurnal, likely because adults are cannibalistic. Tuataras survive in temperatures much lower than those tolerated by most reptiles and hibernate during winter.

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