Tuatara (Sphenodon punctatus) are reptiles endemic to New Zealand. The name tuatara is derived from the Māori language and means "peaks on the back". The single extant species of tuatara is the only surviving member of its order. Their closest living relatives are squamates (lizards and snakes). For this reason, tuatara are of interest in the study of the evolution of lizards and snakes, and for the reconstruction of the appearance and habits of the earliest diapsids, a group of amniote tetrapods that also includes dinosaurs (including birds) and crocodilians.
Tuatara are the largest reptile in New Zealand. Their greenish-brown colour matches their environment and can change over their lifetime. Tuatara shed their skin at least once per year as adults and three or four times a year as juveniles. Tuatara sexes differ in more than size. The spiny crest on a tuatara's back, made of triangular, soft folds of skin, is larger in males and can be stiffened for display. The male abdomen is narrower than the female's. The eyes of tuatara can focus independently and are suggested to have good colour vision, possibly even at low light levels. Tuatara have a third eye on the top of their head called the parietal eye. The parietal eye is visible only in hatchlings, which have a translucent patch at the top centre of the skull. After 4-6 months, it becomes covered with opaque scales and pigment. Its use is unknown, but it may be useful in absorbing ultraviolet rays to produce vitamin D, as well as to determine light/dark cycles, and help with thermoregulation. Tuatara have two rows of teeth in the upper jaw overlapping one row on the lower jaw, which is unique among living species. They are able to hear, although no external ear is present, and have unique features in their skeleton, some of them apparently evolutionarily retained from fish.
Tuatara were once widespread on New Zealand's main North and South Islands. Wiped out from the main islands before European settlement, they were long confined to 32 offshore islands free of mammals. Tuatara inhabit cliff-bound islands with vegetation, shrublands, and temperate forests. They can also be found in sheep pastures and frequently shelter in seabird burrows.
Adult tuatara 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. Juveniles are typically active at night but can be found active during the day. Tuatara use the burrows for the shelter of burrowing seabirds such as petrels, prions, and shearwaters when available, or dig their own. Tuatara thrive in temperatures much lower than those tolerated by most reptiles and hibernate during winter. They remain active at temperatures as low as 5 °C (41 °F), while temperatures over 28 °C (82 °F) are generally fatal. The optimal body temperature for the tuatara is from 16 to 21 °C (61 to 70 °F). The body temperature of tuatara is lower than that of other reptiles, ranging from 5.2-11.2 °C (41.4-52.2 °F) over a day, whereas most reptiles have body temperatures around 20 °C (68 °F). Both the male and the female tuatara defend territories and will threaten and eventually bite intruders. Their bite can cause serious injury. Tuatara will bite when approached, and will not let go easily.
Tuatara are carnivores and mainly feed on beetles, crickets, spiders, wētās (a group of crickets), earthworms, and snails. Their diets also consist of frogs, lizards, and bird's eggs and chicks. Young tuatara are also occasionally cannibalized. The diet of the tuatara varies seasonally and they mainly only consume Fairy prions and their eggs in the summer.
Tuatara are thought to be serially monogamous; the males of this species establish territories and mate and guard one female during the breeding season. These reptiles mate in midsummer and females mate and lay eggs once every 4 years. During courtship, a male makes his skin darker, raises his crests, and parades toward the female. He slowly walks in circles around the female with stiffened legs. The female will either accept the male or retreat to her burrow. Incubation then takes between 12 and 15 months from mating to hatching. The sex of a hatchling depends on the temperature of the egg, with warmer eggs tending to produce male tuatara, and cooler eggs producing females. Eggs incubated at 21 °C (70 °F) have an equal chance of being male or female. However, at 22 °C (72 °F), 80% are likely to be males, and at 20 °C (68 °F), 80% are likely to be females; at 18 °C (64 °F) all hatchlings will be females. Some evidence indicates sex determination in tuatara is determined by both genetic and environmental factors. The hatchlings are independent as soon as they break out of their egg and don’t require parental care. Tuatara reproduce very slowly, taking 10 to 20 years to reach reproductive maturity.
Tuatara live on islands that are difficult to get to, and are colonized by few animal species; this indicates that some animals absent from these islands may have caused tuatara to disappear from the mainland. However, kiore (Polynesian rats) had recently become established on several of the islands, and tuatara were persisting, but not breeding, on these islands. Additionally, tuatara were much rarer on the rat-inhabited islands. These reptiles also suffer from climate changes. They have temperature-dependent sex determination meaning that the temperature of the egg determines the sex of the animal. For tuatara, lower egg incubation temperatures lead to females while higher temperatures lead to males. Since global temperatures are increasing faster than ever, researchers are worried that climate change is skewing the male-to-female ratio of tuatara and that in a few decades, tuatara offspring populations will be all male.
According to IUCN Red List, the total population size of the tuatara is around 55,000 mature individuals. Currently, this species is classified as Least Concern (LC) on the IUCN Red List.
