Tuatara are greenish brown and gray, and measure up to 80 cm (31 in) from head to tail-tip and weigh up to 1.3 kg (2.9 lb) with a spiny crest along the back, especially pronounced in males. Their dentition, in which two rows of teeth in the upper jaw overlap one row on the lower jaw, is unique among living species. They are even more unusual in having a pronounced photoreceptive eye, the "third eye", which is thought to be involved in setting circadian and seasonal cycles. They are able to hear, although no external ear is present, and have a number of unique features in their skeleton, some of them apparently evolutionarily retained from fish. Although tuatara are sometimes called "living fossils", recent anatomical work has shown that they have changed significantly since the Mesozoic era. While mapping its genome, researchers have discovered that the species has between five and six billion base pairs of DNA sequence.
The tuatara Sphenodon punctatus has been protected by law since 1895. A second species, S. guntheri, was recognised in 1989 but since 2009 its use has been discontinued. Tuatara, like many of New Zealand's native animals, are threatened by habitat loss and introduced predators, such as the Polynesian rat (Rattus exulans). They were extinct on the mainland, with the remaining populations confined to 32 offshore islands, until the first mainland release into the heavily fenced and monitored Karori Sanctuary in 2005. During routine maintenance work at Karori Sanctuary in late 2008, a tuatara nest was uncovered, with a hatchling found the following autumn. This is thought to be the first case of tuatara successfully breeding on the New Zealand mainland in over 200 years, outside of captive rearing facilities.
The tuatara is considered the most unspecialised living amniote; the brain and mode of locomotion resemble those of amphibians and the heart is more primitive than that of any other reptile. The lungs have a single chamber and lack bronchi. Both species are sexually dimorphic, males being larger. Adult S. punctatus males measure 61 cm (24 in) in length and females 45 cm (18 in). The San Diego Zoo even cites a length of up to 80 cm (31 in). Males weigh up to 1 kg (2.2 lb), and females up to 0.5 kg (1.1 lb). Brother's Island tuatara are slightly smaller, weighing up to 660 g (1.3 lb). The tuatara's greenish brown colour matches its environment, and can change over its 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.
Sensory Organs Edit
The eyes can focus independently, and are specialized with a duplex retina that contains two types of visual cells for both day and night vision, and a tapetum lucidum which reflects onto the retina to enhance vision in the dark. 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 produce vitamin D, as well as to determine light/dark cycles, and help with thermoregulation. Of all extant tetrapods, the parietal eye is most pronounced in the tuatara. It is part of the pineal complex, another part of which is the pineal gland, which in tuatara secretes melatonin at night. Some salamanders have been shown to use their pineal bodies to perceive polarised light, and thus determine the position of the sun, even under cloud cover, aiding navigation.
Together with turtles, the tuatara has the most primitive hearing organs among the amniotes. There is no eardrum and no earhole, and the middle ear cavity is filled with loose tissue, mostly adipose (fatty) tissue. The stapes comes into contact with the quadrate (which is immovable), as well as the hyoid and squamosal. The hair cells are unspecialized, innervated by both afferent and efferent nerve fibres, and respond only to low frequencies. Though the hearing organs are poorly developed and primitive with no visible external ears, they can still show a frequency response from 100 to 800 Hz, with peak sensitivity of 40 dB at 200 Hz.
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. 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 lowest of any reptile. 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). The low body temperature results in a slower metabolism.
Burrowing seabirds such as petrels, prions, and shearwaters share the tuatara's island habitat during the birds' nesting seasons. The tuatara use the birds' burrows for shelter when available, or dig their own. The seabirds' guano helps to maintain invertebrate populations on which tuatara predominantly prey; including beetles, crickets, and spiders. Their diets also consist of frogs, lizards, and bird's eggs and chicks. The eggs and young of seabirds that are seasonally available as food for tuatara may provide beneficial fatty acids. Tuatara of both sexes defend territories, and will threaten and eventually bite intruders. The bite can cause serious injury. Tuatara will bite when approached, and will not let go easily.
Tuatara reproduce very slowly, taking 10 to 20 years to reach sexual maturity. Mating occurs in midsummer; females mate and lay eggs once every four 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 submit, and allow the male to mount her, or retreat to her burrow. Tuatara eggs have a soft, parchment-like shell. It takes the females between one to three years to provide eggs with yolk, and up to seven months to form the shell. It then takes between 12 and 15 months from copulation to hatching. This means reproduction occurs at two- to five-year intervals, the slowest in any reptile. Wild tuatara are known to be still reproducing at about 60 years of age; "Henry", a male tuatara at Southland Museum in Invercargill, New Zealand, became a father (possibly for the first time) on 23 January 2009, at the age of 111.
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. Tuatara probably have the slowest growth rates of any reptile, continuing to grow larger for the first 35 years of their lives. The average lifespan is about 60 years, but they can live to be well over 100 years old. Some experts believe that captive tuatara could live as long as 200 years.