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© Rune Midtgaard
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Expanded account of the genus Sphenodon
 The genus contains a single species: |
Abbreviations and glossary | ||
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Sphenodon punctatus
Weight: Males up to 1 kg, females up to 0.5 kg. The tuatara is New Zealand's largest terrestrial reptile. Reproduction: Oviparous. 1-19 (usually 4-13) eggs are laid and covered with friable soil in a nest cavity dug by the female, which returns over several nights to fill the cavity. The cavity is about 20 cm deep and 1.5 m long. The eggs are about 30 mm long, oval, white and leathery. They are distinctly more elongate in the subspecies guntheri than in the other subspecies. Adults have a crest of soft spines along the mid-line of the head and body, which is more strongly developed in males and is raised during courtship displays. The male erects the crest, inflates the body and throat, and circles the female with a particular locomotion called the "Stolzer Gang" or "proud walk". The female chooses the male, so only if she is satisfied with the courtship displays, she will allow mating to take place. The male then mounts the female, bites her on the neck, and often turns her over while coiling his tail around her. When positioned correctly, the sperm flows from the male's to the female's cloaca. The copulation may last 90 minutes. Courtship and mating takes place in January-March, but the eggs are not layed until October-December in the following year, one of the longest gestation periods among reptiles. The egg development is slow and stops during winter. The hatching occurs 10-16 months after egg deposition. The yolk sac is still attached to the hatchling until it drops off a few days later. They immediately seek shelter after hatching to avoid dangers such as dehydration, overheating, and cannibalistic adults. Laboratory experiments have shown the optimal incubation temperature to be 18-22°C, the lowest known in living reptiles. Sex determination is temperature dependent. Temperatures of 22.2°C or higher tend to produce more males, whereas temperatures of 21.2°C or lower tend to produce more females. At 22.0°C there is equal probability of an egg developing into a male or a female. The eggs absorb moisture during incubation, resulting in hatchlings weighing 1.2-1.3 times the original egg mass. Females reach maturity at 17 cm after about 13 years, the males at 18 cm after 13-17 years. The females produce a clutch every 2-7 years (every fourth year on average), depending on food resources. The time between clutches includes a vitellogenic cycle usually three years long. Males can mate every year. In summer, males defend territories and fight off rival males. Courting males may travels long distances to fight other males. Combats follow set rituals, including puffing up the throat sac, erecting spines, croaking and lungings. Although they have sharp claws, it is mainly the jaws that are used in the fights, and most older males have scars from these combats. Food: Mainly invertebrates, but also vertebrates are eaten. Prey includes beetles, spiders, weta, isopods, snails, millipedes, centipedes, earthworms, skinks, geckos, frogs, and birds (passerines, seabird chicks and eggs). Cannibalism also occurs, as adults may prey on hatchlings. Tuataras also scavenge on carrion. Seabird nests provide an abundance of arthropod prey for tuataras, and nest burrows are used by tuataras for daily shelter and winter hibernation. Feeding: Forages mainly at night, often at temperatures around 12-16°C, which is too low for proper digestion. The tuatara therefore basks during daytime with warmer conditions. Habitat: Forests, shrublands, herbfields, grasslands, and beaches. Now restricted to small islets in the Cook Strait, the Bay of Plenty and the Hauraki Gulf (but see reintroductions under Conservation status). The tuatara is most abundant in islands with nesting seabird populations, suggesting a lack of or a reduction of rats, the main predators of tuataras. These islands are often cliff-bound and covered with forest. In the northern islands the forests are dominated by Pohutukawa (Metrosideros excelsa), Mahoe (Melicytus ramiflorus), Karo (Pittosporum crassifolium), and Kanuka (Kunzea ericoides). In the Cook Strait islands, the forests are dominated by Kohekohe (Dysoxylum spectabile), Mahoe, Ngaio (Muoporum laetum), Taupata (Coprosma repens), and Akiraho (Olearia paniculata). The vegetation on smaller islands consist of low, windshorn plants, e.g. Taupata. The tuatara has also been found among flax bushes, Harakeke (Phormium tenax), and sheep pasture on Stephens Island. Although capable of digging their own burrows, tuataras frequently inhabit seabird burrows on offshore islands. These seabirds include Fairy Prion (Pachyptila turtur), Fluttering Shearwater (Puffinus gavia), Sooty Shearwater (Ardenna grisea), and Flesh-footed Shearwater (Ardenna carneipes). This co-existence may not always be beneficial for the birds, since tuataras may eat their cohabitants, or at least their eggs and chicks. Abundance: Densities of tuataras in remnant and regrown coastal forest on Stephens Island can reach 2,732 ind/ha, five times the density in paddocks on the island and 20 times that known from any other island. Altitude range: 1-722 m ASL. Habits and activity: Terrestrial, but also climbs lower branches of trees, particularly the juveniles. Although feeding mainly at night, tuataras are not exclusively nocturnal. They bask near their burrow entrances in warm summer months, retreating when they become too hot and emerging when they cool. Active at temperatures down to 5°C, which is lower than what is usually tolerated by other reptiles. Optimal body temperature is 16-21°C. Hatchlings are diurnal and arboreal, probably to reduce the risk of being preyed upon by adults. During hibernation, the heart rate becomes slow and irregular, dropping to about one beat a minute. Metabolism may slow down to a rate of several minutes between breaths. Variation: Three subspecies have been described, but differences seem to be minor. Enemies and defense: Introduced rats - Polynesian Rat or Kiore (Rattus exulans), Black Rat (Rattus rattus), Norway Rat (Rattus norwegicus) - have been the major predators of the tuatara, preying on eggs and youngs. They also compete with tuataras on food ressources. As introduced predators, rats have a negative impact on tuatara populations. In islands with moderate or high rat populations, tuatara populations are dominated by adults, since the eggs and youngs are eaten by rats. Some of these tuatara populations may only survive, because tuataras are long-lived. Other predators include Black-backed Gull (Larus dominicanus), Tasmanian Spotted Owl or Morepork (Ninox novaeseelandiae), harriers (Accipitriformes), and kingfishers (Coraciiformes). Longevity: The tuatara is a long-lived species, reaching an age of up to 91 years, a very high age among reptiles. However, the true longevity and generation length is uncertain, since the longest-studied individuals are still alive. It is likely that the lifespan may exceed 100 years, and the average generation time is probably about 30 years, one of the longest of any known terrestrial reptile. Relationship with man: The tuataras has gained an iconic status among New Zealand animals and plants, similar to the kiwi and other species, symbolising the ancient isolation of the islands. The tuatara is important to Maori culture and feature in legends, traditional paintings and sculptures. There are many stories and proverbs about the tuatara in Maori myth and legends. Both tuataras and lizards are considered god forms, and for some iwi, tuataras represent the guardians of knowledge. In some areas, the tuatara was regarded with awe and seen as a messenger of Whiro, the spirit of death and disaster. Another legend tells that tuataras and lizards are children of Punga, the god of all things ugly, who once lived in the sea. However, the tuatara was also eaten by Maori, but the hunters needed to observe certain rituals. Women never ate tuataras, because they are 'life-givers'. The species has been depicted in several New Zealand coins over the years, including the 5-cent coin that went out of circulation in 2007. Global population: Exceeds 55,000 mature individuals. The total population, including juveniles, has been estimated to 100,000 individuals. The northern subpopulation (Bay of Plenty and Hauraki Gulf) consists of 10,000 mature individuals, and the southern (Cook Strait) consists of 45,000 mature individuals. The Brothers Island Tuatara has a population of only about 400 individuals. Fossil evidence suggest that there were once millions of tuatara, widespread over New Zealand. The current range corresponds to about 0.5% of the former range. Conservation status: IUCN Red List: Least Concern. Tuataras are strictly protected wherever they occur. The first action recovery plans were introduced in 1993, and the most recent in 2001. Although the tuatara was historically much more widespread, the IUCN assessment is justified because the species still occurs in high densities within its current range. The tuatara was in decline until the 1990s, but with the eradication of rats in all populated islands, but one, the populations are now recovering. Presently, it appears that no major ongoing threats exist, following the removal of invasive predators and efforts to restore natural habitat. However, the future of the species relies entirely on conservation management to prevent the establishment of invasive species on the islands populated by tuataras. Subfossil remains show that the tuatara was once widespread on the mainland, but was lost at some point since human arrival about 800 years ago. It has been suggested that it became extinct in the mainland in the 1700s. Within the last 100 years, it has disappeared from 10 islands where it used to occur. About 32 islands still have natural populations, the largest of which is Stephens Island, which has an area of 1.5 km². This island also has the largest subpopulation, consisting of 30,000 individuals. Tuataras have now been introduced to the mainland in sanctuaries that are fenced to exclude mammalian predators, and to seven islands in the Cook Strait (1995-2011) and one in the Bay of Plenty, following local extinction. Other threats include habitat alterations and illegal capture and trade. On Stephens Island, more than 80% of the native forest had been cleared for farmland by the 1920s. Reforestation is ongoing, but native forest covered less than 15% of the island as recently as 2005. Livestock was removed from the island in 2004. A decline in body condition over 54 years (from 1949 to 2003) was reported, based on measurements of mass and body length, which may be a consequence of increased population densities. Trade: The species is not presently considered a trade item, but demand or interest is likely high and smuggling has been attempted in the past. In the past, tuataras have been advertised in in the USA for as much as 10,000 USD for an animal. Animals in transit have also been confiscated by officials, suggesting that poachers have had some success in harvesting the species. Smuggling is regarded with increasing seriousness by New Zealand authorities, and law offenders expose themselves to severe sentences. Etymology: Common name: Tuatara is a Maori name meaning "peaks on the back" (tua = "back", tara = "spiny"), referring to the dorsal crest. Order name: Rhynchocephalus/Rhynchocephalia (Greek) = "beak head"; rhynchus (Greek) = "beak" or "snout", kephale (Greek) = "head", referring to the way the upper jaw projects over the lower, a feature more noticeable in the tuatara's fossil relatives than in the tuatara itself. Genus name: Sphenodon (Greek) = "wedge-toothed"; sphen (Greek) = "wedge", odous (Greek) = "tooth". Species name: punctatus (Latin) = "dotted" or "speckled". Other notes: The tuatara is the sole surviving member of the order Rhynchocephalia of which most species lived during the Triassic and Jurassic, and only few species recorded from the Cretacious. The group evolved more than 200 million years ago, and most species are believed to have died out in other parts of the world about 60-70 million years ago, surviving in South America until about 30 million years ago. The tuatara was first described under the name Sphenodon by John Gray in 1831 on the basis of a skull. Initially, he regarded it as a member of the family Agamidae. In 1840, Gray received the first complete skeleton of a tuatara, but did not realize it was the same species he had described nine years earlier, so he named it Hatteria punctata. Again, he regarded it as an agamid lizard. in 1843, the paleontologist Richard Owen re-examined the remains and realised they had similarities to some fossils from South Africa. Although both he and Gray worked at the British Museum, Owen was unaware of Gray's description and named the tuatara Rhynchocephalus. Not until 1867, Gray's successor at the museum, Albert Gunther, realised that Hatteria and Rhynchocephalus were the same species, and not a lizard. Some parts of the anatomy were more like those seen in birds, turtles, or crocodilians, rather than those seen in lizards. The absence of earholes and copulatory organs are unique to tuataras. As a consequence, Gunther named a new order, Rhynchocephalia, for the tuatara and its fossil relatives. Tuataras have a "third eye" on top of the head, which is assumed to regulate ultraviolet light. The correct dosage of UV is critical to their growth and metabolism. The "third eye" is visible only in juveniles. References: 
 Hitchmough 2019
Jewell 2008
Parkinson 2000
Vitt & Caldwell 2014
Winkel & al. 2018
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 Sphenodon punctatus © Rune Midtgaard  Tuatara depicted on a New Zealand 5-cent coin. |
