Sphenodontia Tuatara

(Sphenodontidae)

Class Reptilia

Order Sphenodontia

Family Sphenodontidae

Number of families 1

Thumbnail description
Neither sex has ear holes, males lack a copulatory organ, both sexes have rear-pointing extensions on ribs, teeth tightly fused to the surface of the jawbone, and a double row of teeth on the upper jaw

Size
Males are about 24 in (60 cm) in length and 2 lb (1 kg) in weight; females are smaller, measuring less than 16 in (40 cm) in length and rarely exceeding 1 lb (454 g) in weight

Number of genera, species
1 genus; 2 species

Habitat
Islands with low-growing, salt-tolerant trees with a relatively open forest floor

Conservation status
Vulnerable: 1 species

Distribution
Islands off the North and South Islands of New Zealand

Evolution and systematics

Tuatara are confusing animals. They certainly confused J. E. Gray of the British Museum, when, in 1831, he described the skull of a tuatara as a species of Agamidae. Then, in 1840, when a whole tuatara came to hand, he placed it in a separate genus from the one assigned to the previously described skull. It was Gray's successor at the British Museum, Albert Günther, who realized, in 1867, that tuatara belonged to an array of fossils in an order of its own, today referred to as Sphenodontia.

The sphenodontians have a fossil record of about 225 million years and, as a group, were most diverse in the late Triassic and Jurassic 180–220 million years ago, when they inhabited Europe, Africa, and North America. Sphenodontians were already in decline during the age of the dinosaurs, and almost all of them became extinct by the early Cretaceous. A single lineage in the family Sphenodontidae survived on a landmass that separated from the southern continent of Gondwana 60–80 million years ago. This lineage gave rise to tuatara, and the Gondwanian fragment now forms the islands of New Zealand. Whether the tuatara have changed significantly from their ancient ancestors is unclear, however. No early sphenodontids have been found in the New Zealand fossil record.

The name tuatara was bestowed by the Maori people when they arrived in New Zealand about 1,000 years ago. In the Maori language, tuatara is both a singular and plural noun. Two living species are recognized: Sphenodon guntheri (Buller, 1877) and Sphenodon punctatus (Gray, 1842). Within S. punctatus there are two distinguishable genetic forms.

Physical characteristics

In general appearance, tuatara resemble some agamid and iguanid lizards, but many morphologic features distinguish tuatara from most or all lizards. For example, tuatara have no ear holes, and the males lack a copulatory organ. Furthermore, tuatara have uncinate processes (rear-pointing extensions) on the ribs and their teeth are acrodont (tightly fused to the surface of the jawbone), and the young have a horny caruncle, or egg-breaker, to assist with hatching. The most

unusual feature, however, is a double row of teeth on the upper jaw into which fit those of the lower jaw.

Tuatara males are distinguishable from females. The males are larger than females and are often twice as heavy. Both sexes have a crest on the midline of the head and back, merging into toothlike projections down the tail. The crest of the males is larger than in females and can be inflated during aggressive and mating behavior. The Maori name, tuatara, refers to these crests on the back.

The largest tuatara are some populations of the northern form, males of which can reach more than 24 in (60 cm) in length and 2 lb (1 kg) in weight. Females are usually less than 16 in (40 cm) in length and rarely exceed 1 lb (454 g) in weight. In the northern form of S. punctatus, size varies broadly with latitude; the largest animals are also the most northern. Nonetheless, differences in mean body size can be found within the same island groups. The cause of this variation is unclear.

Tuatara vary in color from olive green to reddish to gray to almost black. The base color usually is overlaid with irregular darker markings and speckled with paler spots. Compared with S. punctatus, S. guntheri is more often olive green and tends to be more heavily speckled with paler spots. The crest is typically white in both species. Juveniles are often pale gray-brown, with paler V-shaped markings on the upper surface and distinctive darker markings radiating from the eyes. When newly hatched, they sometimes have a large triangular pale patch on top of the head from the snout to the eyes.

Distribution

Subfossil remains of tuatara, not identifiable by species, are present in caves, sand dunes, and Maori midden (rubbish) sites throughout the North and South Islands of New Zealand and on some offshore islands. Only a single wild population of S. guntheri is now known, on North Brother Island in Cook Strait off northern South Island. Populations of the northern form of S. punctatus are confined to about 26 islands off northeastern North Island. The Cook Strait form of tuatara is confined to four islands off northern South Island.

Habitat

Islands inhabited by viable populations of tuatara have four features in common: low-growing, salt-tolerant trees that form a complete canopy over a relatively open forest floor; large invertebrates that include giant flightless orthopterans, carnivorous snails, giant centipedes, and large numbers of flightless tenebrionid beetles; high densities and wide diversities of lizards; and high densities and wide diversities of burrowing seabirds. The seabirds, dominated by petrels, prions, and shearwaters, are regarded as "ecosystem engineers" that soften and aerate the soils, enrich the soils with guano, and provide burrows used by tuatara.

Behavior

In their habitat, the tuatara are active at night. On such islands, tuatara are the top terrestrial predator, reaching densities of up to 810 per acre (2,000 per ha). Encounters between males may be limited to visual displays of animals taking parallel positions in opposite directions, then inflating the lungs and throat, and gaping the mouth and snapping it shut. If one male does not retreat, the two animals resort to combat that includes lunge-and-bite sequences.

Feeding ecology and diet

Tuatara are often sit-and-wait predators but will also actively forage. On average, 75% of the diet consists of invertebrates, especially large, flightless insects, such as orthopterans and tenebrionid beetles. Less than 5% of the diet includes seabirds or lizards. These proportions vary with season.

Reproductive biology

Each tuatara uses a burrow, or series of burrows; on islands densely inhabited by seabirds, these burrows may be less than 3 ft (0.9 m) apart. Males display before combat and

mating. Combat is particularly common in summer and autumn, when males establish territories to include several females. Displays during combat and courtship include stiffening and inflation of the skin and spines down the neck and back.

Mating is in late summer or early autumn every two to five years, depending on form and location. Receptive females are attracted by an exaggerated walk, called "proud walk" or stolzer Gang, during which males with crest erect and throat inflated stiffly and slowly circle females. After about 20 minutes, the females either leave or remain to mate. During mating, a male mounts a female, raising the base of her tail using his hind legs, to align his vent with hers. As in birds, sperm transfer is between the vents of each sex.

The females lay their eggs the spring after mating. Cook Strait populations of S. punctatus lay eight to 15 eggs, whereas the northern populations and S. guntheri lay four to 13 eggs. The eggs are laid in rookeries, where females may gather from distances of at least 200 yards (180 m). The rookeries include unforested areas or gaps in forest canopy exposed to the sun. Each nest is a shallow depression or short burrow up to 20 in (50 cm) deep, with the eggs covered by loose soil. Incubation takes 12–15 months. Temperature in the nest is critical for egg survival and influences the sex of the hatchlings. Laboratory studies show that the highest hatching success is achieved when soil temperatures vary between 64°F and 72°F (18–22°C). Nests at temperatures of about 68°F (20°C) produce a strong female bias among juvenile S. punctatus. Similar studies of S. guntheri found that eggs that hatched at lower temperatures of around 64°F (18°C) produced more females, but those in temperatures that fluctuated between 64°F and 74°F (18–23°C) produced more males. Tuatara take up to 13 years to reach sexual maturity. Unlike the females, males have an annual reproductive cycle. The average life span of adults is at least 60 years.

Conservation status

Currently, S. guntheri is ranked as Vulnerable by the IUCN Red Data Book, whereas S. punctatus has been delisted. The decline of tuatara on the main North and South Islands of New Zealand and on many offshore islands has been attributed to predation by introduced mammals, especially rats. Innovative and aggressive campaigns against introduced mammals by the New Zealand Department of Conservation, combined with research on behavior and reproductive biology by universities and partnerships with zoos in New Zealand and the United States, have led to the beginnings of a resurgence by both species. Within the range of the northern form, introduced rats have been removed from seven islands where tuatara populations were dangerously low or were showing recruitment failure. An additional population has been established on an island within the form's historic range. Attempts are under way to establish two new populations of S. guntheri on islands from which rats have been removed, and additional restoration plans are being formed that will benefit the Cook Strait S. punctatus.

Significance to humans

Tuatara are culturally important to New Zealand indigenous people (Maori), who regard them as living treasures. They are also of particular scientific importance because of their unique biological features. This interest was such in the nineteenth century that tuatara were collected heavily for export to scientific institutions, leading the government to give tuatara full legal protection in 1895.

Resources

Books

Benton, M. Vertebrate Palaeontology. London: Chapman and Hall, 1997.

Gaze, Peter. Tuatara Recovery Plan 2001–2011, Vol. 47. Wellington, New Zealand: Department of Conservation Threatened Species Recovery Plan, 2001.

Newman, Don. Tuatara. Dunedin, New Zealand: John McIndoe and Department of Conservation, 1987.

Robb, Joan. New Zealand Amphibians and Reptiles. Auckland, New Zealand: William Collins, 1986.

Periodicals

Cree, Alison, and Charles Daugherty. "Tuatara Sheds Its Fossil Image." New Scientist (October 20, 1990): 22–26.

Cree, Alison, Michael Thompson, and Charles Daugherty. "Tuatara Sex Determination." Nature 375 (June 15, 1995): 543.

Daugherty, Charles, Alison Cree, J. M. Hay, and Michael B. Thompson. "Neglected Taxonomy and Continuing Extinctions of Tuatara (Sphenodon)." Nature 347 (September 13, 1990): 177–179.

Gillingham, James C., Cristopher Carmichael, and Tracy Miller. "Social Behavior of the Tuatara, Sphenodon punctatus." Herpetological Monographs 9 (1995): 5–16.

Towns, David, Charles H. Daugherty, and Alison Cree. "Raising the Prospects for a Forgotten Fauna: A Review of 10 Years of Conservation Effort for New Zealand Reptiles." Biological Conservation 99, no. 1 (2001): 3–16.

Tyrrell, Claudine, Alison Cree, and David R. Towns. "Variation in Reproduction and Condition of Northern Tuatara (Sphenodon punctatus punctatus) in the Presence and Absence of Kiore." Science for Conservation 153 (August 2000): 1–42.

Organizations

The Tuatara Recovery Group, c/o Department of Conservation. P.O. Box 10 420, Wellington, New Zealand. Phone: 64 (4) 471-0726. Web site: <http://www.doc.govt.nz>

David Robert Towns, PhD