Tuatara, Relics of a Lost Age
by Matt Russell, Dallas Zoo
Originally published in The Desert Monitor, the Journal of the Arizona Herpetological Association, Vol.29, No.1.5, Apr-May, 1998.
As reprinted in the Cold Blooded News, Vol.25, No.8, August 1998.
Approximately 220 million years ago during the Mesozoic era, an order of ancient reptiles known as the Rhynchocephalia ("Beak Heads") ranged throughout Asia, Europe, East Africa, Southern Africa, and the Americas. By the end of this period only one member of this unique group still existed. The tuatara (Sphenodon sp.) has been known to science since J. E. Gray described the first specimen in 1831. However, it is known that these reptiles survived nearly 140 million years ago. Tuataras now belong to a monotypic order, the Sphenodontida (Daugherty et.al., 1990). Two species currently occupy this genus; Sphenodon punctatus, and Sphenodon guntheri. One may wonder why these members of such a primitive order of reptiles survived past the Mesozoic Era leaving all of their ancestors behind. Scientists believe that tuataras avoided an earlier demise when New Zealand separated from Gondwanaland approximately 80 million years ago. This separation allowed the tuatara security from mammalian predators that evolved after this division. However, the tuatara faces many parallel battles today.
Tuatara populations on mainland New Zealand perished over a century ago due to land clearing, and hunting by humans, rats, pigs, and cats. As early as the late 1800's scientists were noting the absence of tuataras throughout New Zealand. Today tuataras are believed to only exist on 30 small offshore islands and rock stacks in the Cook Strait and the Bay of Plenty. The islands where tuataras live today are mainly cliff-bound, with boulder beaches. The waters surrounding these areas can be treacherous, making these small localities difficult to access for humans. Within this century alone though, nearly ten island populations of tuataras have become extinct mainly due to the introduction of the Polynesian rat. These pest rodents have proven to prey on the eggs and young of tuataras (Daugherty et. at., 1990). Fortunately, awareness of this issue is strong and several measures have been taken to eliminate rodents and other pest species from tuatara habitat (Towns, 1991).
The tuatara has remained virtually unchanged over the past 140 million years. These living fossils make important study subjects for scientists that are trying to learn how ancient reptiles survived. Several field studies have been performed to better understand the biology of these animals (Cree et. al., 1990, 1992; Guillete et. al., 1990). Adult tuataras have an olive-green, gray, or pink ground color with white or gray spotting throughout the body. The ventrals are cream white. Males have a well-developed crest along the back and tail, while the crest of females is less pronounced. The word "tuatara" is a Maori term which means "peaks on the back" (Halliday et.al., 1986). Like many lizards, the tail of the tuatara is autotomic. Males achieve a length of up to 61 cm (24 in.) total length (TL) and females reach physical maturity at up to 45 cm (18 in.) TL. Weights over 1 kg (2.2 lb.) are rarely seen. Hatchlings are normally brown or gray, with a pale head shield, striped throat, and often with distinct tail markings. Physical growth may not finish until 50-60 years of age, and longevity can be as long as 120 years.
To the unknowing eye, the tuatara appears no different than any medium-sized lizard. However, a number of characters clearly distinguish these reptiles from their saurian relatives. The skull of the tuatara has two arches like the Crocodilia, small projections are found on the posterior of each rib (uncinate process), and the vertebrae are hollow at each end. The jaw teeth are serrations of the jawbone and not separate structures. Male tuataras do not possess a copulatory organ like other reptiles and these animals also lack both an eardrum and a middle ear. The tuatara brain and mode of locomotion resemble that of amphibians and the heart is more primitive than all other reptiles. The eye is specialized with a "duplex retina" that contains two types of visual cells for vision by both day and night. The metabolic rate of tuataras is low, which allows them to endure cool temperatures (Halliday et.al., 1986).
Similar to the lizards, the pineal body of the tuatara's brain is connected to a parietal eye on the top of the head. This feature is covered by the parietal scale in adults, but is visible in young specimens. Early hypotheses stated that this character may warn lizards and tuataras of a predatory threat from above, but this has never been proven true. Other scientists have suggested that this third eye may be sensitive to light, but even this is not believed to be the case for tuataras. Essentially, the parietal eye serves little purpose for the tuatara (Bogert. 1953).
Tuataras inhabit moderately cold areas. Their low metabolic rate allows them to survive in temperature conditions that may reach near freezing during some parts of the year. The majority of the day is normally spent in burrows, but animals will bask on sunny days. At night, tuataras will actively search for food, which includes beetles, spiders, small lizards, seabird chicks, and eggs. Adults have even been known to cannibalize younger tuataras as well. Movement is normally slow. However, tuataras can make a hasty retreat if threatened. When alarmed, males will normally stiffen their crests and gape their mouths. Females tend to be more shy and will seek refuge if in jeopardy. If captured, tuataras will emit loud medium pitched vocalizations for defense. Animals may also attempt to bite.
Reproductive maturity is achieved at around twenty years of age, and an entire cycle may take up to five years to complete. In the field, reproductive behavior occurs in October/November (Spring), and in January (Summer). After breeding, females will dig burrows 10 cm (4 in.) in depth or greater for oviposition. Six to fifteen eggs are deposited per clutch. Eggs are oval (25-3Omm/1-1.2 in.) and are enclosed in a parchment-like covering. Incubation for tuatara eggs lasts 15 months, the longest of any reptile (Halliday et.al., 1986). The embryo develops a thorny egg breaker on the top of its nose. This apparatus is used to slit the egg shell before emerging. The egg breaker will fall off within two weeks after hatching. Hatchlings are around 54mm (2.2 in.) TL. Upon hatching, young tuataras are completely independent. Young specimens are very active in finding food and will set up shelter under logs or in small burrows.
The eradication of introduced rodents and success with captive reproduction and/or captive rearing may be the only hope for some isolated tuatara populations (Goetz et.al. 1994). During the last 1980's, the Dallas Zoological Society (Dallas, Texas) began financially supporting wildlife organizations in New Zealand toward the efforts of rodent elimination. Although the extermination of introduced rodents has proven effective, there was still a need to establish additional captive tuatara populations outside of New Zealand. In 1992, the Dallas Zoo was selected to maintain one of these groups. Due to complications in exhibit construction though, animals that were intended for the Dallas Zoo were placed on loan to the Toledo Zoo until recently. There are currently four North American zoos housing tuataras; St. Louis, San Diego, Dallas, and Toledo. These zoos, with the exception of San Diego, are housing S. punctatus. The San Diego Zoo is working with S. guntheri. Both the Dallas Zoo and Toledo Zoo are exhibiting tuataras for public display, while the remaining institutions are caring for their animals behind the scenes. Tuataras have been maintained with some success in captivity, but reproduction has been sparse. This species has reproduced in New Zealand zoos and at least one university in Australia. However, captive reproduction has not been achieved in a North American zoo. One reason for this is the sexual immaturity of specimens currently housed in these collections. The Dallas Zoo currently maintains 0.0.5 tuataras, S. punctatus. Two animals are maintained in a spacious exhibit terrarium, while the remaining three specimens are kept in clear wood framed Plexiglas cages off display. The exhibit is divided in half to not allow cagemate interaction. Tuataras are fairly solitary throughout most of the year, except for the reproduction season. Therefore, it is unwise to house multiple animals together except for reproductive purposes. Specimens maintained at the Dallas Zoo are not sexually mature, therefore it is not necessary to house animals together.
A dry peat moss substrate is provided with various sized slabs of cork bark for refugia. Hide boxes are available to reserve animals and those specimens housed on exhibit can burrow in the substrate. A variety of tropical plants are also used to furnish the exhibit. Tuataras at the Dallas Zoo currently experience ambient temperatures between 12.8°-15.6°C (55°-60°F). However, there are future plans to vary temperatures to meet natural seasonal thermoclines. The St. Louis Zoo keeps tuataras between 10°-21°C (50°-70°F) depending upon the season (Ettling, 1997, pers. comm.). Captive tuataras at the Dallas Zoo receive a variety of food items. Newborn mice, crickets, mealworms, waxworms, earthworms, and roaches are all accepted. Care must be taken to not overfeed these specimens. It is very difficult for these animals to shed excess body weight with such low metabolism. Waterbowls are provided at each enclosure, and fresh water is added daily.
It is not difficult to understand how tuataras are truly living fossils that are well worth saving. The extreme differences, both morphological and physiological, that separate these animals from the remaining three reptilian orders are truly unique. Also, the knowledge that the Sphenodontida existed before the reign of the mammals is fascinating to any reptile enthusiast. A number of measures have been implemented throughout the world to assure the future existence of the tuatara. Further efforts must be made towards the eradication of pest rodents and other local threats to these animals. Additional progress is also necessary to advance the success of captive reproduction. It would be a shame to think that our era was the one that led to the fall of these ancient reptiles.