Species of the day one hundred and six: Brothers Island Tuatara (Sphenodon guntheri)
Species: Sphonodon guntheri
Description: The Brothers Island Tuatara is one of the oldest animals in the world today. It may look like a lizard but it belongs to the order Sphenodontia, which includes ancient reptiles that existed 200 million years ago. All other species in this order, apart from the tuataras, declined and eventually became extinct about 60 million years ago. Tuataras are therefore of huge interest to biologists as they represent the only living link to these ancient reptiles. The Brothers Island Tuatara is one of two species of tuatara, the other being the more common Sphenodon punctatus species, which is found on the Northern Islands. The Brothers Island Tuatara has a lizard-like body, and a long tail, stocky legs, long claws and a large head. A crest of spines runs along its back, neck and head: a characteristic which led to its Maori name, meaning ‘peaks on the back,’. Males are larger than females, with larger spines, though they look similar with olive green, grey, or dark pink body colouration, and speckles of grey, white or yellow. Newly hatched young are brown or grey, with pink tinges and a striped throat. This reptile has the unusual feature of a third pineal eye: This eye has a retina, a rudimentary lens and is connected to the brain by a nerve. While it is apparent in infants, it becomes covered by opaque scales in adults, so it is unknown whether the eye serves any function.
Habitat and Distribution: The Brothers Island Tuatara inhabits low forest, scrub areas and rock stacks on Brothers Island, and is found between zero and three hundred meters above sea level, where the climate is cool. This rare species is only found on Brothers Island, off the coast of New Zealand. It has survived there for 200 million years because there are no natural predators. A recent survey estimated that only 400 individuals exist here.
Biology and Ecology: The Brothers Island Tuatara is terrestrial and primarily nocturnal, though, as an ectotherm, it spends part of its day basking in the sun outside its burrow to warm up. It does not drink water, and feeds at night on insects, worms, snails, birds eggs, chicks and occasionally even its own young. They often take over and live in bird burrows, though tuatara can and frequently do construct their own burrows. Several may use the same burrow, although at different times, and residents can be quite aggressive to intruders. Females only reproduce once every two to five years, and males compete for the right to mate, with territorial displays, aggressive fights and erected crests to make them appear larger than they are. Mating occurs between January and March and eggs are laid from October to December. About 8 to 15 eggs are deposited in small, specially constructed chambers, covered with soil and abandoned. They hatch after 12 to 15 months, which is the longest hatching time for any reptile. The sex of the offspring is determined by the temperature: warm soil temperatures produce males, while cool soil temperatures produce females. Individuals reach maturity between 9 and 13 years of age, which may seem late, but these fascinating reptiles are believed to live for over 100 years. Their long life is the product of having an extremely low metabolic rate, and a slow growth rate, which is due to their tolerance to extremely cool weather. Indeed the activity levels of the tuatara peak at body temperatures of 12 to 17 degrees Celsius, the lowest for any reptile.
Status and Threats: The Brothers Island Tuatara is classified as Vulnerable under the IUCN Redlist and listed on Appendix I of CITES. Evidence suggests that the tuatara cannot persist in areas where rats are present due to competition with these fast breeding rodents. They do not occur on Brothers Island but there is concern that Polynesian rats, Rattus exulans, which occur on other islands, may spread by boats and on driftwood to Brothers Island. Tuatara are also predated on by introduced animals such as dogs and cats. Furthermore, scientists warn that climate change could have significant impact on this species as the eggs are sensitive to small changes in temperature that could alter the sex ratio and unbalance the reproductive success of a population. Tuatara have, however, survived 200 million years, so may have mechanisms to cope with climate change, though it is feared that the climate change of the future may occur at a faster rate than tuatara can adapt, physiologically or behaviourally.