What is a Tuatara?
Tuatara (Sphenodon punctatus) are medium-sized reptiles found only in New Zealand. Adult tuatara weigh from about 300g to 1kg, and can reach up to half a meter in length. Although they look superficially like lizards, they are not lizards, but are the only surviving members of the reptilian order Rhynchocephalia. The closest relatives of Rhychocephalids are the squamates (lizards and snakes), but the two groups diverged early in reptilian evolution, around 250 million years ago. Many different species of Rhynchocephalid were found across Europe, Africa and the Americas during the dinosaur era, 230-65 million years ago. However, they went extinct everywhere except New Zealand around 65 million years ago, and natural populations are now found only on offshore islands around New Zealand.
Where Does the Word “tuatara” Come From?
Tuatara is a Maori word, meaning “spiny-back”. Maori is the indigenous language of New Zealand. In Maori, nouns do not have a plural form, so the plural of tuatara is simply “tuatara”.
How Many Species of Tuatara are There?
Two species of tuatara are formally recognized: Sphenodon guntheri, found only on North Brother Island in Cook Strait, between the North and South Islands of New Zealand; and Sphenodon punctatus, found on islands in Cook Strait and off the north-eastern coast of New Zealand. However, genetic studies published in 2009 by researchers at Victoria University of Wellington and Massey University, New Zealand, found that the population on North Brother Island is not as genetically distinct as first thought, and suggests that only a single species of tuatara (Sphenodon punctatus) should be recognised.
Tuatara Distribution and Habitat
Tuatara live in burrows in forested areas of offshore islands of New Zealand. Fossil remains indicate that tuatara were once widespread across the New Zealand mainland, but natural populations are now confined to 31 different offshore islands, located in Cook Strait and off the Northland, Coromandel and Bay of Plenty regions of the North Island. The largest population, on Stephens Island in Cook Strait, has 30,000 – 50,000 individuals. However, all other populations are considerably smaller, with population sizes ranging from less than ten to a few thousand individuals.
Tuatara habitat comprises coastal forest or scrub with a relatively open understorey. On many islands, tuatara share their habitat (and sometimes even their burrows) with nesting seabirds such as fairy prions and petrels.
Tuatara Diet and Lifestyle
Tuatara primarily feed on invertebrates such as weta (a native cricket-like insect), beetles, spiders and millipedes. Their diet also includes lizards, seabird eggs and chicks and sometimes even their own young.
Tuatara are most active at night, but also come out to bask in the sun during the day. Like all reptiles, tuatara are ectothermic, meaning that their body temperature fluctuates according to the environmental temperature, rather than being maintained internally. Tuatara prefer surprisingly low temperatures compared with other reptiles, with an optimal body temperature of 16 to 21°C, but remaining active at temperatures as low as 5°C.
Tuatara are also extremely long-lived. The oldest known tuatara is at least 88 years old, but individuals may live to over 100.
Reproductive Biology of Tuatara
The age at which tuatara become reproductively active is determined largely by their size. Males are sexually mature once they reach 180mm in length (measured from their snout to the base of their tail), while females breed at around 170mm in length, which is reached at around 10 to 15 years of age. Tuatara are sexually dimorphic, with males being larger than females. Their mating system is characterized by intense competition between males and only about 25% of males are successful in mating. During the mating season in January to March, males hold a territory in which they court receptive females using a ritualized display known as a stolzer Gang, or “proud walk”, where the male slowly encircles the female with an exaggerated walk with his head up and crest erect. Male mating success is largely determined by their body size as large males are more able to successfully defend their territory against intruders.
In the wild, females breed only once every 2 to 4 years and the egg incubation period is long. Eggs are laid in spring (October to December), about 8 to10 months after mating, and hatch 11 to 16 months later. Females lay their eggs in small excavated burrows in open areas away from their home burrows. Although females may guard their eggs for a short period of time after laying, they soon return to their home burrows and there is no further parental care.
The sex of the hatchlings is determined by the temperature at which the egg is incubated. Warmer temperatures produce males, and cooler temperatures produce females. In the laboratory the difference in temperature required to produce males vs females is very small: at 21°C all of the hatchlings are female, while at 22°C all are male. Although temperature-dependent sex determination is common in reptiles, production of males rather than females at high temperatures is unusual. This mechanism of sex determination may also put tuatara at risk from global warming, as some tuatara populations already have skewed sex ratios with more males than females, and this situation will worsen with warmer temperatures.
Tuatara were once widespread across New Zealand but their numbers began to decline with the arrival of humans about 750 years ago, primarily due to the introduction of rats. They were functionally extinct on mainland New Zealand by about 200 years ago, and their present-day distribution on offshore islands comprises only a tiny fraction of their former range. Habitat destruction and mammalian predators have continued to be a threat to tuatara over the last 200 years and many remaining populations are small and primarily composed of old adults.
Tuatara are now fully protected by law in New Zealand and are the subject of intense and ongoing conservation efforts, which have halted the decline of the species. Rats have been removed from virtually all offshore islands with tuatara populations, and new populations have been established, including the first breeding population on the New Zealand mainland in over 200 years at Zelandia Wildlife Sanctuary in Wellington. Current tuatara conservation efforts are focused on the continuing survival of existing populations in order to maintain genetic diversity across the species, and the establishment of new self-sustaining wild populations throughout their pre-human range.