Tuatara were once widespread and abundant on the New Zealand mainland, but after humans and predatory mammals arrived, they gradually became restricted to 32 nearshore islands. Many of these islands were tiny – less than 10 hectares, and some as small as only 1 hectare. A few, such as the Poor Knights Islands off the Northland coast, or Stephens Island in Cook Strait, were never invaded by rats, and had few of the other mammals that threaten native animals.
These islands have become refuges for many native species, especially lizards, birds and invertebrates.
The common tuatara (Sphenodon punctatus) occurs on islands off the north-eastern coast of New Zealand, and on some islands in Cook Strait. The Brothers Island tuatara (Sphenodon guntheri) survived only on the tiny, 4-hectare North Brother Island, in Cook Strait. Two new populations of the Brothers Island tuatara have been created on Tītī Island in the Marlborough Sounds and Matiu (Somes Island) in Wellington Harbour.
The two tuatara species look similar, but have genetic differences. Tuatara bones have been found the entire length of New Zealand. Where dated, they are usually a few hundred to 5,000 or more years old. It is not known whether these bones are from the two living species, or others now extinct.
A tuatara in your trousers
One cold, wet night on Stephens Island in Cook Strait, researcher Alison Cree was bitten by a tuatara. It let go after 15 minutes, leaving her with bruised fingers. She felt she got off lightly: ‘My pain, though, seemed small compared with the terror that must have seized a male colleague who had a tuatara run up his trouser leg. Fortunately, the adventurous reptile was subdued before causing any major harm.’ 1
Tuatara can live in remarkably dense populations. Most tuatara islands have 50–100 tuatara per square hectare – so an island of only 10 hectares may have a population of hundreds. Larger islands with many seabirds and invertebrates, which tuatara eat, may have greater densities. The largest population is on Stephens Island (Takapourewa), where there are estimated to be as high as 2,500 per square hectare in some places, and a total of at least 30,000. The total number of tuatara on all the islands is probably between 50,000 and 100,000.