Bulletin

Issue: June 2005

Research in Action

The heat is on for survival of this living fossil

It’s curtains, eventually, for the tuatara if those who model global warming are right and temperatures in New Zealand’s Cook Strait rise 1.5 degrees C in the next century.

Eventually is the key word. The change will happen oh-so-slowly because the large lizard-like reptile lives to about 100, breeds only once in four years and takes its time in every facet of its life - even taking a year to hatch from its egg. Extremely primitive, tuataras were widespread around the world more than 100 million years ago but have survived only in New Zealand, mainly through lack of predators.

Since human habitation of New Zealand about 1,000 years ago, two species - Sphenodon punctatus and Sphenodon guntheri - have survived, mainly on offshore islands.

Scientifically predicting the effect of temperature increase on future tuatara breeding rates is a major ongoing part of the research of La Trobe University Post-doctoral Fellow, Dr Nicola Mitchell, who now works in the Department of Zoology.

Formerly a Royal Society fellow working with Professor Charles Daugherty and colleagues at Victoria University of Wellington, Dr Mitchell explains that, like Madagascar's coelacanth fish and eastern America's horseshoe crab, tuatara are one of the few 'living fossils' - creatures which have survived more or less unchanged from the time of the dinosaurs.

They share another characteristic of those other survivors from prehistoric times - crocodiles and turtles. Females bury their eggs and the temperature of the earth in which they are buried determines the sex of the offspring, which dig themselves out upon hatching.

For reasons not yet determined, warmer temperatures generally produce female crocodiles and turtles but conversely, produce male tuataras. That fact raises the question about the future of the tuatara given predictions of global warming.

During her research, Dr Mitchell discovered the exact pattern of 'temperature-dependent sex determination' that enabled modelling to take place to predict the fate of the tuatara in the face of global warming.

Working for more than two years with a population of the smaller and rarer Sphenodon guntheri species on North Brother Island in Cook Strait, she and her colleagues discovered that when females buried their eggs in soil at a temperature of 21.5 degrees C, the ratio of male to female offspring was even. But below 21 degrees C, all offspring were females and above 22 degrees C, all were males.

Arriving at this figure was the result of both delicate science and long and painstaking physical endurance. It involved Dr Mitchell and teams of volunteers spending 100 nights on North Brother Island - a four hectare rock stack dominated by a 100-year-old lighthouse - collecting eggs from the 350 tuatara residents, of which only 10 to 20 females were breeding in any one year.

She dug out nests which contain between three and eight eggs, and the eggs were taken by helicopter to the laboratory in Wellington and incubated at four constant temperatures, 18, 21, 22 and 23 degrees C.

There it took up to nine months for the soft-shelled eggs to hatch, and the babies' eventual arrival presented another problem. It is impossible to tell males from females externally until they are around 15 years of age - but Dr Mitchell did not have that much time.

So colleague Dr Nicola Nelson used a laparoscope to view the internal gonads that showed which were males and which were females.

Using the 21.5 degree C threshold and predictions of global warming in Cook Strait, Dr Mitchell calculates that the population will be safe if the proportion of males produced at hatching remains below 70 per cent.

'But if it goes above 85 per cent - and that might not happen for several hundred years - then the tuatara would face eventual extinction on North Brother Island if no other factors come into play.'

However, Dr Mitchell says that there may well be other factors which have already caused discrepancies in the ratio of males to females in the tuatara population.

'For example on North Brother Island, previous research had shown that at present 60 per cent of the population are males. This may be because females choose the warm northern face of the island to breed or that the construction of the lighthouse and keeper's houses may have created more open areas for nesting.

'All is not gloom and doom. In future females may instinctively breed on cooler parts of the island and there is always the possibility of deliberate human intervention. Some North Brother Island tuataras have already been transferred to other islands where cooler temperatures might ensure the survival of the species.'