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Tuatara Sex Find

Researchers have discovered that soil temperature is crucial in determining the sex of tuatara, which has major implications for conservation efforts. In most animals, sex is determined by the nature of the sex chromosomes obtained from the parents, but not so in the tuatara.

Dr Alison Cree, a zoologist at the University of Otago, says the discovery was made by chance. Juvenile tuatara reared in several captive-breeding institutions were being sexed for studies on nutrition. The young tuatara from one clutch of eggs were found to be all male, whereas juveniles from two other clutches were all female.

Further study with Dr Michael Thompson from the University of Sydney and Dr Charles Daugherty from Victoria University of Wellington revealed that tuatara offspring from eggs incubated at cool temperatures were always female, whereas those from eggs incubated at warm temperatures were almost always male.

Tuatara are found on about 30 offshore islands of New Zealand. Their ancestors first appeared in the fossil record about 220 million years ago, and they are the last survivors of the sphenodontid lineage, which makes their conservation of great importance.

Cree says that the finding is one of the most significant advances in tuatara conservation for decades. Tuatara eggs are often incubated in captivity for captive-breeding programmes, and some of the offspring will form the nucleus of new wild colonies.

"The good news is that egg incubation programmes raising tuatara for reintroduction to the wild have used a variety of incubation temperatures. Sex ratios are likely to be about 70% female, an attractive percentage for ensuring the new populations grow rapidly," she says.

Cree praises the involvement of captive breeding institutions in the research.

"Tuatara take a decade or more to reach sexual maturity. If we'd waited until a significant number reached maturity to obtain these results, captive breeding programmes could have been producing offspring largely of one sex for many years."

The phenomenon, known as temperature-dependent sex determination, has been reported in some other reptiles, including turtles and crocodilians, but until now was not thought to occur in tuatara. Its presence in tuatara increases confidence in the suggestion that it appeared very early in the evolution of reptiles.