Tuatara living on an island in Cook Strait may be doomed to a lifetime as bachelors thanks to climate change.
Tuatara eggs hatch out as male or female depending on the temperature. The hatchlings come out as females if it's cold and males if it's warm.
The problem is that the climate has warmed in recent times thanks to greenhouse gas emissions, leading to 70 percent of tuatara hatchlings on North Brother Island in the Cook Strait being born as males - and the ratio is worsening.
Researchers at Victoria University have been monitoring the roughly 500 tuatara living on North Brother Island for 25 years. Nicky Nelson, from the university's School of Biological Sciences, said population modelling showed there would eventually be no female tuatara left on the island, and the population would die out.
However the species as a whole is not facing extinction yet, with tuatara on other islands maintaining a 50/50 male to female split.
Dr Nelson said tuatara on North Brother Island appeared to be particularly vulnerable because they nested on the sunny, north-facing slopes of the island.
Tuatara on other islands could lay their eggs in shady areas to keep them cooler and ensure more females were born, she said.
Dr Nelson said all was not lost, as tuatara and other species whose gender was determined by temperature, such as turtles and crocodiles, had survived major changes in climate in the past.
Tuatara are the only surviving members of an ancient class of reptile known as sphenodons.