Published online 1 July 2008 | Nature | doi:10.1038/news.2008.927

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Condemned to single-sex life by climate change

All tuatara could be born male — and thus doomed to extinction — within decades.

tuataraMale delivery? Within 80 years there might be no female tuataras born at all.N. Mitchell et al.

Rising temperatures look set to produce male-only offspring in the tuatara, condemning the ancient reptile species to extinction by 2085, computer modelling predicts.

Researchers studying tuatara (Sphenodon spp.) — the ancient relatives of which once roamed the world alongside dinosaurs more than 200 million years ago — made their doomsday prediction using digital terrain maps detailing the consequences for the reptiles' nesting sites of a 4°C hike in average temperature.

The entire tuatara population is now effectively trapped on about 30 small islands in New Zealand's north, having been wiped out elsewhere by predators. They therefore have no chance of adapting by fleeing to cooler climes, the researchers say. The study is reported in Proceedings of the Royal Society B1.

"Since the mid 1990s, people have been talking about the vulnerability of reptiles to climate change because they have temperature-dependent sex determination. But no one has been able to model it in this type of complexity before," says research leader Nicola Mitchell of the University of Western Australia in Perth.

Temperature threshold

"By integrating the climate and the physiology of the animal we are able to make spatially explicit predictions about a particular spot on the island," says Mitchell. "We can say if this is the climate, this is what will happen at this particular location."

With the aid of computer software, the researchers combined the physics of heat transfer with terrain data for the four-hectare North Brother Island in New Zealand's Cook Strait.

Using the exact coordinates of 52 known nesting sites of the rarest species of tuatara (Sphenodon guntheri), along with a database of current soil properties and their constant temperature equivalent, the researchers simulated climate change and then monitored its effect on specific points across the island.

They found that based on maximum warming predictions, tuatara, which hatch as males when nest temperature during development moves above 21.5°C, will be incapable of producing females.

This type of software may to used to look into the future of other animals, such as marine turtles, whose offspring's sex are also determined by the surrounding temperature, says Mitchell.

Consistent with reality

While some argue that this type of model can make unnecessarily sweeping assumptions, Mitchell's colleague Michael Kearney at the University of Melbourne in Australia says that the data were road-tested to predict the soil temperature and sex ratios of natural nests where the researchers already knew the results.

"We asked the model what the ultimate sex ratio would be on nests where we knew the observed temperature fluctuations and it was very consistent with what we had observed," says Kearney.

"In the present study, one validation test suggests that the model underestimates the negative influence of warming on sex ratio," says Raymond Huey, a biologist at the University of Washington in Seattle.

Taking cover

Often described as 'living fossils', the tuatara is a cold-climate reptile that can reach nearly a metre in length.

Although during the past 200 million years tuatara have survived ice ages and previous bouts of global warming, escape routes to colder climes were available to them before.

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"Relative to the past, tuatara now have few places to hide, if anything their genetic inertia is now elevated. Moreover, they face a rate of temperature change that is unprecedented over the last 50 million years," Huey says.

So what hope is left for the tuatara? Its greatest chance for survival could literally come from us putting a cover over it, says Mitchell.

"We can put shade cloth over their nesting sites to effectively change their sex ratio back to a 1-to-1 ratio even if the planet warms — or start translocating them to other places that would be more suitable, " Mitchell adds. "Translocations are already occurring to the mainland and we now have a tool to identify which locations would produce favourable sex ratios." 

  • References

    1. Mitchell, N. J., Kearney, M. R., Nelson, N. J. & Porter, W. P. Proc. R. Soc. B doi:10.1098/rspb.2008.0438 (2008).
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