Tuatara do things a little differently to other reptiles when it comes to sex determination – not because they have temperature-dependent sex determination (thats common to lots of reptiles), but because their pattern of temperature-dependent sex determination (or TSD) is different from most other reptiles. For tuatara, incubating eggs at higher temperatures (over 22°C) produces males, while lower temperatures (under 21°C) produce females. In other reptiles with TSD, you generally either get a pattern of females being produced at high temperatures and males at low temperatures, or females being produced at both high and low temperatures, and males produced at intermediate temperatures.
Dr Nicola Nelson at Victoria University has experimented with switching tuatara eggs between male and female-producing temperatures in an effort to determine which part of the incubation period temperature is critical for sex determination. She found that sex is set early on - by the time the incubation period is about one third of the way though. However, incubating eggs in captivity at constant temperatures only tells part of the story, as of course temperatures are not constant in the wild, where eggs are laid in shallow burrows in the soil. Nelson and colleagues have also collected temperature data from natural nests and found that warmer nests produce males and cooler nests produce females, but what isn’t known is how long eggs have to remain above or below the critical temperature in order to produce males vs females. Its possible that the critical period for sex determination is actually quite short – for example an egg may actually only need to spend a few days above 22°C in order to turn out male.
Having more males produced at warmer temperatures could be bad news for tuatara in the light of global warming. Some tuatara populations, like the small, genetically distinct population on North Brother Island, already have more males than females. A recent study by Nicola Mitchell of the University of Western Australia predicted that, under current “worst-case” global warming scenarios, populations like North Brother Island will produce all-male clutches by the mid 2080s.
Of course, tuatara have survived changes in climate in the past, but this time around the climate is changing faster than ever before – perhaps too fast for a species like tuatara with its long generation times and low levels of genetic variation to be able to evolve to compensate. Tuatara may be able to adapt behaviourally to the higher temperatures by nesting earlier, digging deeper nests, or choosing cooler nest sites. However, on many islands the choice of nest sites is limited, and as tuatara are now confined to offshore islands or ringed in by predator-proof fences on the mainland, they will be unable to simply move south to seek cooler temperatures. It seems likely that tuatara will need our help if they are to survive the threat of global warming.
Mitchell NJ, Kearney MR, Nelson NJ, Porter WP (2008) Predicting the fate of a living fossil: how will global warming affect sex determination and hatching phenology in tuatara? Proceedings of the Royal Society B-Biological Sciences 275: 2185-2193
Huey RB,Janzen FJ (2008) Climate warming and environmental sex determination in tuatara: the Last of the Sphenodontians? Proceedings of the Royal Society B: Biological Sciences 275: 2181-2183
Mitchell NJ, Nelson NJ, Cree A, Pledger S, Keall SN, Daugherty CH (2006) Support for a rare pattern of temperature-dependent sex determination in archaic reptiles: evidence from two species of tuatara (Sphenodon). Frontiers in Zoology 3: 9