The Tuatara’s tale

Everyone has heard of the tuatara. It’s as famous as the Coelocanth for being a living fossil (an oxymoron if ever there was one). The tuatara is the sole surviving member of an entire order of reptiles that emerged in the Triassic. It’s a cute little thing that is definitely out of its depth in the unprecedented conditions of the Anthropocene. And, as are many cute and rare things, it’s doomed because of climate change.

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

Nature News

This story was widely covered in 2008, and originated with a study by Nicola Mitchell and colleagues in the formerly prestigious Proceedings of the Royal Society B series: “Predicting the fate of a living fossil: how will global warming affect sex determination and hatching phenology in tuatara?”

According to Mitchell et al, rising temperatures will eventually ensure that all tuatara hatch as males, which will presumably become increasingly frustrated and lonely, before the species finally dies out altogether. This might take some time: there’s a tuatara called Henry who holds court at Southland Museum, Invercargill, and must be 120 by now.^

The notion that tuatara might be doomed passes an elementary sanity check; here’s how the story goes. We all know that reptiles have temperature-dependent sex ratios. For tuatara, this means that when eggs are incubated in warmer-than-normal conditions, males predominate in the offspring. An entirely neutral, and factual, observation, unburdened by any climate-related portents of impending doom. But now let an enterprising researcher ask an innocent question: what happens to the sex ratio in tuatara under the projected conditions of anthropogenic climate change? We already know the answer: it will become male-biased. We intuitively grasp the notion that if we keep pushing the temperature higher, sooner or later there will only be male offspring. Ergo, the species is doomed because of impending climate change.

The tuatara species in question was down to a few hundred individuals on a small islet called North Brother Island. Small in this case really does mean small – about the area of a football stadium (it’s bigger than a footy pitch, but if you tried to play footy on it, you’d rapidly lose your ball off the edge of a cliff into the Cook Strait).

Let me just rewind a bit here and ask what we as a species would be worried about if we were down to 350 individuals stuck on an island the size of Wembley Stadium: would it be global warming, or maybe something else, like why are we excluded from all the rest of the land habitat?

Why might we have been driven to the precipice of extinction on a tiny rocky island? Did carbon dioxide have a role in it? Here we have a rather spooky echo of the Puffin’s Tale. Our old friend the rat is largely to blame. As with the chicks of puffins, the eggs of tuatara are rat fast food. Where there are rats, if there are tuatara, no juveniles are seen. Humans liked to eat them as well, once upon a time, another small reminder that poor people will eat anything out of necessity, while the wealthy will pop down to Waitrose and elbow one another out of the way for a free coffee. Anyway thanks to humans and their friends the rats, tuatara were soon driven off the large islands of New Zealand, persisting only here and there, and thriving only in rare places were there were no rats.

At this point I must ask the reader to consider what measures might be taken in the following circumstances: a species restricted to a single islet thanks to rat predation of eggs; a temperature-dependent sex ratio; a warming climate, potentially warming enough to swing that sex ratio to infinity males per female.

“Let’s find another island,” the reader might say, “further south, in a colder direction; let’s eliminate all the rats there, then translocate some tuatara – ooh, and let’s make it a tad larger then 4 hectares.”

Now the title of the Nature News page quoted above is “Condemned to single-sex life by climate change.” Of course unless you are a species that can reproduce parthenogenetically, or one whose individuals are immortal, then “single-sex life” means you have bought a ticket on the bus to extinctionville. Driven out of its entire range, bar a tiny nugget, by humans and rats, clinging on by its claw tips there, the extinction of the tuatara, when it occurs, will be blamed instead on human civilisation itself, via its exhalations of carbon dioxide.

A surprising taxonomic intervention

Taxonomists can be lumpers or splitters. Lumpers combine multiple species into one when they judge that the differences between them do not amount to much. Splitters take an individual species and divide it into two or more. Originally this might have been based on minor morphological features but of course it is increasingly done via DNA sequencing. A long time ago the tuatara was considered to be several species of the genus Sphenodon. Then they were all lumped together as Sphenodon punctatus. Then in 1990 S. punctatus was split once more into S. punctatus (sensu stricto) and S. guntheri. The last surviving population of S. guntheri is that on North Brother Island, hence the real fear of its imminent extinction in 2008.

However, in 2010 Hay et al did some genetic jiggery-pokery, and came to the conclusion that: “Without conducting formal species descriptions here, it now seems most appropriate to consider tuatara as a single species, S. punctatus, that contains distinctive and important geographic variation.”

So in 2010, by the stroke of a pen, S. guntheri was made extinct, or ceased to be, or rather it never was. Should the North Brother Island population of tuatara go extinct, a species will not be lost (although a significant part of the genetic diversity of a species will be).

Since the tuatara is one species instead of two, the North Brother Island population is now part of a population of c. 40,000 instead of 400. That still does not sound like very many even though it is a hundred times larger. And where do the rest live? Well, it might be surprising to discover that there are numerous small populations on islands 4 degrees further north than North Brother Island (i.e. in a warmer direction). The Hen and Chickens Islands are one such island group. In the 1990s, rats were eliminated on Whatupuke (1993), Lady Alice (1994) and Coppermine (1997) (the crossing to the first mentioned is rather rough for western stomachs*). And as you might expect, once the rats were gone, juveniles were seen again.

Finally a recent paper in Oryx by Price et al reported on tuatara translocated to sites 2-4°C warmer than the source site (Stephens Island, near North Brother Island). Their conclusion:

“The fact that several tuatara populations have so far demonstrated high survival rates, growth, and some evidence of reproduction at sites that can exceed mean temperatures on Stephens Island by 2–4°C suggests that these warmer climates have not negatively affected the survival of translocated individuals, that possible local adaptations to the Stephens Island climate have not impeded their ability to establish at new sites, and that tuatara may be capable of tolerating the warmer air temperatures predicted for the 2100s.”

So although it’s early days for a naïve species that has been around for >200 million years, the signs so far are good.


Mitchell et al 2008

The fateful Nature News report, 2008

Hay et al 2010

Price et al 2020

^ Henry the tuatara is still alive, unless the website needs updating. Mind you he is not yet listed as 120 years old despite having been 110 years old a decade ago: he’s “over 110 years old.”

Southland Museum’s breeding program has been so successful that they have supplied  the other New Zealand zoos with sufficient tuatara for their needs and populated an outdoor enclosure to supply individuals for recolonisation efforts elsewhere.

* With apologies to our New Zealand readers.

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via Climate Scepticism

April 15, 2021 at 12:23PM

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