North Brother Island lies at 41° south and some 35 km north-west of Wellington, broadside to the full wrath of the Cook Strait seas and the glare of the northern sun. The strait is famously quick to anger, gathering up gales and hurling them at the steep cliffs of this sparse 4 ha outcrop.

As tuatara habitat, it’s what an ecologist might call “suboptimal”. Salt-scorched shrubs give up somewhere around waist-height, their roots gripping for dear life in shallow, spent soils against the rampaging northerlies. All the more surprising, then, to find tuatara here. North Brother’s shallow soils have always made life difficult for the burrow-nesting “living fossil”, but in a warming world, they might just finish it off altogether.

As the mercury passes 21.7°C, something fascinating happens inside a tuatara egg; the developing embryo stops becoming a female and pursues masculinity instead. Under artificial incubation at 22°C, says Nicky Nelson, a tuatara researcher at Wellington’s Victoria University, “we got 100 per cent males. At 21°C, we got three males out of 80 eggs.” Between lies the pivotal equipoise of 50/50 sex ratio, and the future of the tuatara.

But on North Brother, that balance has been lost. Sixty per cent of the island’s 350 tuatara are male, and researchers believe that under an ever-warmer sun, every egg on the island will hatch a male by the mid-2080s. That’s because the animals can’t dig any deeper to find cooler soil temperatures; the island bedrock lies just five to 25 cm down. Nor can they seek out forest shade. Only the island’s south-facing cliff-tops, where few tuatara venture, offer the cooler temperatures necessary to restore gender balance.

More males, says Nelson, might even drive the extinction vortex faster, as competition between them for food and mates becomes more pitched. Studies on lizards have shown such aggression harms females as well, and breeding success falls. Nelson says there are signs that may already be happening on North Brother, where females are only nesting once every seven to nine years. On the western horizon, on Takapourewa, or Stevens Island, females are laying eggs every four years on average.

Tuatara have been through hard times before. Their ancestors walked the ancient supercontinent of Pangaea 200 million years ago. They watched the dinosaurs rise, and survived their cosmic oblivion. They endured ice ages and heat waves, saw mountains come and go. But now, as little as a 1.5°C temperature rise could tip them into extinction on this, the most southerly outpost of their relict range.

That’s because warming is happening so rapidly, says Nelson, who points out that tuatara can live for more than a century. “So these temperature changes are happening within the lifespan of a single generation. That means that if they are going to deal with it, it has to be behaviourally, not genetically. We’re not talking adaptation; we’re talking about the ability of individuals to survive.”

Once, tuatara were widespread throughout the country, but rats and fires and axes drove them to the lifeboats of offshore islands, where they are now effectively marooned. In times past, when the climate changed, the creatures simply moved. But now, Nelson says, “They’re restricted to islands, which means they don’t have the options they once did. They can’t change their behaviour; they can’t go uphill, they can’t move further south to cooler temperatures.”