You know New Zealand geckos. They are the ones with the big eyes, the soft skin, the sticky toes that let them walk upside down across a window. They are slow, deliberate, almost friendly – the kind of lizard that will sit on your hand and look at you with an expression of mild philosophical curiosity. They are also small. A Duvaucel's gecko – our largest living species – fits in the palm of your hand. But there used to be a giant. A gecko the size of a small rat. A rock lizard that hunted in the dark and owned the crevices. It was the boulder sentinel, and it is gone. What made it special? Size and hunting strategy. A 35-centimetre gecko is not a gecko. It is a small monitor lizard in gecko clothing. It had the same big eyes, the same soft skin, the same sticky toes – but scaled up to terrifying proportions. Its jaws were powerful enough to crush a wētā's exoskeleton or a small skink's skull. Its claws were thick and curved, perfect for gripping rock. It was a predator of the night, and at its size, very little in the leaf litter was safe. What did it do? It hunted. Nocturnally. It emerged from its crevice after dark, patrolling the boulder field, searching for insects, spiders, small skinks, and perhaps the eggs or chicks of ground-nesting birds. It was an ambush predator – waiting motionless on a rock, then striking with a speed that belied its bulk. It also ate fruit – geckos are omnivores – playing a role in seed dispersal on the rocky slopes. Breeding? Geckos are slow. They give birth to live young – usually one or two at a time – rather than laying eggs. The gestation period is long. The young grow slowly. A giant gecko would have taken years to reach maturity, and a female would produce only a handful of offspring in her lifetime. That strategy works when the world is stable and the predators are few. It fails catastrophically when rats arrive. Why did it vanish? Rats. Specifically, the kiore (Pacific rat) brought by Polynesian settlers. Rats are nocturnal. Geckos are nocturnal. They shared the night – and the rats won. Rats eat geckos. They eat them in their crevices, on their basking rocks, wherever they find them. A slow-moving, soft-skinned, night-active lizard is a perfect rat meal. At the same time, habitat disturbance – fire, clearing, the introduction of dogs and pigs – destroyed the boulder fields and scree slopes that the giant gecko called home. Without deep crevices to hide in, without abundant insect prey to eat, the giant could not survive. The smaller geckos held on. They are faster, more cryptic, better at hiding in shallow crevices. Some are still threatened, still teetering on the edge. But the giant rock lizard is extinct. A few bones in a cave, a few fragments in a museum drawer, and the memory of a predator that used to stalk the night rocks. We brought the rats. Then we wondered why the sentinel fell.