On a knob of rock in the Cook Strait known as North Brother Island, a population of the lizardlike creature called the tuatara is quickly becoming all male. When scientists examined the imbalance in the late 1990s, the sex ratio was already 62.4 percent male, and it has rapidly worsened since then, to more than 70 percent. Researchers say climate change is the cause: ground temperature determines the sex of tuatara embryos, with cooler temperatures favoring females and warmer ones favoring males. When climate pushes the sex ratio to 85 percent male, the North Brother tuataras will slip inescapably into what biologists call the extinction vortex.