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Davis has spent three years tracking Gilas implanted with radio transmitters and miniature temperature recorders. His study site near Picacho Peak State Park, northwest of Tucson, samples a single square mile of Sonoran Desert. It contains at least two dozen Gilas. In an adjoining square mile, another reptile researcher estimates the western diamondback rattlesnake population at 200. That's not counting the tiger rattlers, Mojave rattlers, and sidewinders. A lot of Sonoran wildlife shifts to nighttime activity in summer to cope with the heat, and while tracking Gilas under the stars, we often come upon diamondbacks. Each rests in a motionless coil, scanning the darkness with infrared sensors in pits on its face, until a kangaroo rat or pocket mouse happens by. Scorpions and tarantulas scuttle underfoot as we ease past cactuses furry with long spines. One careless moment, and you jump back with pieces pinned to your hide. Meet teddy bear cholla, possibly the worst-named, least cuddly plant in the world.

One by one, the pools vanish. Shriveled tadpole bodies cover the pebbles like tar. Where a few puddles no bigger than a dinner plate persist near the source of the seep, so do squirming masses of young amphibians, their bodies still growing, backs breaking the surface at times. Although great, luminous clouds billow overhead most afternoons, the showers they yield are spotty. Finally, it does rain on the mountainside. Not much; just enough to swell the pools a fraction. . . .

Even in early May, this desert can draw three gallons of water out through your skin in a day. But if the region is so arid, why are there angelfish swimming with me and curious sea lions swooping by? Half of the Sonoran ecosystem lies within 50 miles of a sea. Salt water surrounds one of the most fascinating portions, which takes in nearly all of the 34 major islands and more than 850 smaller islands and islets in the Gulf of California. Some have labeled this archipelago Galápagos North because so many different species arose from common ancestors as they adapted to island environments, providing a laboratory for the study of evolution.

Steep-sided Isla San Pedro Mártir, a square mile in size, towers among the remote Midriff Islands at the center of the gulf, shimmering ghostly white from guano, blurred by a haze of wings, and moated by cold, upwelling currents where great whales feed on krill and sardines. Large birds, including the world's densest colony of brown boobies and largest colony of blue-footed boobies, rule this strange castle alongside side-blotched lizards, which abound almost a thousand to an acre. The court jesters are striped gnats known as bobitos, or little boobies.

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