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Rising with the calls of doves and quail before dawn, he reaches in to grab the animal and scan her with a portable ultrasound unit. A wake-up cup of coffee first is a good idea; the Gila and its closest kin, the Mexican beaded lizard, are both venomous, and their bites can cause severe pain, faintness, or worse. An image of the animal's bladder appears on the screen. This organ has banked a fortune in water, the currency of the Sonoran Desert ecosystem. "Gilas can drink enough in one day to boost their body weight 20 percent," he tells me. "Just like that, they'll go from looking awful to looking great. This is the kind of year that gets everybody through."

Spanning 100,000 square miles, the Sonoran Desert encompasses southeasternmost California, southwestern Arizona, at least half the Mexican state of Sonora, and nearly all of the Baja California peninsula. Although some sections average barely 3 inches of rain yearly, others see 10 to 12, more than parts of Wyoming. Winter storm systems from the Pacific typically supply about half the annual total, but the winter of 2004-05 brought record deluges, breaking the grip of a several-year drought. By spring, the region looked and smelled like a florists' convention. Even plants no one had seen for decades erupted from long-patient seeds. Then summer came. The thermometer hit 100°F or more for 39 straight days in Tucson. Phoenix recorded afternoons of 115° and more than two dozen heat-related deaths. Heatstroke and dehydration were killing at least that many illegal immigrants from Mexico monthly as they trekked north through remote Arizona borderlands.

Lowest and warmest of North America's four major deserts (the others being the Mojave, Great Basin, and Chihuahuan), the Sonoran is also the only one with two distinct rainy seasons. That tremendous summer heat rising off the countryside draws moist air from the Gulf of California and occasionally the Gulf of Mexico. Thunderclouds take over the blazing sky many afternoons, delivering intense downpours, complete with flash floods. The monsoons, as locals call the hot season storms, were late this year. But they have arrived, slaking thirsts and spurring another round of plant growth. Which means more births among rabbits, rodents, and birds, whose young are all favorite meals of the Gila monster. It may eat enough at one nest to increase its weight as much as 50 percent, then retire to its burrow for a week or more. A hormone recently discovered in the monsters' venomous saliva may help regulate this on-and-off pattern of activity. In 2005, a synthetic version of the chemical won approval as a drug that has proved very effective in controlling type 2 diabetes. It could aid patients in losing weight at the same time.

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