The desert tortoise has a simple solution for coping with Death Valley's extreme heat: It avoids it. The slow-moving creature hibernates during the winter and stays in its tunnel for much of the summer, meaning that it spends more than 90 percent of its life immobile. In fact, the tortoise usually only surfaces after a good rain. Then, it gets to work. The tortoise stocks up on water by eating plants and digging holes to collect rain. But to stay supplied with water through its extended hibernation, the reptile relies on something else ─ its highly sophisticated bladder. Unlike most animals, the tortoise's bladder acts as a holding tank, allowing it to reabsorb water back into its body. Incredibly, a desert tortoise can go a full year without taking in any freshwater at all. And because its bladder is so important to a tortoise's survival, park rangers often remind visitors not to stop and help the slow-movers across the road. Tortoises become so terrified when people pick them up that they empty their bladders, losing their precious water reserves.