Cold works in a more merciful way. When a man finds himself caught outdoors, exposed to very cold weather, his body urges him to slow down, to hibernate. He falls asleep in the snow and slips away.
Heat drives a man forward. It challenges him and awakens his mind. It makes him stagger forward, instead of falling asleep.
"People tend to fight for it, in heat," said David Pascoe, a professor at Auburn University and an authority on how the body reacts to temperature change. "Your body wants to unload that heat, which means increasing circulation. Which means movement."
When Evan Tanner woke up that morning he was probably already dehydrated. During the previous day's long motorcycle ride from the coast, the wind had secretly siphoned off a great deal of his fluid. When the sun came up and the temperature began to rise, a little cone-shaped part of his brain, the hypothalamus, sensed the change and sent word to his heart. The heart beat a little faster to move blood from his core to his skin, where the heat could dissipate. His skin was cooler because, in the meantime, the hypothalamus had directed the sweat glands to excrete water. Water that came from his blood.
It's an ingenious system: Blood circulates both heat and water. But the system is expensive. It costs fluid. Without incoming water the blood becomes more concentrated, which means there's less volume, which means the heart has to pump faster to move ever-thicker blood.
Pascoe conducts experiments with people's temperatures, dialing up the heat in a controlled way to see what happens. In one experiment, for example, he planned to raise his subjects' temperatures two degrees centigrade.
"The first degree was easy," Pascoe said. "We had them doing a sort of walking workload, no problem. Then in the next half-degree, suddenly people weren't so jovial. Not so happy. Then came the last half-degree."
People got aggressive, he said. They grew profane. They snapped, "Shut up! Leave me alone!"
At higher temperatures, he said, people lose their mental acuity. "I've seen people mumble about things with no idea what they were saying," he said. "It's bizarre behavior."
Dizziness sets in. Then disorientation.