Heat (3 page)

Another strategy, useful for plants out in the open, away from easy water: grow quickly after rain, shed seeds, and wait for the next rain. Or leaf out following rain, then shed leaves after exhausting the rainwater to stand dormant with bare stems, like the thorny ocotillo,
Fouquieria splendens,
which, live and wet, stands as an upright green tassel but, live and dry, loses its leaves and turns brown and thorny and looks dead. Another strategy: curl leaves during the dry season and uncurl them during the wet. Or develop leaves with thick cuticles that hold water like waxed bags. Or alter the leaf pores, the stomata, by hiding them in pits or by growing fringing hairs around them, in both cases protecting them from dry winds. Or reduce the size of leaves, leaving less surface area through which to bleed moisture. Or, like the cactus, convert leaves to spines and relegate photosynthesis to the stem.

Some adaptations cannot be seen. Water is always needed to convert light and carbon dioxide to carbohydrates, the energy molecules of life. But how the light and carbon dioxide and water interact varies. Most plants—even desert plants like the creosote bush—follow a common path, a path that demands a constant supply of water, a path that demands open pores throughout the day to supply carbon dioxide. Without water, plant pores close, and photosynthesis stops. With water, a three-carbon carbohydrate forms.

Some plants—corn and saltbush—vary this theme, leaving their pores open during the day but making more efficient use of water, demanding high light and only photosynthesizing in inner cells, forming a four-carbon carbohydrate. And a few plants—succulents and cacti and some orchids—open their pores only at night, when the air cools. In the cool darkness, they pick up carbon dioxide through the pores and convert it to an acid. During the day, the pores close, but the reaction used to convert carbon dioxide to an acid reverses, releasing the carbon dioxide within the plant, allowing it to interact with water and light to form carbohydrates.

William McGee died in 1912, but if he were around today, I would write a fan letter, thanking him for “Desert Thirst as Disease,” his story of Pablo Valencia. An article like his, I would tell him, changes the way one sees the world. It opens one’s eyes. But, I would tell him, desert plants are not water-
​storing
monstrosities. They are not any one thing at all. They are many plants, with many tricks, thriving in conditions that at first glance appear insufferable and deadly.

A half billion years ago, a sea washed through this part of Nevada and California. It was a time before dinosaurs, before warm blood, a time when backbones debuted in jawless fish, a time when plants were struggling for a foothold on land. It was a time of a distinctly different geography.

A few hundred million years pass, and a desert replaces the sea, a desert hotter and drier than the deserts we know today. The wind lifts red sand to form Sahara-like dunes. Time passes, and the climate changes again. Water moves through, calcifying the dunes, forming sandstone. Two hundred million years come and go. Four thousand years ago, the Gypsum People show up, inhabiting this place at a wetter time, a cooler time. Another two thousand years pass, and the Basketmaker People arrive, sandal wearers, contemporaries of Iron Age villagers in western Europe, contemporaries of Christ, contemporaries of Roman emperors. Flash forward again to nine hundred years ago and the Southern Paiutes show up, nomadic people moving with the changing seasons of increasingly dry country, builders of roasting pits. Flash forward to now, to today. The climate change experts say that these deserts will grow warmer and drier. Marginal habitat will become less habitable.

In our wanderings, my companion and I see rock art. Most of it is graffiti of obscure meaning, straight lines and squiggly curves and crescents, but also stick figures of people and animals and even rounded-out figures of animals that could be fat goats or bighorn sheep. Some of the rock art may have been left by Basketmakers. Some is younger, the work of Southern Paiute. It is light on dark, made by scraping away the blackened surface of the rock, leaving behind an image like that of a black-and-white negative, but without any sense of three dimensions. If the taggers who drew here meant to leave a message, that message remains encoded, unexplained, maybe nothing more than a record of their passing, homage to the animals and the earth and the plants and the sky.

An antelope squirrel darts across the rocks, frantic sudden movement in a timeworn landscape. It scales a nearly vertical cliff, running across brilliantly lit rock art that glares in the sun.

I shoot a temperature of the rock art: 144 degrees.

In the 1930s, a University of California biology professor squatted in the desert watching lizards. The lizards were leashed in a manner that allowed them to move toward shade but not to reach it.

The professor discovered the obvious. He discovered that lizards died in the heat. “Here was I,” he wrote, “a heat-generating animal with a naked, unprotected skin, surviving even longer exposure while dozens of reptiles were killed in minutes by overheating.” He discovered that the smaller animals died quickly, that it takes less time to roast a small turkey than a large turkey. “Some of the smallest reptiles,” he wrote, “died within sixty seconds after being scooped out of their underground shelters into the blazing sun of the surface.”

He also discovered that the animals lost their ability to move well before they died. Self-rescue became impossible. He discovered too that their tolerance of cold far exceeded their tolerance of heat. They could cool thirty degrees below their optimal body temperature without loss of their abilities, but warm them more than ten degrees above their optimal body temperature, and they would suffer, in his words, “crippling effects.”

All of this, it turns out, is also true for humans.

Lizards survive the heat by avoiding it, hiding in burrows or under bushes. Humans sweat. For humans, life at high temperatures returns again and again to sweating. Humans evolved in the tropics, a place that was hot but where water was abundant. We sweat to shed heat. We sweat to expose water to air, and that water evaporates, taking with it unwanted and potentially deadly heat. Adult humans have something like three million sweat glands. Individuals who grow up in extreme heat have more sweat glands than those who grow up in cold climates, but three million is a typical number.

Perspiration separates humans from dogs and cats and pigs, which shed heat by panting. It separates us, too, from birds, also panters. In terms of dumping heat, we primates stand closer to the other sweaters, to donkeys, camels, and horses.

We are warm-blooded. By virtue of breathing, of a beating heart, of firing neurons, we generate our own heat.
La respiration est donc une combustion.
At rest, we generate enough energy to power a hundred-watt lightbulb. Moving, contracting and relaxing muscles, we convert the chemical energy of food into the work of motion, but most of the energy—more than three-quarters of it—becomes heat. Moving, we generate enough heat to power ten lightbulbs. The chemical reactions in our cells produce enough energy to power a well-lit room, and just as in a well-lit room, most of that energy is lost to heat.

In the desert, the air and the sun and the ground add heat. Stand still and feel convection currents in the air moving heat across your skin. Stand in the sun and feel radiant heat. Sit on a rock and feel conductive heat.

Fail to dump heat, and the body’s temperature rises. Add six degrees in core temperature, and suffer heatstroke. The skin goes dry, coordination diminishes, the mind becomes delirious, the body convulses with epileptic seizures. Self-rescue becomes impossible. Coma follows. Without help, only death remains.

“As far as I am aware,” wrote the medical doctor W. Hale White in 1891, “no explanations have been offered of the mode in which in the process of evolution cold-blooded animals became warm-blooded.”

The difference between the cold-blooded and the warm-blooded is not as clear as Doctor White suspected. Take fish: cold-blooded, with temperatures matching those of the surrounding water. Unless the fish you take is the swordfish, whose innards include musclelike tissues that serve no purpose other than warming the blood supply to the brain and eyes. Or unless you take bluefin tuna, whose constant swimming generates heat, and whose blood vessels are arranged in such a way that the heat is conserved, with warm blood coming from the muscles donating heat to cold blood coming from the gills.

Body temperature of a swordfish, around the brain and eyes: as much as twenty degrees higher than that of the surrounding water.

Body temperature of a bluefin tuna: as much as twenty-five degrees higher than that of the surrounding water.

Then we have the camel, with a body temperature that drifts between the low 90s and 110 degrees, its thermostat adjusting to save water, to limit the need to sweat. Or certain bats, with temperatures when active at just under ninety-nine degrees, about the same as humans, but with temperatures dropping to those of the surrounding air when resting, and dropping close to freezing during hibernation. Or the arctic ground squirrel, with a body temperature when active similar to that of humans, but during hibernation dropping into the high twenties, below the freezing point of fresh water, not warm-blooded at all when curled in winter burrows. The camel, certain bats, the ground squirrel: warm-blooded, sort of, if one does not look too closely.

The earliest mammals pumped warm blood. They appeared two hundred million years ago, denizens of the late Triassic, a time when the supercontinent of Pangaea was just beginning to break apart. They probably came from cynodont therapsids, beasts that in appearance looked half lizard and half dachshund, egg laying but probably with hair, more warm-blooded than cold.

Warm blood flowed through the arteries of birds, too, appearing in the Jurassic, fifty million years after the early mammals gained a foothold. The birds probably descended from the theropod dinosaurs, which themselves may have sported warm blood.

Body temperature of the once living but forever extinct cynodont therapsids and theropods: unknown and unknowable.