"Capsaicin demonstrates the incredible elegance of evolution," says Tewksbury. The specialized chemical deters microbes—humans harness this ability when they use chilies to preserve food—but capsaicin doesn't deter birds from eating chili fruits and spreading seeds. "Once in a while, the complex, often conflicting demands that natural selection places on complex traits results in a truly elegant solution. This is one of those times."
William Foley, a nutritional ecologist at the Australian National University in Canberra, says Tewksbury is "working on the right system to answer tricky ecological questions people have been asking for a long time." Denise Dearing, an ecologist at the University of Utah, calls Tewksbury's research "the most in-depth work on a plant-frugivore [fruit eater] interaction." Tewksbury's success comes in part from his fearless exploration of the chili's motherland. Adds Foley: "You can't expect to understand complex interactions between plants and animals unless you're actually in the field."
It takes a special kind of perseverance to conduct fieldwork in the Gran Chaco, a dry forest wilderness that covers 500,000 square miles in Paraguay, Argentina, Brazil and Bolivia. In Bolivia, only 6 percent of the roads are paved and gasoline and accurate maps are hard to come by. Weather fluctuates between oppressive heat and torrential rain—turning roads into mud wallows. The researchers' truck is outfitted with two spare tires, but it was once stopped by a third flat. In the course of their research, the scientists have lost a wheel (loose lug nuts), snapped an axle (inexperienced driver) and cracked the engine block (river crossing). Their sponsoring organization, the Fundación Amigos de la Naturaleza, had to replace the entire vehicle when it was stolen in 2003.
Tewksbury says his fieldwork runs on three compounds: coca, caffeine and capsaicin. But his fascination with natural products and his seeming indefatigability may have deeper roots. His father, Peter Tewksbury, was the director of the 1950s TV sitcoms "My Three Sons" and "Father Knows Best" and the short-lived but acclaimed series "It's a Man's World," which featured Josh's mother, Cielle, acting under the name Ann Schuyler. Peter would eventually direct Elvis Presley in Stay Away, Joe and The Trouble With Girls and worked briefly with J. D. Salinger in a failed attempt to bring one of his short stories to the screen. Eventually fed up with the constraints of Hollywood, Peter came home one day in the 1970s, pulled his Emmy Award from the closet and chucked it into the trash can. "It was one of his bitter moments," Cielle recalls. "He had an incredible sense of morality and ethics, and it just didn't match with a Hollywood career." As Peter himself would later tell the New York Times, "Peter Tewksbury the director is dead."
The couple packed up the car and drove with their two children to Vermont, where they adopted the names Henry and Mary Jane. "Henry" became a renowned cheese expert and the author of The Cheeses of Vermont: A Gourmet Guide to Vermont's Artisanal Cheesemakers. They home-schooled Josh and his sister, Marintha, and moved among Vermont, Quebec, Oregon and a ranch in California. Peter Tewksbury died in 2003 at age 79. "When [Peter] would glom onto something he wanted to do, it was just [at] a dead run," says Cielle, 71, who teaches the Taoist philosophy tai chi and Chinese sword and saber techniques in Brattleboro, Vermont. "There's quite a bit of him in Josh: the excitement, the drive and the complete dedication and focus."
Indeed, some mornings in Bolivia, Josh Tewksbury was in such a rush to get out to the field that he'd put his shirt on inside out or backward. Talking about science, he would get a faraway look and say, "that would be slick." From the back of the truck one day, he yelled to colleagues riding in the cab about a new experiment he was contemplating. Carlos Manchego, a student at Bolivia's Natural History Museum, and Tomás Carlo, an ecologist at Pennsylvania State University, spent the next hour hashing it out with Tewksbury as they hung out the windows, clutching the roof rack.
His zeal can sometimes get the better of him. About eight years ago, he and Doug Levey of the University of Florida, an expert on plant and animal interactions, were visiting Ilha do Cardoso off the coast of Brazil. The duo became convinced they had uncovered a novelty: a fungus whose spores were dispersed by a bird. They spent several days frantically collecting samples with hopes of culturing the fungus back in the lab. They hoped to submit their findings to a prestigious journal. But when they finally examined the "fungus" under a microscope, they noticed it had legs—and there's nothing unusual about birds eating insects.
But Levey points out that even Tewksbury's misguided enthusiasms may prove fruitful: "There's a long history in science of the most important discoveries being made by accident or by following a whim."
"I think this is insane right now," says Machnicki as she plucks a huge spine out of the back of her knee. Her nylon quick-dry pants are no match for the Bolivian forest. At her feet, a snake-like cactus winds among thickets of spiny ground bromeliads ("my nemesis," she calls them), thorny shrubs and the bulbous stem of the devil nettle.
We had camped the night before on a low plateau overlooking the Paraguayan border. Our breakfast—and the last of our food supplies—consisted of a thimbleful of cold coffee, a boxed juice drink and a modest bag of trail mix. Instead of moving on as planned, Tewksbury's wandering legs took him to an unexpected patch of chilies. He was soon stringing a measuring tape through the woods to count every one of them in a plot some 200 yards on each side. After two years of laboratory work, Machnicki, a fungus expert, is finally getting a chance to see the natural habitat where her seed-killing fungus thrives. At the moment, though, she would rather be eating lunch. "Everything with him is by the seat of your pants," she would tell me later.