Sled Dogs, Ultrarunners, and Couch Potato Genes
The aluminum sign for Comeback Kennel is nailed haphazardly to an evergreen tree north of Fairbanks, Alaska, off the Elliott Highway and two miles in on a dirt road. The gravel driveway is packed hard from the cold and steep enough to make entry without an SUV precarious. This solitary spot befits Alaskan taste. If you can see the smoke from your neighbor’s chimney, he probably lives too close.
It’s an unlikely address for a collection of the planet’s greatest and most steel-willed endurance athletes. But there, on a sloped clearing framed by black spruce, are 120 of dogsled racing’s most distinguished Alaskan huskies. Comeback Kennel is really just the name of the frosted front yard belonging to Lance Mackey.
Mackey is an icon in the dogsled racing world, where he essentially invented the thousand-mile double. That is, in both 2007 and ’08, Mackey won the thousand-mile Yukon Quest, and then, just weeks later, the world’s other thousand-miler: the Iditarod, known to the faithful as “The Last Great Race on Earth.” Prior to Mackey’s back-to-back doubles, the feat was thought to be impossible. A musher was lucky to escape even one of the races without illness or serious injury to himself or his dogs. Even if he did, there is the problem of will, for both the dogs and their master.
Eminent mushers have had to withdraw from the Iditarod when their dogs simply lie down in the snow and refuse to go another step. And the freezing cold and sleep deprivation of the long Alaska nights are famous for divorcing Iditarod mushers from their better judgment. From time to time, a musher crossing atop the frozen Bering Sea will gaze into the bright sunlight after a deep black night and start removing his jacket and gloves, only to be greeted by -50 degree air, and instant frostbite. Mackey himself has heard voices. Once, after a long, cold stretch with no sleep he was pleased to see an Inuit woman beside the trail smiling at him. He turned and started waving, and only then realized that she was gone. Or, rather, that she had never been there at all.
Prior to Mackey’s runs, just to attempt to finish the Yukon Quest and the Iditarod back-to-back was considered foolhardy. Even if the musher survived the Quest with his vital signs intact, what about the dogs? Assuming they were healthy, would they want to keep running? Sled dogs, like their masters, must have the will to forge ahead.
“These aren’t house dogs. Food will not work as a training device for sled dogs,” says Eric Morris, a musher and biochemist who created Redpaw dog food for canine athletes. “Negative reinforcement will not work as a training device for sled dogs either. To go that distance, it’s like a bird dog sniffing down a pheasant, it has to be the one thing in their life that brings them the greatest amount of pleasure. They have to have the innate desire to pull [the sled] . . . and you will find varying degrees of that in different dogs.”
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Each of the Alaskan huskies in Mackey’s yard is chained to a metal ring that is looped around a pole, restricting its movement to a circle several meters in diameter that includes entrance into its own wooden house. Each dog, that is, except for Zorro.
On top of the hill in the yard is Zorro’s fenced-in pen. He has more space, and no chain. It’s his “condo on the hill,” Mackey jokes. From here, Zorro looks down on the nighttime lights of Fairbanks far below, and also on his nieces, nephews, sisters, brothers, and sons and daughters, all here in this yard.
As Mackey walks toward Zorro, he pauses to point. “That’s my main bitch right there,” Mackey says, gesturing to one of Zorro’s granddaughters, a female dog named Maple whose golden brown coat is the color of cinnamon toast. In 2010, Maple led Mackey’s team—meaning she was at the head of the group of dogs—and won the Golden Harness Award for the most outstanding performer in the Iditarod. Like Maple, all of Mackey’s champion dogs are in Zorro’s line. “It was pretty ballsy,” Mackey says, “to base the whole kennel around one dog.” He leans down to nuzzle the blond rings of fur around Zorro’s eyes, the ones that resemble the mask his namesake wore.
After communing with Zorro, Mackey walks back to the half-constructed house that he and his wife Tonya share. It’s full of exposed wiring, and still partially wrapped in Tyvek sheets, but it belongs to them, along with the garage that holds a limited-edition Dodge Charger and three Dodge trucks, all prizes for Iditarod wins. “The dogs bought all of this,” Mackey says. None more so than Zorro.
Zorro is the genetic nexus of the kennel, and not because he was a particularly fast husky. (He wasn’t.) Rather, Mackey bred for the genes of work ethic. He had no other choice. In 1999, when Mackey began his breeding program, he couldn’t afford the fastest, sleekest dogs.
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Lance Mackey’s father, Dick, was one of the cofounders of the Iditarod Trail Sled Dog Race, first run in 1973. In his first five attempts at the race, Dick never finished higher than sixth. In his sixth attempt, in 1978, something so unexpected occurred that the fledgling race had no rule to adjudicate it.
Lance was seven years old, standing near the burled arch that marks the finish, when his father, running alongside his sled and almost suffocating in his parka, sprinted down Front Street in a dead heat with defending champion Rick Swenson, also on foot beside his sled. As Dick Mackey’s lead dog crossed the finish first, by a nose, Mackey collapsed to the ground, leaving his team straddling the line as Swenson’s sled zoomed past. At the end of 14 days, 18 hours, 52 minutes, and 24 seconds, the Iditarod had come down to whether race marshal Myron Gavin would rule that the first musher with a single dog across the line won or whether it was the musher with all his dogs across the line. “They don’t take a picture of the horse’s ass, do they?” Gavin asked, rhetorically. And so Dick Mackey won the Iditarod, and became a full-fledged hero to his son.
“I was standing right at the finish line,” says Lance, who grew up in Wasilla, Alaska. “It was exciting. It was dramatic. It was emotional. It was embedded in my head. I have no doubt that something in that moment, in that one second, affected my passion or my drive or my commitment. It not only changed my dad’s life, it changed mine.” From that moment on, Lance Mackey always told himself that one day he would win the Iditarod too. But the path would be tortuous.
Three years after his father won the Iditarod, Mackey’s parents divorced. He began to see little of his father, an ironworker who was off building up the remote reaches of Alaska. His mother, Kathie, worked as a bush pilot and dishwasher to support the family, so Lance had all the unsupervised time in the world to seek out trouble. He excelled at finding it.
By fifteen, Mackey was a one-boy crime wave: fighting, consumption of alcohol by a minor, more fighting, drunk and disorderly, public urination, and a little more fighting. Before he had a driver’s license, he stole Kathie’s checkbook, used it to buy a ’68 Dodge Charger and drove it north to pawn three firearms he’d swiped from the family gun cabinet.
So Kathie sent her son above the Arctic Circle to spend some quality time with his father, who was selling food out of a converted school bus to truckers passing along the Trans-Alaska Pipeline. That operation would become a restaurant and service station and then the town of Coldfoot, Alaska, population: a dozen.
Working at his father’s service station, Mackey learned to barter truck repairs for drugs. “Truck drivers are as bad of junkies as anybody you ever met,” he says. “So I had access to just about every drug I could get my hands on.” Mackey returned to Wasilla just before his eighteenth birthday and picked up his life of petty crime where he had left off—until one Saturday, when Kathie refused to bail her son out of jail.
When Lance got out, he headed to the Bering Sea, where he spent the next decade as a commercial fisherman on long-liners. Even then, Mackey would tell crewmates on fishing vessels—many of whom were from Mexico and had never heard of the Iditarod—that one day he would win the race that his father cofounded. “You ain’t nothing as a musher unless you win the Iditarod,” Mackey would recount his father saying.
By 1997, Mackey was living with Tonya in Nenana, Alaska, and both were addicted to cocaine. They occasionally used Amanda, Tonya’s daughter from a previous marriage, as a designated driver. “She had a cushion so she could see over the wheel,” Mackey says. “She thought it was cool as hell, being nine years old driving down the highway.”
On June 2, 1998, Mackey’s twenty-eighth birthday—and not long after he’d nearly gotten himself killed in a gun-filled bar brawl—he and Tonya decided to go cold turkey. On one night’s packing, they moved 465 miles south to Alaska’s Kenai Peninsula and left their drug habits behind. There, Lance and Tonya lived with Amanda and Brittney—Tonya’s other daughter, then eight—beneath a tarp on the beach. A pup tent served as the master bedroom. For dinner, Tonya made a campfire and cooked flounder that the girls plucked from the sand. Lance started working for a construction crew and at a local sawmill. It was enough to make a down payment on a plot of land where he and Tonya built a timber house and stuffed the walls with clothing from the Salvation Army for insulation. With cocaine behind him, Mackey threw himself into a new addiction: breeding and raising sled dogs.
He had no money to buy the trim and powerful huskies that had already distinguished themselves in races, so he took in mutts from the street or adopted the castoffs of other mushers. Mackey accepted that his motley band of dogs would never be the sprinters of the canine world, so he decided to breed for other qualities, and that’s when he met Rosie.
Rosie was a tiny female dog that once belonged to sprint racer Patty Moran. Moran decided that Rosie was too slow, so she sold her for a pittance to Rob Sparks, a musher who raced longer distances. When Sparks saw that Rosie refused to switch from a trot to a lope, he, too, decided that little Rosie was just too slow to race. At Sparks’s offering, Mackey took Rosie out for a test drive. Sure, she wasn’t fast, but Mackey saw something else: hook Rosie up to a sled harness and she’ll trot, as Mackey puts it, until she bores a hole through the earth. He was glad to take her off Sparks’s hands. His “trotting tornado,” he called her.
Mackey bred Rosie with Doc Holliday, another husky that would never win a sprint but that yearned for nothing more than to run, eat, and run some more. From the union of Rosie and Doc Holliday, Mackey got Zorro.
Even elite-bred and trained sled dogs will regularly coast on a long run. That is, they’ll slyly back off the pace when other members of the team are working hard. An experienced musher can tell when a dog is backing off because the rope—known as the “tug line”—that connects the dog to the sled’s main line won’t be perfectly taut. But Zorro was always pulling. From his very first race Zorro had to be restrained at the starting line and kept right on pulling even after the finish. Though Zorro was on the heavy side for a racing dog, “I told my brother Rick,” Mackey says, “I’m breeding Zorro to every dog I own.”
In 2001, Mackey picked a team from his band of rejects and hand-me-downs and put them together with Zorro—the lone dog he’d bred and raised—and entered the Iditarod. It took Mackey 12 days, 18 hours, 35 minutes, and 13 seconds to finish the race, good enough for thirty-sixth place. Zorro, not yet two years old, was the youngest dog in the entire field to complete the 1,100 miles, and he did it in great shape, barking and yanking the sled across the finish.
Mackey himself was less chipper. He had pushed through the pain of what multiple doctors had told him, erroneously, was an abscessed tooth. During the race he suffered from blurry vision, headaches, and blackouts. After the finish, he collapsed. Tonya took him straight to the hospital. The following week Mackey was in emergency surgery for throat cancer. It was the kind of surgery before which the doctor tells the patient to make sure there’s nothing he’d regret having left unsaid to his wife and family. Mackey’s normally staid father, Dick, was inconsolable.
Surgeons removed a grapefruit-sized tumor from Mackey’s throat, along with the skin, muscle tissue, and salivary glands with which it was entangled. From then on, Mackey had to sip constantly from a water bottle or on the juice from a fruit cup in order to keep his throat moist enough that he could breathe. Radiation treatments that damaged Mackey’s nerves left him with pulsing pain in his left index finger, so he went from doctor to doctor until he persuaded one to just cut the thing off.
Through it all, even when it seemed as though Mackey might not survive, Tonya kept his breeding plan going. At Mackey’s direction, she bred Zorro with females in the yard. By the winter after his surgery, Mackey was well enough to return to work with sixty-six of Zorro’s tongue-and-tail-wagging puppies to greet him.
Mackey returned to the Iditarod in 2002—with a feeding tube in his stomach—but withdrew after 440 miles. He skipped the next Iditarod, and for the next few years concentrated on raising and training Zorro’s children and grandchildren. Mackey’s training plan was tailored to his initial breeding strategy of mating the hardest-working dogs—the strategy foisted upon him because he couldn’t afford the fastest dogs. Knowing he would never outrace his Iditarod competitors between checkpoints, Mackey developed what he calls his “marathon style,” a technique that would transform long-distance dogsled racing. Rather than sprint between rest stops—as many successful mushers did at speeds up to 12, or occasionally 15 miles per hour—Mackey had dogs that were slower but would trot till they bored a hole in the earth. “At seven miles per hour, that’s poking,” Mackey says, “but if the dogs will go seven miles per hour for nineteen straight hours, then you’re going to go places.”
In 2007, Mackey started the Iditarod with a sixteen-dog team that consisted almost entirely of Zorro’s progeny. Those that weren’t directly from Zorro included his half-brother Larry, his nephew Battel, and Zorro himself. Just more than nine days later, with tears frozen to his face, Mackey passed beneath the burled arch in first place. “Life just changed,” Mackey told his dogs. And so did dogsled racing.
Suddenly, Mackey’s competitors wanted to copy the marathon style. Overnight his kennel went from home of cheap castoffs to a yard full of coveted bloodlines, with dogs worth four figures each, at least. (According to Tonya Mackey, Zorro’s son Hobo was purchased by another musher and then “tossed all around Norway breeding, at a couple grand for each breeding.”) In 2008, Mackey won the Iditarod again, his second of four straight. In a race a few weeks later, a drunk snowmobile driver rammed his team. Zorro had to be airlifted to Seattle with three broken ribs, a bruised lung, internal bleeding, and spinal damage so severe he couldn’t stand.
Zorro survived, but a veterinarian ordered that he be retired from breeding and racing. Mackey built Zorro a doghouse in the yard, but quickly saw that his relentless runner would whine and tug at his chain if he was passed over while other dogs were taken out on a run. So Mackey built him the “condo on the hill,” his fenced-in area in front of the house. “He’s still the badass in the kennel,” Mackey says. “He’s my main man even though he doesn’t run. He holds a very, very special place in my life and heart.” And, more important, in the gene pool in his front yard.
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The idea that dogs can be bred to win a race is no revelation. Darwin himself marveled at the ability of dog breeders to cultivate almost any trait they wanted. Breeding for speed in racing whippets has been so intense that over 40 percent of the dogs in the top division have what is normally an exceedingly rare myostatin gene mutation (the “Superbaby” mutation).
In the late nineteenth and early twentieth centuries—particularly during the Klondike Gold Rush—when the seaports and rivers of Alaska were frozen solid, sled dogs were the main source of transport for everything from mail to gold ore. Breeding for strength, endurance, and resistance to cold proceeded in earnest, until snowmobiles came into fashion. When dogsled racing gained popularity with the rise of prize money following the first Iditarod in 1973, breeding for athleticism became a serious business. Pointers, salukis, and a bundle of other breeds were mixed into a genetic stew that had traditionally included Alaskan malamutes and Siberian huskies. It worked.
The winners of the first two Iditarod races took more than twenty days to finish. Two decades of breeding later, mushers were finishing in half that time. Alaskan huskies morphed into athletes unique on the planet. Even before training, an elite Alaskan husky can move four to five times as much oxygen as a healthy, untrained adult man. With training, top sled dogs reach a VO2max about eight times that of an average man, and more than four times higher than a trained Paula Radcliffe, the women’s marathon world record holder.
Sled dogs were bred for everything from a voracious appetite—they eat ten thousand calories a day during the Iditarod—to webbed toes ideal for traveling atop snow, to a pulse rate that settles quickly at a moment’s rest. Perhaps the most remarkable bit of biology bred into Alaskan huskies is the ability to adapt almost instantly to exercise. As in humans, when sled dogs start training, they deplete the energy reserves in their muscles, undergo an increase in stress hormones, and damage cells. Human athletes experience this as fatigue and soreness, and must rest to allow the body to adapt to the exercise before coming back to training or racing. But the best sled dogs adapt on the run. Whereas humans have to alternate exercise and rest to get fit, premier Alaskan huskies get fit while barely stopping to recuperate. They are the ultimate training responders.
In 2010, Heather Huson, a geneticist then studying at the University of Alaska, Fairbanks—and a dogsled racer since age seven—tested dogs from eight different racing kennels. To Huson’s surprise, Alaskan sled dogs have been so thoroughly bred for specific traits that analysis of microsatellites—repeats of small sequences of DNA—proved Alaskan huskies to be an entirely genetically distinct breed, as unique as poodles or labs, rather than just a variation of Alaskan malamutes or Siberian huskies.
Huson and colleagues discovered genetic traces of twenty-one dog breeds, in addition to the unique Alaskan husky signature. The research team also established that the dogs had widely disparate work ethics (measured via the tension in their tug lines) and that sled dogs with better work ethics had more DNA from Anatolian shepherds—a muscular, often blond breed of dog originally prized as a guardian of sheep because it would eagerly do battle with wolves. That Anatolian shepherd genes uniquely contribute to the work ethic of sled dogs was a new finding, but the best mushers already knew that work ethic is specifically bred into dogs.
“Yeah, thirty-eight years ago in the Iditarod there were dogs that weren’t enthused about doing it, and that were forced to do it,” Mackey says. “I want to be out there and have the privilege of going along for the ride because they want to go, because they love what they do, not because I want to go across the state of Alaska for my satisfaction, but because they love doing it. And that’s what’s happened over forty years of breeding. We’ve made and designed dogs suited for desire.”
Several mushers I spoke with suggested that sled dogs may have maxed out their physiological capacity and are no longer getting faster or hardier, and that the improvement in race times is now entirely down to how long the dogs are eager to pull without rest. “The dogs are in control,” says Eric Morris, the biochemist and musher. “That’s why we breed dogs that want to do it . . . it’s something I had to learn through trial and error, and time, and speaking and working with other mushers, to find out what all the great ones know. The great mushers know how to breed a dog that has drive and the desire to pull, and then they foster and develop that desire.”*
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Scientists who breed rodents for their desire to run have proven that work ethic is genetically influenced. One of the leaders in that field has been Theodore Garland, a physiologist at UC Riverside. For more than a decade, he has been offering mice a wheel that they may hop on or shun at their discretion.
Normal mice run three to four miles each night. Garland took a group of average mice and separated them into two subgroups: those that chose to run less than average each night, and those that chose to run more than average. Garland then bred “high runners” with other high runners, and “low runners” with other low runners. After just one generation of breeding, the progeny of the high runners were, of their own accord, running even farther on average than their parents. By the sixteenth generation of breeding, the high runners were voluntarily cranking out seven miles each night. “The normal mice are out for a leisurely stroll,” Garland says. “They putz around on the wheel, while the high runners are really running.”
When mice are bred for endurance capacity—not voluntary running, but when they are forced to run as long as they physically can—successive generations have more symmetrical bones, lower body fat, and larger hearts. In his voluntary-runner breeding program, Garland saw body changes, “but at the same time,” he says, “clearly the brains are very different.” Like their hearts, the brains of the high runners were larger than those of average mice. “Presumably,” Garland says, “the centers of the brain that deal with motivation and reward have gotten larger.”
He then dosed the mice with Ritalin, a stimulant that alters levels of dopamine. Dopamine is a neurotransmitter, a chemical that conveys messages between brain cells. The normal mice, once doped, apparently derived a greater sensation of pleasure from running, so they started doing it more. But the high runners, when doped, did not run more. Whatever Ritalin does in the brains of normal mice is already occurring in the brains of the high-running mice. They are, quite literally, running junkies.*
“Who says motivation isn’t genetic?” Garland asks, rhetorically. “In these mice, it’s absolutely the case that motivation has evolved.”
Researchers around the world have begun to explore locations on the genome that differ between marathon mice and their normal counterparts, and specifically to home in on genes related to dopamine processing that might impact the sense of pleasure or reward a mouse gets from a particular behavior.
Of course, they aren’t doing this simply to understand why rodents want to run. The ultimate goal is to learn about human gym rats.
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Pam Reed was up on top of the parking garage at LaGuardia Airport in Queens, again. Her flight out of New York City was delayed, and she was never one for sitting still. While disgruntled travelers jostled for electrical outlets and cushioned seats, their bags trundling behind them, the fifty-one-year-old Reed popped in her earbuds and headed for the top deck of the parking garage.
She breathed in the thick summer air. Reed stashed her luggage in a corner and started running. Immediately, a placid calm dripped through her body. For a good hour, she ran around and around in tight circles, each lap no more than 200 meters. It certainly wasn’t because she needed the fitness.
Just the previous day, Reed had finished the U.S. championship Ironman triathlon in New York City in 11 hours, 20 minutes, and 49 seconds, good enough to qualify for the world championship in Hawaii. A week before that, she participated in a relay race in which her leg consisted of eight continuous hours of circling a track. Two weeks before that, she spent 31 hours running en route to becoming the second female finisher at the 2012 Badwater Ultramarathon, a 135-mile race that starts in Death Valley, and that Reed has won, twice.
Reed’s flight out of LaGuardia eventually left, and the next weekend she completed the Mont-Tremblant Ironman in Québec in 12 hours, 16 minutes, and 42 seconds. The weekend after that, she had “only a marathon,” she says, never mind that it was through the Tetons, in her home of Jackson Hole, Wyoming.
This isn’t some masochistic running binge, it’s life for a woman who once ran three hundred miles without sleeping, and in 2009 spent six days running 491 laps around a drab one-mile loop in a park in Queens.
When she was an eleven-year-old in Michigan, Reed was smitten by her first sports love while watching the 1972 Olympics on TV: gymnastics. “I was obsessed,” Reed later wrote in her autobiography, The Extra Mile. “I practiced gymnastics every minute that I could, in the basement, off the couch, wherever I happened to be.” In high school, Reed turned to tennis, and, as usual, threw herself into it the way a Navy SEAL throws himself out of a plane—with gusto. Part of her training was a minimum of one thousand sit-ups a day. She went on to play varsity tennis at Michigan Tech. When she later moved to Arizona—she owns and directs the Tucson Marathon—she worked as an aerobics instructor so that she could have access to the health club’s pool. Naturally (for Reed), she fell in love with her second husband as the pair trained together for an Ironman triathlon. Reed has often wondered about the source of her relentless drive to be in motion.
Her father was tireless. He used to rise at 3:30 A.M. to head to work at an iron mine, and when he returned home in the afternoon he would go straight to building an addition to the house or tinkering on the car. According to family lore (“absolutely true,” Reed says), her grandfather Leonard once got into an argument at a family gathering in Merrill, Wisconsin, and stormed out in a huff. He kept walking. The entire three hundred miles back home to Chicago.
“Running for three hours every day might put some people in the hospital,” Reed writes in her book, while noting that she finds peace of mind in extreme activity. “I am certain that not running for three hours every day would very quickly make me ill. . . . While nobody’s forcing me to do this, it’s not really a choice, either. There’s something in my nature that makes it really hard for me to sit still . . . being temperamentally attuned to perpetual motion makes me pretty uncomfortable on long car trips or in sedate social settings.” (Reed’s son Tim contrasts himself to his mother: “I only like to run for maybe two or three hours max.”) One of Reed’s current goals is to set the women’s world record for running across America, which she plans to do at a pace of two marathons a day.
“When I don’t do this,” Reed says—and by “this” she means running three to five times a day—“I feel horrible. I had C-sections, and three days after them I was running. . . . It’s who I am. I totally love it. As I get older, I have to say, I can sit still a bit longer, but it’s not comfortable.”
In her book, Reed astutely ponders whether she might be the human version of the rodents from an experiment at the University of Wisconsin in which mice bred for voluntary running were restricted from running, and then had their brain activity measured. Brain circuitry similar to that which is active when humans crave food or sex, or when addicts crave drugs, was activated in the high-running mice that were denied the chance to run, and they became agitated. The researchers presumed that when the mice were deprived of running their brain activity would decline. Instead, it went into overdrive, as if the mice needed exercise to feel normal. The longer the distance a particular mouse was used to running, the more frenetic its brain activity became when it was made to sit still. As with Garland’s mice, these rodents were genetic junkies for exercise.
Pam Reed is an outlier by any measure. But a seemingly compulsive drive to exercise is hardly unique among distinguished athletes. Consider Ethiopian Haile Gebrselassie, who has set twenty-seven distance running world records: “A day I don’t run, I don’t feel good,” he says. Or Floyd Mayweather Jr., the undefeated boxing champion, who has been known to jolt awake in the middle of the night and force his bloated entourage to meet him at the gym for a workout. Or Steve Mesler, a member of the 2010 Olympic four-man bobsled team that won the first U.S. gold in sixty-two years. He retired afterward, but says he “feels anxious” when he takes a break from working out even now. Or Ironman triathlete Chrissie Wellington or high jumper Stefan Holm, both of whom claim addictive personalities that they channeled to their training.
Or Herschel Walker, best known as the 1982 Heisman Trophy–winning running back and twelve-year NFL veteran. Now fifty-one, Walker is 2-0 as a professional mixed martial artist. Walker has trained in ballet, taekwondo (he’s a fifth-degree black belt), and, in 1992, was an Olympic bobsled pusher. Most indicative of Walker’s drive to be active, though, is the workout regimen he started at age twelve, before he was involved in organized sports, and which he has continued every day since. “I would start doing sit-ups and push-ups at seven P.M.,” he says, “and go until eleven. It was every night, on the floor. It was about five thousand sit-ups and push-ups.” These days, Walker says he “only” does 1,500 push-ups and 3,500 sit-ups a day—in sets of 50 to 75 push-ups and 300 to 500 sit-ups or crunches—but he also has his martial arts training.
Walker says the push-ups and sit-ups routine will remain, even after he stops competing. “It has nothing to do with my competitions,” he says. “It becomes a drug, or a medicine. Even if I’m sick, I do it. It’s like there’s something saying, ‘Herschel, you gotta get up. You gotta do it.’”
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Variations in the brain’s dopamine system make certain individuals more likely to feel reward when using particular drugs, and they are more likely to become addicted. Is it possible that, like sled dogs and lab mice, some people are biologically predisposed to get an outsized sense of reward or pleasure from being constantly in motion?* All sixteen human studies conducted as of this writing have found a large contribution of heredity to the amount of voluntary physical activity that people undertake.
A 2006 Swedish study of 13,000 pairs of fraternal and identical twins—fraternal twins share half their genes on average, while identical twins essentially share them all—reported that the physical activity levels of identical twins were twice as likely to be similar as those of fraternal twins. That study used a survey to measure physical activity, though, and people chronically overestimate their own physical activity levels. But another, smaller study of twin pairs that used accelerometers to measure physical activity directly found the same difference between fraternal and identical twin pairs. The largest study, of 37,051 twin pairs from six European countries and Australia, concluded that about half to three quarters of the variation in the amount of exercise people undertook was attributable to their genetic inheritance, while unique environmental factors, like access to a health club, had a comparatively puny influence.
It is entirely clear that the dopamine system responds to physical activity. This is one reason that exercise can be used as part of treatment for depression and as a method to slow the progression of Parkinson’s disease, an illness that involves the destruction of brain cells that make dopamine. And there is evidence that the reverse is true as well, that physical activity levels respond to the dopamine system. Several lines of scientific evidence have begun to implicate genes that control dopamine.
Particular versions of dopamine receptor genes have been associated with higher physical activity and lower body mass index. Multiple studies—including a meta-analysis of all published studies—have also replicated the finding that one of those variants, the 7R version of the DRD4 gene, increases an individual’s risk for attention deficit hyperactivity disorder, or ADHD. Tim Lightfoot, director of the Sydney and J. L. Huffines Institute for Sports Medicine and Human Performance at Texas A&M, has authored papers on voluntary physical activity in rodents and humans, and he sees a connection between ADHD, exercise, and dopamine genes. “The high active mice we bred in the lab,” Lightfoot says, “they mimic ADHD kids, at least as far as the dopamine system goes. . . . They’re low on [a particular kind of] dopamine receptors, and if you can drive the amount of dopamine up, their physical activity decreases.”
Ritalin drives dopamine up in hyperactive children, and their activity decreases. Obviously, this is a good thing for a child who is having difficulty sitting still in school. But, Lightfoot suggests, it might have unintended consequences. “These may be kids that have a very strong drive to be active, and maybe we’re blunting it with medications.”
“Our society is so scared right now of kids being fat,” Lightfoot continues. “Well, what if we’re putting some of these kids on drugs that actually may be contributing to this by driving their activity levels down?” In any case, that’s exactly what happened in Lightfoot’s mice.
A set of scientists have proposed the controversial idea that hyperactivity and impulsivity may have had advantages in the ancestral state of man in nature, leading to the preservation of genes that increase ADHD risk. Interestingly, the 7R variant of the DRD4 gene is more common in populations that have migrated long distances, as well as those that are nomadic, compared with settled populations.
In 2008, a team of anthropologists genetically tested Ariaal tribesmen in northern Kenya, some of whom are nomadic and some recently settled. In the nomadic group—and only in the nomadic group—those with the 7R version of the DRD4 gene were less likely to be undernourished. One of several hypotheses the researchers offered: “It might also be that higher activity levels in [the 7R] nomads are translated into increased food production.” In other words, it could be that carriers of that version of the gene are harder workers when it comes to physical activities.
“One of the issues with our field is when we’ve looked at activity, and what controls activity, we’ve forgotten that we know very clearly there are biological mechanisms that actually influence people to be active or not,” Lightfoot says. “You can have a predisposition to be a couch potato.”
Quite obviously, as is the case with Kenyan children, the necessity of transportation by foot and the aspiration for a better life can have profound influences on physical activity levels. But those environmental factors do not exclude the significant contribution of genetics that has shown up in every study ever conducted on the heritability of voluntary physical activity.
Those consistent findings are reminiscent of a famous quote by Wayne Gretzky, the greatest hockey player in history: “Maybe it wasn’t talent the Lord gave me, maybe it was the passion.”
Or maybe the two are inextricable.
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Even as it has been demonstrated in study after study that genetic inheritance influences physical activity, scientists are only beginning to discern the specific biological processes that play a role. Plus, every scientist knows full well that extreme environments can dramatically alter how much an individual trains. While dopamine plays a role in the drive to be in motion, there are certain, more obvious enticements.
When Floyd Mayweather Jr., renowned for his furious training, dropped by the Sports Illustrated office in 2007, fresh off his victory over Oscar De La Hoya, he described an unhappy period in his past when he was constantly concerned about money. “But I’m happy now,” he said with a mile-wide smile, referring to the $25 million he made for the fight.
All told, the tangle of nature and nurture is so complex as to prompt the question: can there possibly be any practical use at all for genetic testing in sports right now, in the present day?
The answer, despite all the complexity: absolutely.
The aluminum sign for Comeback Kennel is nailed haphazardly to an evergreen tree north of Fairbanks, Alaska, off the Elliott Highway and two miles in on a dirt road. The gravel driveway is packed hard from the cold and steep enough to make entry without an SUV precarious. This solitary spot befits Alaskan taste. If you can see the smoke from your neighbor’s chimney, he probably lives too close.
It’s an unlikely address for a collection of the planet’s greatest and most steel-willed endurance athletes. But there, on a sloped clearing framed by black spruce, are 120 of dogsled racing’s most distinguished Alaskan huskies. Comeback Kennel is really just the name of the frosted front yard belonging to Lance Mackey.
Mackey is an icon in the dogsled racing world, where he essentially invented the thousand-mile double. That is, in both 2007 and ’08, Mackey won the thousand-mile Yukon Quest, and then, just weeks later, the world’s other thousand-miler: the Iditarod, known to the faithful as “The Last Great Race on Earth.” Prior to Mackey’s back-to-back doubles, the feat was thought to be impossible. A musher was lucky to escape even one of the races without illness or serious injury to himself or his dogs. Even if he did, there is the problem of will, for both the dogs and their master.
Eminent mushers have had to withdraw from the Iditarod when their dogs simply lie down in the snow and refuse to go another step. And the freezing cold and sleep deprivation of the long Alaska nights are famous for divorcing Iditarod mushers from their better judgment. From time to time, a musher crossing atop the frozen Bering Sea will gaze into the bright sunlight after a deep black night and start removing his jacket and gloves, only to be greeted by -50 degree air, and instant frostbite. Mackey himself has heard voices. Once, after a long, cold stretch with no sleep he was pleased to see an Inuit woman beside the trail smiling at him. He turned and started waving, and only then realized that she was gone. Or, rather, that she had never been there at all.
Prior to Mackey’s runs, just to attempt to finish the Yukon Quest and the Iditarod back-to-back was considered foolhardy. Even if the musher survived the Quest with his vital signs intact, what about the dogs? Assuming they were healthy, would they want to keep running? Sled dogs, like their masters, must have the will to forge ahead.
“These aren’t house dogs. Food will not work as a training device for sled dogs,” says Eric Morris, a musher and biochemist who created Redpaw dog food for canine athletes. “Negative reinforcement will not work as a training device for sled dogs either. To go that distance, it’s like a bird dog sniffing down a pheasant, it has to be the one thing in their life that brings them the greatest amount of pleasure. They have to have the innate desire to pull [the sled] . . . and you will find varying degrees of that in different dogs.”
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Each of the Alaskan huskies in Mackey’s yard is chained to a metal ring that is looped around a pole, restricting its movement to a circle several meters in diameter that includes entrance into its own wooden house. Each dog, that is, except for Zorro.
On top of the hill in the yard is Zorro’s fenced-in pen. He has more space, and no chain. It’s his “condo on the hill,” Mackey jokes. From here, Zorro looks down on the nighttime lights of Fairbanks far below, and also on his nieces, nephews, sisters, brothers, and sons and daughters, all here in this yard.
As Mackey walks toward Zorro, he pauses to point. “That’s my main bitch right there,” Mackey says, gesturing to one of Zorro’s granddaughters, a female dog named Maple whose golden brown coat is the color of cinnamon toast. In 2010, Maple led Mackey’s team—meaning she was at the head of the group of dogs—and won the Golden Harness Award for the most outstanding performer in the Iditarod. Like Maple, all of Mackey’s champion dogs are in Zorro’s line. “It was pretty ballsy,” Mackey says, “to base the whole kennel around one dog.” He leans down to nuzzle the blond rings of fur around Zorro’s eyes, the ones that resemble the mask his namesake wore.
After communing with Zorro, Mackey walks back to the half-constructed house that he and his wife Tonya share. It’s full of exposed wiring, and still partially wrapped in Tyvek sheets, but it belongs to them, along with the garage that holds a limited-edition Dodge Charger and three Dodge trucks, all prizes for Iditarod wins. “The dogs bought all of this,” Mackey says. None more so than Zorro.
Zorro is the genetic nexus of the kennel, and not because he was a particularly fast husky. (He wasn’t.) Rather, Mackey bred for the genes of work ethic. He had no other choice. In 1999, when Mackey began his breeding program, he couldn’t afford the fastest, sleekest dogs.
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Lance Mackey’s father, Dick, was one of the cofounders of the Iditarod Trail Sled Dog Race, first run in 1973. In his first five attempts at the race, Dick never finished higher than sixth. In his sixth attempt, in 1978, something so unexpected occurred that the fledgling race had no rule to adjudicate it.
Lance was seven years old, standing near the burled arch that marks the finish, when his father, running alongside his sled and almost suffocating in his parka, sprinted down Front Street in a dead heat with defending champion Rick Swenson, also on foot beside his sled. As Dick Mackey’s lead dog crossed the finish first, by a nose, Mackey collapsed to the ground, leaving his team straddling the line as Swenson’s sled zoomed past. At the end of 14 days, 18 hours, 52 minutes, and 24 seconds, the Iditarod had come down to whether race marshal Myron Gavin would rule that the first musher with a single dog across the line won or whether it was the musher with all his dogs across the line. “They don’t take a picture of the horse’s ass, do they?” Gavin asked, rhetorically. And so Dick Mackey won the Iditarod, and became a full-fledged hero to his son.
“I was standing right at the finish line,” says Lance, who grew up in Wasilla, Alaska. “It was exciting. It was dramatic. It was emotional. It was embedded in my head. I have no doubt that something in that moment, in that one second, affected my passion or my drive or my commitment. It not only changed my dad’s life, it changed mine.” From that moment on, Lance Mackey always told himself that one day he would win the Iditarod too. But the path would be tortuous.
Three years after his father won the Iditarod, Mackey’s parents divorced. He began to see little of his father, an ironworker who was off building up the remote reaches of Alaska. His mother, Kathie, worked as a bush pilot and dishwasher to support the family, so Lance had all the unsupervised time in the world to seek out trouble. He excelled at finding it.
By fifteen, Mackey was a one-boy crime wave: fighting, consumption of alcohol by a minor, more fighting, drunk and disorderly, public urination, and a little more fighting. Before he had a driver’s license, he stole Kathie’s checkbook, used it to buy a ’68 Dodge Charger and drove it north to pawn three firearms he’d swiped from the family gun cabinet.
So Kathie sent her son above the Arctic Circle to spend some quality time with his father, who was selling food out of a converted school bus to truckers passing along the Trans-Alaska Pipeline. That operation would become a restaurant and service station and then the town of Coldfoot, Alaska, population: a dozen.
Working at his father’s service station, Mackey learned to barter truck repairs for drugs. “Truck drivers are as bad of junkies as anybody you ever met,” he says. “So I had access to just about every drug I could get my hands on.” Mackey returned to Wasilla just before his eighteenth birthday and picked up his life of petty crime where he had left off—until one Saturday, when Kathie refused to bail her son out of jail.
When Lance got out, he headed to the Bering Sea, where he spent the next decade as a commercial fisherman on long-liners. Even then, Mackey would tell crewmates on fishing vessels—many of whom were from Mexico and had never heard of the Iditarod—that one day he would win the race that his father cofounded. “You ain’t nothing as a musher unless you win the Iditarod,” Mackey would recount his father saying.
By 1997, Mackey was living with Tonya in Nenana, Alaska, and both were addicted to cocaine. They occasionally used Amanda, Tonya’s daughter from a previous marriage, as a designated driver. “She had a cushion so she could see over the wheel,” Mackey says. “She thought it was cool as hell, being nine years old driving down the highway.”
On June 2, 1998, Mackey’s twenty-eighth birthday—and not long after he’d nearly gotten himself killed in a gun-filled bar brawl—he and Tonya decided to go cold turkey. On one night’s packing, they moved 465 miles south to Alaska’s Kenai Peninsula and left their drug habits behind. There, Lance and Tonya lived with Amanda and Brittney—Tonya’s other daughter, then eight—beneath a tarp on the beach. A pup tent served as the master bedroom. For dinner, Tonya made a campfire and cooked flounder that the girls plucked from the sand. Lance started working for a construction crew and at a local sawmill. It was enough to make a down payment on a plot of land where he and Tonya built a timber house and stuffed the walls with clothing from the Salvation Army for insulation. With cocaine behind him, Mackey threw himself into a new addiction: breeding and raising sled dogs.
He had no money to buy the trim and powerful huskies that had already distinguished themselves in races, so he took in mutts from the street or adopted the castoffs of other mushers. Mackey accepted that his motley band of dogs would never be the sprinters of the canine world, so he decided to breed for other qualities, and that’s when he met Rosie.
Rosie was a tiny female dog that once belonged to sprint racer Patty Moran. Moran decided that Rosie was too slow, so she sold her for a pittance to Rob Sparks, a musher who raced longer distances. When Sparks saw that Rosie refused to switch from a trot to a lope, he, too, decided that little Rosie was just too slow to race. At Sparks’s offering, Mackey took Rosie out for a test drive. Sure, she wasn’t fast, but Mackey saw something else: hook Rosie up to a sled harness and she’ll trot, as Mackey puts it, until she bores a hole through the earth. He was glad to take her off Sparks’s hands. His “trotting tornado,” he called her.
Mackey bred Rosie with Doc Holliday, another husky that would never win a sprint but that yearned for nothing more than to run, eat, and run some more. From the union of Rosie and Doc Holliday, Mackey got Zorro.
Even elite-bred and trained sled dogs will regularly coast on a long run. That is, they’ll slyly back off the pace when other members of the team are working hard. An experienced musher can tell when a dog is backing off because the rope—known as the “tug line”—that connects the dog to the sled’s main line won’t be perfectly taut. But Zorro was always pulling. From his very first race Zorro had to be restrained at the starting line and kept right on pulling even after the finish. Though Zorro was on the heavy side for a racing dog, “I told my brother Rick,” Mackey says, “I’m breeding Zorro to every dog I own.”
In 2001, Mackey picked a team from his band of rejects and hand-me-downs and put them together with Zorro—the lone dog he’d bred and raised—and entered the Iditarod. It took Mackey 12 days, 18 hours, 35 minutes, and 13 seconds to finish the race, good enough for thirty-sixth place. Zorro, not yet two years old, was the youngest dog in the entire field to complete the 1,100 miles, and he did it in great shape, barking and yanking the sled across the finish.
Mackey himself was less chipper. He had pushed through the pain of what multiple doctors had told him, erroneously, was an abscessed tooth. During the race he suffered from blurry vision, headaches, and blackouts. After the finish, he collapsed. Tonya took him straight to the hospital. The following week Mackey was in emergency surgery for throat cancer. It was the kind of surgery before which the doctor tells the patient to make sure there’s nothing he’d regret having left unsaid to his wife and family. Mackey’s normally staid father, Dick, was inconsolable.
Surgeons removed a grapefruit-sized tumor from Mackey’s throat, along with the skin, muscle tissue, and salivary glands with which it was entangled. From then on, Mackey had to sip constantly from a water bottle or on the juice from a fruit cup in order to keep his throat moist enough that he could breathe. Radiation treatments that damaged Mackey’s nerves left him with pulsing pain in his left index finger, so he went from doctor to doctor until he persuaded one to just cut the thing off.
Through it all, even when it seemed as though Mackey might not survive, Tonya kept his breeding plan going. At Mackey’s direction, she bred Zorro with females in the yard. By the winter after his surgery, Mackey was well enough to return to work with sixty-six of Zorro’s tongue-and-tail-wagging puppies to greet him.
Mackey returned to the Iditarod in 2002—with a feeding tube in his stomach—but withdrew after 440 miles. He skipped the next Iditarod, and for the next few years concentrated on raising and training Zorro’s children and grandchildren. Mackey’s training plan was tailored to his initial breeding strategy of mating the hardest-working dogs—the strategy foisted upon him because he couldn’t afford the fastest dogs. Knowing he would never outrace his Iditarod competitors between checkpoints, Mackey developed what he calls his “marathon style,” a technique that would transform long-distance dogsled racing. Rather than sprint between rest stops—as many successful mushers did at speeds up to 12, or occasionally 15 miles per hour—Mackey had dogs that were slower but would trot till they bored a hole in the earth. “At seven miles per hour, that’s poking,” Mackey says, “but if the dogs will go seven miles per hour for nineteen straight hours, then you’re going to go places.”
In 2007, Mackey started the Iditarod with a sixteen-dog team that consisted almost entirely of Zorro’s progeny. Those that weren’t directly from Zorro included his half-brother Larry, his nephew Battel, and Zorro himself. Just more than nine days later, with tears frozen to his face, Mackey passed beneath the burled arch in first place. “Life just changed,” Mackey told his dogs. And so did dogsled racing.
Suddenly, Mackey’s competitors wanted to copy the marathon style. Overnight his kennel went from home of cheap castoffs to a yard full of coveted bloodlines, with dogs worth four figures each, at least. (According to Tonya Mackey, Zorro’s son Hobo was purchased by another musher and then “tossed all around Norway breeding, at a couple grand for each breeding.”) In 2008, Mackey won the Iditarod again, his second of four straight. In a race a few weeks later, a drunk snowmobile driver rammed his team. Zorro had to be airlifted to Seattle with three broken ribs, a bruised lung, internal bleeding, and spinal damage so severe he couldn’t stand.
Zorro survived, but a veterinarian ordered that he be retired from breeding and racing. Mackey built Zorro a doghouse in the yard, but quickly saw that his relentless runner would whine and tug at his chain if he was passed over while other dogs were taken out on a run. So Mackey built him the “condo on the hill,” his fenced-in area in front of the house. “He’s still the badass in the kennel,” Mackey says. “He’s my main man even though he doesn’t run. He holds a very, very special place in my life and heart.” And, more important, in the gene pool in his front yard.
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The idea that dogs can be bred to win a race is no revelation. Darwin himself marveled at the ability of dog breeders to cultivate almost any trait they wanted. Breeding for speed in racing whippets has been so intense that over 40 percent of the dogs in the top division have what is normally an exceedingly rare myostatin gene mutation (the “Superbaby” mutation).
In the late nineteenth and early twentieth centuries—particularly during the Klondike Gold Rush—when the seaports and rivers of Alaska were frozen solid, sled dogs were the main source of transport for everything from mail to gold ore. Breeding for strength, endurance, and resistance to cold proceeded in earnest, until snowmobiles came into fashion. When dogsled racing gained popularity with the rise of prize money following the first Iditarod in 1973, breeding for athleticism became a serious business. Pointers, salukis, and a bundle of other breeds were mixed into a genetic stew that had traditionally included Alaskan malamutes and Siberian huskies. It worked.
The winners of the first two Iditarod races took more than twenty days to finish. Two decades of breeding later, mushers were finishing in half that time. Alaskan huskies morphed into athletes unique on the planet. Even before training, an elite Alaskan husky can move four to five times as much oxygen as a healthy, untrained adult man. With training, top sled dogs reach a VO2max about eight times that of an average man, and more than four times higher than a trained Paula Radcliffe, the women’s marathon world record holder.
Sled dogs were bred for everything from a voracious appetite—they eat ten thousand calories a day during the Iditarod—to webbed toes ideal for traveling atop snow, to a pulse rate that settles quickly at a moment’s rest. Perhaps the most remarkable bit of biology bred into Alaskan huskies is the ability to adapt almost instantly to exercise. As in humans, when sled dogs start training, they deplete the energy reserves in their muscles, undergo an increase in stress hormones, and damage cells. Human athletes experience this as fatigue and soreness, and must rest to allow the body to adapt to the exercise before coming back to training or racing. But the best sled dogs adapt on the run. Whereas humans have to alternate exercise and rest to get fit, premier Alaskan huskies get fit while barely stopping to recuperate. They are the ultimate training responders.
In 2010, Heather Huson, a geneticist then studying at the University of Alaska, Fairbanks—and a dogsled racer since age seven—tested dogs from eight different racing kennels. To Huson’s surprise, Alaskan sled dogs have been so thoroughly bred for specific traits that analysis of microsatellites—repeats of small sequences of DNA—proved Alaskan huskies to be an entirely genetically distinct breed, as unique as poodles or labs, rather than just a variation of Alaskan malamutes or Siberian huskies.
Huson and colleagues discovered genetic traces of twenty-one dog breeds, in addition to the unique Alaskan husky signature. The research team also established that the dogs had widely disparate work ethics (measured via the tension in their tug lines) and that sled dogs with better work ethics had more DNA from Anatolian shepherds—a muscular, often blond breed of dog originally prized as a guardian of sheep because it would eagerly do battle with wolves. That Anatolian shepherd genes uniquely contribute to the work ethic of sled dogs was a new finding, but the best mushers already knew that work ethic is specifically bred into dogs.
“Yeah, thirty-eight years ago in the Iditarod there were dogs that weren’t enthused about doing it, and that were forced to do it,” Mackey says. “I want to be out there and have the privilege of going along for the ride because they want to go, because they love what they do, not because I want to go across the state of Alaska for my satisfaction, but because they love doing it. And that’s what’s happened over forty years of breeding. We’ve made and designed dogs suited for desire.”
Several mushers I spoke with suggested that sled dogs may have maxed out their physiological capacity and are no longer getting faster or hardier, and that the improvement in race times is now entirely down to how long the dogs are eager to pull without rest. “The dogs are in control,” says Eric Morris, the biochemist and musher. “That’s why we breed dogs that want to do it . . . it’s something I had to learn through trial and error, and time, and speaking and working with other mushers, to find out what all the great ones know. The great mushers know how to breed a dog that has drive and the desire to pull, and then they foster and develop that desire.”*
•
Scientists who breed rodents for their desire to run have proven that work ethic is genetically influenced. One of the leaders in that field has been Theodore Garland, a physiologist at UC Riverside. For more than a decade, he has been offering mice a wheel that they may hop on or shun at their discretion.
Normal mice run three to four miles each night. Garland took a group of average mice and separated them into two subgroups: those that chose to run less than average each night, and those that chose to run more than average. Garland then bred “high runners” with other high runners, and “low runners” with other low runners. After just one generation of breeding, the progeny of the high runners were, of their own accord, running even farther on average than their parents. By the sixteenth generation of breeding, the high runners were voluntarily cranking out seven miles each night. “The normal mice are out for a leisurely stroll,” Garland says. “They putz around on the wheel, while the high runners are really running.”
When mice are bred for endurance capacity—not voluntary running, but when they are forced to run as long as they physically can—successive generations have more symmetrical bones, lower body fat, and larger hearts. In his voluntary-runner breeding program, Garland saw body changes, “but at the same time,” he says, “clearly the brains are very different.” Like their hearts, the brains of the high runners were larger than those of average mice. “Presumably,” Garland says, “the centers of the brain that deal with motivation and reward have gotten larger.”
He then dosed the mice with Ritalin, a stimulant that alters levels of dopamine. Dopamine is a neurotransmitter, a chemical that conveys messages between brain cells. The normal mice, once doped, apparently derived a greater sensation of pleasure from running, so they started doing it more. But the high runners, when doped, did not run more. Whatever Ritalin does in the brains of normal mice is already occurring in the brains of the high-running mice. They are, quite literally, running junkies.*
“Who says motivation isn’t genetic?” Garland asks, rhetorically. “In these mice, it’s absolutely the case that motivation has evolved.”
Researchers around the world have begun to explore locations on the genome that differ between marathon mice and their normal counterparts, and specifically to home in on genes related to dopamine processing that might impact the sense of pleasure or reward a mouse gets from a particular behavior.
Of course, they aren’t doing this simply to understand why rodents want to run. The ultimate goal is to learn about human gym rats.
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Pam Reed was up on top of the parking garage at LaGuardia Airport in Queens, again. Her flight out of New York City was delayed, and she was never one for sitting still. While disgruntled travelers jostled for electrical outlets and cushioned seats, their bags trundling behind them, the fifty-one-year-old Reed popped in her earbuds and headed for the top deck of the parking garage.
She breathed in the thick summer air. Reed stashed her luggage in a corner and started running. Immediately, a placid calm dripped through her body. For a good hour, she ran around and around in tight circles, each lap no more than 200 meters. It certainly wasn’t because she needed the fitness.
Just the previous day, Reed had finished the U.S. championship Ironman triathlon in New York City in 11 hours, 20 minutes, and 49 seconds, good enough to qualify for the world championship in Hawaii. A week before that, she participated in a relay race in which her leg consisted of eight continuous hours of circling a track. Two weeks before that, she spent 31 hours running en route to becoming the second female finisher at the 2012 Badwater Ultramarathon, a 135-mile race that starts in Death Valley, and that Reed has won, twice.
Reed’s flight out of LaGuardia eventually left, and the next weekend she completed the Mont-Tremblant Ironman in Québec in 12 hours, 16 minutes, and 42 seconds. The weekend after that, she had “only a marathon,” she says, never mind that it was through the Tetons, in her home of Jackson Hole, Wyoming.
This isn’t some masochistic running binge, it’s life for a woman who once ran three hundred miles without sleeping, and in 2009 spent six days running 491 laps around a drab one-mile loop in a park in Queens.
When she was an eleven-year-old in Michigan, Reed was smitten by her first sports love while watching the 1972 Olympics on TV: gymnastics. “I was obsessed,” Reed later wrote in her autobiography, The Extra Mile. “I practiced gymnastics every minute that I could, in the basement, off the couch, wherever I happened to be.” In high school, Reed turned to tennis, and, as usual, threw herself into it the way a Navy SEAL throws himself out of a plane—with gusto. Part of her training was a minimum of one thousand sit-ups a day. She went on to play varsity tennis at Michigan Tech. When she later moved to Arizona—she owns and directs the Tucson Marathon—she worked as an aerobics instructor so that she could have access to the health club’s pool. Naturally (for Reed), she fell in love with her second husband as the pair trained together for an Ironman triathlon. Reed has often wondered about the source of her relentless drive to be in motion.
Her father was tireless. He used to rise at 3:30 A.M. to head to work at an iron mine, and when he returned home in the afternoon he would go straight to building an addition to the house or tinkering on the car. According to family lore (“absolutely true,” Reed says), her grandfather Leonard once got into an argument at a family gathering in Merrill, Wisconsin, and stormed out in a huff. He kept walking. The entire three hundred miles back home to Chicago.
“Running for three hours every day might put some people in the hospital,” Reed writes in her book, while noting that she finds peace of mind in extreme activity. “I am certain that not running for three hours every day would very quickly make me ill. . . . While nobody’s forcing me to do this, it’s not really a choice, either. There’s something in my nature that makes it really hard for me to sit still . . . being temperamentally attuned to perpetual motion makes me pretty uncomfortable on long car trips or in sedate social settings.” (Reed’s son Tim contrasts himself to his mother: “I only like to run for maybe two or three hours max.”) One of Reed’s current goals is to set the women’s world record for running across America, which she plans to do at a pace of two marathons a day.
“When I don’t do this,” Reed says—and by “this” she means running three to five times a day—“I feel horrible. I had C-sections, and three days after them I was running. . . . It’s who I am. I totally love it. As I get older, I have to say, I can sit still a bit longer, but it’s not comfortable.”
In her book, Reed astutely ponders whether she might be the human version of the rodents from an experiment at the University of Wisconsin in which mice bred for voluntary running were restricted from running, and then had their brain activity measured. Brain circuitry similar to that which is active when humans crave food or sex, or when addicts crave drugs, was activated in the high-running mice that were denied the chance to run, and they became agitated. The researchers presumed that when the mice were deprived of running their brain activity would decline. Instead, it went into overdrive, as if the mice needed exercise to feel normal. The longer the distance a particular mouse was used to running, the more frenetic its brain activity became when it was made to sit still. As with Garland’s mice, these rodents were genetic junkies for exercise.
Pam Reed is an outlier by any measure. But a seemingly compulsive drive to exercise is hardly unique among distinguished athletes. Consider Ethiopian Haile Gebrselassie, who has set twenty-seven distance running world records: “A day I don’t run, I don’t feel good,” he says. Or Floyd Mayweather Jr., the undefeated boxing champion, who has been known to jolt awake in the middle of the night and force his bloated entourage to meet him at the gym for a workout. Or Steve Mesler, a member of the 2010 Olympic four-man bobsled team that won the first U.S. gold in sixty-two years. He retired afterward, but says he “feels anxious” when he takes a break from working out even now. Or Ironman triathlete Chrissie Wellington or high jumper Stefan Holm, both of whom claim addictive personalities that they channeled to their training.
Or Herschel Walker, best known as the 1982 Heisman Trophy–winning running back and twelve-year NFL veteran. Now fifty-one, Walker is 2-0 as a professional mixed martial artist. Walker has trained in ballet, taekwondo (he’s a fifth-degree black belt), and, in 1992, was an Olympic bobsled pusher. Most indicative of Walker’s drive to be active, though, is the workout regimen he started at age twelve, before he was involved in organized sports, and which he has continued every day since. “I would start doing sit-ups and push-ups at seven P.M.,” he says, “and go until eleven. It was every night, on the floor. It was about five thousand sit-ups and push-ups.” These days, Walker says he “only” does 1,500 push-ups and 3,500 sit-ups a day—in sets of 50 to 75 push-ups and 300 to 500 sit-ups or crunches—but he also has his martial arts training.
Walker says the push-ups and sit-ups routine will remain, even after he stops competing. “It has nothing to do with my competitions,” he says. “It becomes a drug, or a medicine. Even if I’m sick, I do it. It’s like there’s something saying, ‘Herschel, you gotta get up. You gotta do it.’”
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Variations in the brain’s dopamine system make certain individuals more likely to feel reward when using particular drugs, and they are more likely to become addicted. Is it possible that, like sled dogs and lab mice, some people are biologically predisposed to get an outsized sense of reward or pleasure from being constantly in motion?* All sixteen human studies conducted as of this writing have found a large contribution of heredity to the amount of voluntary physical activity that people undertake.
A 2006 Swedish study of 13,000 pairs of fraternal and identical twins—fraternal twins share half their genes on average, while identical twins essentially share them all—reported that the physical activity levels of identical twins were twice as likely to be similar as those of fraternal twins. That study used a survey to measure physical activity, though, and people chronically overestimate their own physical activity levels. But another, smaller study of twin pairs that used accelerometers to measure physical activity directly found the same difference between fraternal and identical twin pairs. The largest study, of 37,051 twin pairs from six European countries and Australia, concluded that about half to three quarters of the variation in the amount of exercise people undertook was attributable to their genetic inheritance, while unique environmental factors, like access to a health club, had a comparatively puny influence.
It is entirely clear that the dopamine system responds to physical activity. This is one reason that exercise can be used as part of treatment for depression and as a method to slow the progression of Parkinson’s disease, an illness that involves the destruction of brain cells that make dopamine. And there is evidence that the reverse is true as well, that physical activity levels respond to the dopamine system. Several lines of scientific evidence have begun to implicate genes that control dopamine.
Particular versions of dopamine receptor genes have been associated with higher physical activity and lower body mass index. Multiple studies—including a meta-analysis of all published studies—have also replicated the finding that one of those variants, the 7R version of the DRD4 gene, increases an individual’s risk for attention deficit hyperactivity disorder, or ADHD. Tim Lightfoot, director of the Sydney and J. L. Huffines Institute for Sports Medicine and Human Performance at Texas A&M, has authored papers on voluntary physical activity in rodents and humans, and he sees a connection between ADHD, exercise, and dopamine genes. “The high active mice we bred in the lab,” Lightfoot says, “they mimic ADHD kids, at least as far as the dopamine system goes. . . . They’re low on [a particular kind of] dopamine receptors, and if you can drive the amount of dopamine up, their physical activity decreases.”
Ritalin drives dopamine up in hyperactive children, and their activity decreases. Obviously, this is a good thing for a child who is having difficulty sitting still in school. But, Lightfoot suggests, it might have unintended consequences. “These may be kids that have a very strong drive to be active, and maybe we’re blunting it with medications.”
“Our society is so scared right now of kids being fat,” Lightfoot continues. “Well, what if we’re putting some of these kids on drugs that actually may be contributing to this by driving their activity levels down?” In any case, that’s exactly what happened in Lightfoot’s mice.
A set of scientists have proposed the controversial idea that hyperactivity and impulsivity may have had advantages in the ancestral state of man in nature, leading to the preservation of genes that increase ADHD risk. Interestingly, the 7R variant of the DRD4 gene is more common in populations that have migrated long distances, as well as those that are nomadic, compared with settled populations.
In 2008, a team of anthropologists genetically tested Ariaal tribesmen in northern Kenya, some of whom are nomadic and some recently settled. In the nomadic group—and only in the nomadic group—those with the 7R version of the DRD4 gene were less likely to be undernourished. One of several hypotheses the researchers offered: “It might also be that higher activity levels in [the 7R] nomads are translated into increased food production.” In other words, it could be that carriers of that version of the gene are harder workers when it comes to physical activities.
“One of the issues with our field is when we’ve looked at activity, and what controls activity, we’ve forgotten that we know very clearly there are biological mechanisms that actually influence people to be active or not,” Lightfoot says. “You can have a predisposition to be a couch potato.”
Quite obviously, as is the case with Kenyan children, the necessity of transportation by foot and the aspiration for a better life can have profound influences on physical activity levels. But those environmental factors do not exclude the significant contribution of genetics that has shown up in every study ever conducted on the heritability of voluntary physical activity.
Those consistent findings are reminiscent of a famous quote by Wayne Gretzky, the greatest hockey player in history: “Maybe it wasn’t talent the Lord gave me, maybe it was the passion.”
Or maybe the two are inextricable.
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Even as it has been demonstrated in study after study that genetic inheritance influences physical activity, scientists are only beginning to discern the specific biological processes that play a role. Plus, every scientist knows full well that extreme environments can dramatically alter how much an individual trains. While dopamine plays a role in the drive to be in motion, there are certain, more obvious enticements.
When Floyd Mayweather Jr., renowned for his furious training, dropped by the Sports Illustrated office in 2007, fresh off his victory over Oscar De La Hoya, he described an unhappy period in his past when he was constantly concerned about money. “But I’m happy now,” he said with a mile-wide smile, referring to the $25 million he made for the fight.
All told, the tangle of nature and nurture is so complex as to prompt the question: can there possibly be any practical use at all for genetic testing in sports right now, in the present day?
The answer, despite all the complexity: absolutely.
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