This story appears in the Oct. 17 issue of ESPN The Magazine.

SHAUN O'HARA IS TOTALLY NUTS.

When the New York Giants made Eli Manning the No. 1 overall draft pick in 2004, O'Hara -- the team's Pro Bowl center at the time -- welcomed the $54 million quarterback to the big leagues with a prank he first concocted in Cleveland on another naive No. 1 pick, QB Tim Couch. The stunt is of such shockingly brilliant, adolescent grotesqueness that it might just explain, once and for all, why Couch failed to matriculate in the NFL.

Before practice, O'Hara would use the trainer's razor-sharp scissors to make a small incision to the undercarriage of his skin-tight football pants. Like most old-school linemen, he wore a backless jockstrap under his pants and nothing else. After breaking the huddle, on his way to the line of scrimmage, the 300-pound O'Hara would reach around and Houdini his testicles through their trap door before bending over for the snap. When the gung-ho Manning eagerly snuggled up under center, he was greeted by O'Hara's naked dangling orbs.

Today, when asked to elaborate on his testicle tricks, O'Hara pauses for a moment before offering up this cheeky response: "Which one?" Peek-a-ball wasn't the big man's only puerile pastime. Anyone dumb enough to leave a cell phone unattended inside the Giants locker room would likely discover his screensaver had been replaced with a snapshot of the O'Hara boys. As former teammate and defensive end Michael Strahan puts it, "I think Shaun's got ball issues."

Everyone in sports does, actually. Viewed from a different (and safer) angle, O'Hara's pranks expose something far more significant and, at the same time, far more primal. Think about it: What is the most influential, yet least discussed, biological component relating to male sports performance? Yep ... balls.

They weigh less than an ounce each, are barely 1.5 inches long, and -- let's face it -- ain't much to look at, but thanks to the testosterone contained therein, testicles are no less than the symbolic plumbs of masculinity and dominance in our society. They're like precious little crystal balls, revealing answers to many of the mysteries behind the physiology and sociology of sports.They are about fearlessness and recklessness more than bravery. They're about power, aggression and control more than gender, accolades or money. They're about the eagerness to fight more than they're about the outcome itself. Ultimately, balls are about the willingness to risk it all. "A lot of having balls has to do with what's above your shoulders," says MMA legend Dan Henderson. "It means you'll do whatever it takes, go through any kind of pain, to get the job done."

"Balls literally determine winners," says Brendon Ayanbadejo, the All-Pro special teams ace for the Baltimore Ravens. "Nothing else does that. Not just on a molecular level, but the personality needed to win. Balls come down to this: Running down the field on a kickoff, headed for two 300-pound blockers, do you go around or do you speed up and try to hit them as hard as you possibly can? Anytime you see a touchdown on special teams, it's because of one thing. Balls."

“ Balls literally determine winners. ” -- Baltimore ravens linebacker Brendon Ayanbadejo

More specifically, it's that testicles are home to leydig cells, the male body's main production plant for testosterone. Each day, testicles produce between 6 mg and 8 mg of "T," the substance that compels the body to develop male characteristics as well as the essential tools of an athlete: muscle mass and strength, bone density, energy levels and muscle repair. A 2011 study in the Journal of Strength and Conditioning Research tested 12 wrestlers from NCAA champion Penn State before and after a series of five matches. Winning wrestlers were found to have nearly 10 percent more testosterone prior to the match and levels that were 16 percent higher afterward. Other studies have shown a correlation between high T levels and dominant, aggressive behavior in men.

That link between testicles, power and winning is as old as athletic competition itself. The ancient Greeks invented the Olympics as a way to honor their gods, deities often equipped with magical, unruly testicles. The Greeks exercised and competed in the nude, and the word "testify" comes from the tradition of competitors swearing an oath by placing their hands on, or near, each other's genitals. Some 4,000 years later, calling a person "ballsy" or saying that an action "took balls" or that an athlete played "balls out" remains the highest sporting compliment.

Sometimes, it also means a pretty sweet nickname. After LSU captured the 2007 BCS title, fans paid tribute to coach Les Miles by referring to him as Lesticles. "If anybody really cares about something, it's worth risking for," Miles says. "So there are a lot of guys who have proverbial testicles." And many of them, it turns out, are women. Cristiane "Cyborg" Santos, the world's best female MMA fighter, has heard the term "balls" used so often to describe herself and other women -- Serena Williams in tennis, soccer's Abby Wambach, surfer Bethany Hamilton, jockey Julie Krone, to name a few -- that it's no wonder the term has begun to lose its gender specificity. "Women will never be as strong as men," Santos says, "but women are tougher in many, many ways."

Especially where it counts. As nature would have it, the fount of so much biological and psychological power is also the most vulnerable spot on the male body. Because of an evolutionary imperative and a developmental quirk in the womb (more on that later), testes are excruciatingly sensitive to pain and exceedingly difficult to protect. And that makes them a very popular target. In August, Abner Mares practically roshamboed his way to the IBF bantamweight championship by hitting Joseph Agbeko with 28 low blows in the first six rounds of their title fight. NFL players readily admit that when they're at the bottom of a pile, fighting for a fumble, they target balls almost as much as the ball.

The same mentality applies in the political arena. Moments before his famous presidential debate against Richard Nixon in 1960, John F. Kennedy received a final, five-word missive from brother and adviser Bobby: "Kick him in the balls."

ONE OF THE best-kept secrets in sports is the story of athletes who've lost testicles in the heat of battle. From the self-explanatory karate move known as Monkey Steals the Peach, to the rugby ruck phenomena of scrotal "degloving," to the NHL's Stanley (shattered) Cup playoffs, severe testicle trauma is more common than you might suspect. The limited statistics available say that 1 percent of all sports injuries are below the belt, but most experts believe that athletes under-report due to embarrassment. "It's kind of like how concussions used to be laughed off, and now we know how serious they are," says Michael Koester, a doctor at the Slocum Center for Orthopedics and Sports Medicine, at the University of Oregon. "Taking a shot in the balls can be very serious."

And yet, jocks are decidedly laissez-faire when it comes to guarding their goods. Freud would have a field day with this paradox, but modern athletes find old-fashioned cups restricting, bulky and uncool. The jock and cup were patented in 1927 by Jack Cartledge of Guelph, Ontario, and in the 80-plus years since, the cup's design has barely changed from its original, cumbersome V-shape trough, which an early ad pitched as the best way to protect "the floppy man parts."

So, despite the fact that the force of a hit from an NFL defensive tackle measures 3,200 pounds of force (or 16 times what's required to squash a testicle like a grape), hardly anyone in the league bothers to cup it up. Titans linebacker Barrett Ruud says he tried wearing one in his first high school game, but the resulting rash on his inner thighs was so bad that he ended up walking bow-legged like John Wayne for a week before deciding that a "shot to the nuts" would be preferable to any more atomic chaffing. He's not alone. College football players don't regularly wear cups, either. Same goes for guys in the NBA and about half of all pro baseball players.

Even athletes who do wear cups run into frequent testes trouble. During the 2010 NHL playoffs, Vancouver defenseman Sami Salo took a slapper to the groin and was hospitalized. Within hours, fans had created a Facebook page in honor of his ruptured testicle, including inspirational messages such as "Hang in there, buddy" and encouraging chants of "BALLS OF STEEL" upon Salo's return to the ice, just two days later. It was the third year in a row that an NHL player nearly lost a testicle while blocking a shot during the playoffs. Hockey players wear cups, but when they slide, so does their protection, exposing their scrotums to a six-ounce piece of frozen rubber traveling at 100 mph.

“ The connection between men and their testicles is very real. They're the repository for strength, vigor and vitality. ” -- Matt Higgins, New York Jets executive vice president of business operations

Castration has been one of man's greatest fears since Australopithecus first stood up on two feet four million years ago. Testicles are located outside the protective skeletal structure because one of their main functions, sperm production, requires a setting at least two degrees cooler than normal body temperature. The left testicle hangs lower than the right in 65 percent of men, a scrotal asymmetry that keeps the crown jewels from banging into one another, like a Newton's Cradle desktop toy. When our ancient ancestors lifted their hands off the ground and evolved into bipeds, one of the trade-offs for upward mobility was less protection for the testicles.

The clammy, queasy feeling that incapacitates men after they take a shot to the groin is nature's way of reminding them to protect their packages. In the embryonic state, testicles start out in the stomach cavity next to the kidneys. As the body develops, they eventually descend, but the nerve endings remain connected deep within the retroperitoneum -- hence the sensation that someone just crowbarred your kidneys.

In 2007, Matt Higgins felt a similar kind of shooting pain while waiting in a Dunkin' Donuts drive-thru. And just like that, the New York Jets' vice president of business operations was, he says, "down a ball three days later." At 33, Higgins was diagnosed with testicular cancer, the most common form of the disease for men 18 to 35. The surgery saved his life and now, cancer-free, wears a pendant from his wife that reads, Half the Balls, Twice the Man. "The connection between men and their testicles is very real," Higgins says. "They're the repository for strength, vigor and vitality. But for me, losing that symbol of manhood is what gave me the opportunity to discover what it is to be a real man."

He's not alone. Lance Armstrong famously returned from advanced testicular cancer to win the Tour de France seven times; other survivors include figure skater and 1984 Olympic gold medalist Scott Hamilton, former miler and Olympian Steve Scott, retired Phillies first baseman John Kruk and current NFL wideout Kevin Curtis. "I don't think any man likes to hear the news," Curtis says. "The word cancer is scary enough. When part of the answer is removing the testicle, it's, like, boom."

Cringing that way is a natural reaction known as the cremasteric reflex. In times of danger, anxiety or cold water, the testicles are reflexively drawn back up toward the protective pelvic cavity. According to papal legend from the first century, this response (also known as "shrinkage") was especially troublesome for potential popes, who were required to sit on a chair with a hole cut out of the seat, placing their manhood on display. The Old Testament says that men without testicles will not be permitted into heaven. So the cardinals would gather around, take a gander down under, then -- if everything was in its right place -- sing out, "Habet ova noster papa!" Our pope has balls!

For athletes, these gifts from god often inspire similar proclamations because, in the end, testicles govern athletic achievement far more than triceps or any other muscle. When someone has "guts" or "heart," it doesn't mean bigger intestines or a larger aorta. But when we say the Saints displayed major cajones for their on-sides kick in Super Bowl XLIV, there is an elemental truth to the statement. Because of that, there is a strong and unique link between balls, sports and the modern-day definition of manhood in America. With fewer ways to prove and celebrate masculinity, one set of balls has become profoundly connected and dependent on the other set of balls -- the baseballs, basketballs and footballs.

Few athletes understand this better than former Bears cornerback Virgil Livers, the Joe Theismann of testicle trauma. In 1976, Livers was blocking on a punt return against the Raiders when he took a knee to the groin. He wasn't wearing a cup, and the force of the impact caused his right testicle to explode on contact. Somehow, Livers went back out for the next defensive series, until the swelling in his scrotum began obstructing his stride. Trainers called for an X-ray; on film, Livers' testicle looked like a shattered light bulb. He was immediately sent to the hospital for surgery -- and drove himself. Before putting him under, the surgeon told Livers not to worry, that there was a reason most vital organs come in sets of two. Livers recovered, fathered two kids and went on to play five more seasons with the Bears and the Chicago Blitz of the USFL, wearing a cup for only a short time after his injury.

"Losing a testicle wasn't too much to give up to reach my goals, and I bet most pro athletes would say the same thing," says Livers, now a 59-year-old assistant high school principal in Indiana, who still signs dozens of cards sent to him every year by Bears fans. "That's what you do in sports: You give it all up for your team and leave it all out on the field."

Or, sometimes, on the operating room floor.

David Fleming is a senior writer for ESPN The Magazine.

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