For a glaring manifestation of biological sex differences in strength, look no farther than the pull-up. The process of hoisting oneself eye-level with an overhead bar is no big deal to plenty of men. Not so, however, for women. In fact, Marines require male recruits to complete at least three chin-ups in order to pass their physical entrance exam, while female hopefuls aren't asked to execute a single one [source: Parker-Pope]. That isn't letting military women off the hook easily; the female body simply isn't optimally built -- what with weight distribution and less testosterone-fueled muscle mass -- for the exercise.
The short bursts of energy required for weight lifting might not be the forte of the female body, but as more women have begun participating in endurance sports, such as running marathons, conflicting research has prompted a debate over whether they're better tailored for the long haul than men. The fastest male runners are swifter than the fastest female runners due to innate factors including muscle mass, higher oxygen intake and lower resting heart rates. That said, some studies have indicated that in ultradistance running -- beyond 30 miles (48 kilometers) -- the fattier female body can keep moving more efficiently than the muscular male frame since the fat represents more lasting, slower-burning energy stores [source: Maharam]. Estrogen may also offer an advantage of protecting against muscle fatigue, although its effects can vary by athlete and running conditions [source: Crowther]. Those biological benefits may help explain women's sudden surge in Iditarod races, the grueling Alaskan dog sledding competition, bringing home championships four years straight from 1985 through 1988 [source: Library of Congress].
Meanwhile, what isn't up for argument is that on average, women win at the ultimate endurance competition: life. Even as public health improvements have increased lifespans around the world, women still tend to live longer than men by five or six years [source: Kirkwood]. Evolutionary biology points to women's responsibility as child bearers for why the female body ultimately may be more resilient, having evolved heartier healing capacities on a cellular level [source: Dillner]. Another explanation for that consistent gender gap circles back to testosterone, the hormonal culprit behind gender unequal upper body strength. In addition to being anabolic (muscle-building), testosterone also is characterized as androgenic (or masculinizing), which can take the form of men indulging in riskier behaviors that could eventually curb their life expectancies. Not that it has to signal an early death sentence for men; by keeping those muscular bodies in shape, courtesy of testosterone, they can possibly ward off its more deleterious effects.