Q) Which species has the fastest-growing population?
Harry Ottinger, Northbrook, Illinois

There's been no pan-species census to give us clear numbers on which critters are today's champion breeders, though the trillions of microorganisms no doubt fill the top spots. According to Tom Stohlgren, director of the National Institute of Invasive Species Science, when it comes to the higher phyla, invasive organisms are particularly fecund. Homo sapiens is one of the fastest-replicating mammals—we gain 6.2 million souls a month—and our invasive activities make way for most intruders. Other notable populaters include Mus musculus, the house mouse, which goes almost anywhere we do, and the feral pig, which can be found in 39 U.S. states, up from 18 in 1982. Then there's the dreaded cane toad, which was widely introduced in the Pacific islands for pest control in the 20th century and can drop 30,000 eggs at once. "It's Darwin on steroids," says Stohlgren. "In the U.S. alone, invaders cost us $120 billion per year. We just plain have to be better prepared."

Hit or Myth?

A famished reader asks, can you really survive by eating shoe leather? Many wayward explorers have munched on cowhide as a diet of last resort. Magellan's starving mates turned to leatherphagy during a 1520 crossing of the Pacific; the 1846 Dahmer, er, Donner Party supped on boiled shoe leather before turning to cannibalism; and in 1998, six Chinese miners trapped for 27 days devoured their belts before being rescued. But while such morsels may satiate a starving psyche, they do little to keep you alive. According to Wayne Askew, a nutrition researcher at the University of Utah who focuses on diets for extreme conditions, tanned leather is mainly collagen, a fibrous protein that's almost completely indigestible, and the chromium used in the tanning process can actually be poisonous in high concentrations. If your hankering for hide proves insatiable, allow us to suggest the hydrolyzed leather, a.k.a. "leather meal"—ground scraps processed with enzymes—which is used as a supplement in food for cattle, chickens, and dogs. Bon appétit!

Q) Will listening to music improve my training?
David Tilton, Lefifi, South Africa

It depends on your sport. When it comes to fine motor skills, like those used in a golf swing, music can actually worsen execution. New research also suggests that music offers little to no benefit for endurance athletes. But a recent study by kinesiologist Jimmy Smith, of Southwestern Texas University, indicates that, at least during five-to-ten-minute intervals of intense exertion, the right tunes can pump you up. Smith played music for subjects instructed to furiously pedal stationary bikes and found that it allowed them to last nearly twice as long and improved their tolerance to lactic-acid buildup. The key is to cue up something you dig. "Surprisingly, the tempo doesn't matter," says Smith, who favors working out to heavy metal. "Play music that you connect to, be it Mozart or AC/DC, and it will help improve performance." In other words, one person's "Achy Breaky Heart" is another's "Eye of the Tiger."

Q) Can bullets shot straight up in the air kill people?
Jayne Kelly, San Francisco, California

In 1920, the U.S. military concluded that bullets fired into the sky dropped to earth at less-than-lethal speeds; 30 years ago, many ballistics experts still believed this. Today, all respect the deadly potential of plummeting projectiles. According to Michael Haag, president of Forensic Science Consultants, in Albuquerque, New Mexico, a typical handgun shot rises about a mile. How fast it falls depends on variables like size, shape, and weather. Small rounds are likely to inflict minor injuries, but heavy pointed ones, like rifle bullets, can be fatal. The big danger is any shot fired at an angle, as gravity can only minimally slow its initial velocity (in the range of 800 miles per hour). A study by the King/Drew Medical Center, in L.A., estimated that falling bullets killed 38 city residents between 1985 and 1992. In fact, so many Angelinos go Yosemite Sam on New Year's that the LAPD has to broadcast public-safety-awareness messages each year in the days before Auld Lang Syne.