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Outside magazine, July 1999

Why are saltwater fish unable to survive in freshwater, and vice versa?

Jason Brewer, Waddy, Kentucky

The underlying science can be a little complicated—involving divalent ions and osmotic gradients and other unspeakable torments from advanced chemistry class—but the crux of the problem comes down to piscine physiology and osmosis. The kidneys of saltwater fish are designed to discharge a concentrated, viscous urine that's laden with the excess salt they constantly take in through the gills and mouth. If a red snapper ventured into a river or lake, it would quickly swell up and die from fluid retention, since its ill-equipped kidneys would be unable to pass the strangely thin urine quickly enough. Because the kidneys of freshwater fish, on the other hand, process an extremely dilute, saline-free urine, a brown trout that blundered into the Atlantic would dump too much precious H2O while simultaneously ingesting parching amounts of salt—a deadly combination that would cause the beleaguered piscine to shrivel up and expire from acute dehydration. Of course, there are anadromous species such as steelhead and salmon that can go both ways, but before a salmon smolt can head into the open sea, it spends a week or so acclimatizing in the brackish water of an estuary, its thyroid gland and gills undergoing a metamorphosis that enables it to withstand the ocean's high salt concentrations—33 parts per thousand. "All those salts are trying to suck the water out of a fish's body," explains marine biologist Richard Rosenblatt of the Scripps Institution of Oceanography. "Throw a freshwater fish into the sea and it's like putting a jungle orchid in the Sahara."


What are the Seven Natural Wonders of the World?

—Mary Murphy, Maynard, Massachusetts

This just in: though practically every roadside waterfall or freak rock formation in America touts itself as one of the seven natural wonders of the world, there is no sanctioned roster and never has been. It's all just marketing hokum. "From a tourism point of view, it's advantageous to advertise your claim to be such a site," explains Roger Payne of the U.S. Board on Geographic Names. "But the fact is, there is no official list." The concept does, how-ever, play off a venerable designation: the Seven Wonders of the Ancient World, a classical septet of ancient architectural achievements, including the Hanging Gardens of Babylon, the Colossus of Rhodes, and the Pyramids of Giza. The concept of such a list was first suggested by the Greek historian Herodotus around 400 b.c. and went through several subsequent refinements. Although the Greeks failed to follow up with a pantheon of natural wonders, there are several contemporary lists that do nature's star attractions some justice. With the recent addition of the Golden Mountains of Russia, the United Nations roster of World Heritage Sites (locales having "universal value from the point of history, art, or science") has swelled to an unruly 582 and counting. And the National Park Service has tapped some 587 significant places in America, such as the Palisades of the Hudson, as national natural landmarks. Despite these seemingly comprehensive registers, some sites still cling to the notion that they belong among a mystic and essential Big Seven. When we asked Natural Bridge of Virginia marketing director David Parker which authority bestowed the span's lofty superlative ("One of the Seven Natural Wonders of the World!" proclaims its glossy brochure), he conceded, somewhat sheepishly, "I couldn't tell you in my wildest dreams."


How does bug repellent work?

—Grant Burnett, Austin, Texas

Experts will tell you that only one thing repels mosquitoes with any lasting efficacy—a synthetic chemical called N, N- diethyl-m-toluamide, more commonly known as deet. Developed by Agriculture Department chemists in the late 1940s to protect army troops in malarial jungles, deet is the active ingredient in most insect repellents sold today—especially the stronger "sportsman" formulas that last for up to four hours and are marketed for serious forays into buggy outbacks. (Deet's potency is infamous: After several widely reported cases of severe skin rashes in the 1980s, EPA tests concluded that the chemical, in large quantities, may cause allergic reactions but is noncarcinogenic.) Strictly speaking, deet is not a repellent at all, but a sly and sophisticated chemical saboteur that effectively shuts down a mosquito's onboard radar system. By using acute antennae receptors implanted in its mouth and eyes, a blood-feeding insect can detect the invisible miasma of carbon dioxide that hovers over you from as far away as 70 feet. Rising off your body in steady fumes, deet clogs the bug's receptors, assailing its tiny brain with hundreds of confusing nerve signals. Befuddled, the pest hunts for a warm spot on your skin where the blood thrums close to the surface—to no avail. "A mosquito drunk on deet is suffering from information overload," explains University of Florida entomologist Jonathan Day, "and it may take a few hours to clear its head."

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