The Wild File

Q: While walking on beaches I've heard the sand squeak with every step, like tires squealing on pavement. What causes this, and why? —Peter Kellner, Telluride, Colorado

A:Generally, for sand to "sing" the grains must be fine, well sorted by the wind, and nicely rounded—like the sand found on the beaches of Baja or Australia, for example. Here's why, and how: On a soft strand, the weight of a footstep displaces the granules; if they're spherical they roll against each other, and the friction makes them vibrate and squeak. Larger particles, even gravel, make something like a low-pitched bark, while smaller particles make a high-pitched whistle because they vibrate at a higher frequency. Odd-size gunk like superfine clay or a shard of seashell interrupts the percussion and quiets the sound. And soggy or muddy beaches are often too firm and compact to vibrate and sing at all. Remarkably, this is merely one note in the repertoire of sabulous music. Sand can actually boom under rarer circumstances—namely, when a windstorm builds an unstable cornice on a 500- to 1,000-foot-high dune, like those of Colorado's Great Sand Dunes National Monument. As soon as the gale subsides, the dune avalanches a couple of inches, compressing under the weight of the sand until it finds a comfortable angle of repose. Acting like a giant kettledrum, the body of the dune amplifies the noise and makes a thunderous clap—at about the frequency and volume of a howitzer cannon.
Q:When a spider builds a web from tree to tree, how does it string the initial thread across such a wide distance? —James B. Moon, Oak Creek, Colorado

A:It doesn't take much legwork. All the arachnid has to do is release an ultralight silk strand--one of seven varieties the average web-building North American temperate forest spider manufactures in its abdominal silk glands—and the wind, which can carry the fine thread up to six feet away, does the rest. Electrostatic forces (the same ones that stick balloons to TVs) or a simple, breeze-induced tangle affixes the thread to a tree branch, blade of grass, flower stem, or whatever else it happens upon. After this nearly invisible temporary line is set, the spider uses it as scaffolding to walk along and unspool what entomologists call the "bridge thread," the first structurally solid span, upon which the rest of the web will hang. It then walks back on the bridge thread while cutting the initial thread with its fangs and tidying it up into a ball that will eventually be consumed and recycled for future tightrope jaunts. Finally, the enterprising spider puts up the web's circumference, attaches spokelike radii, and erects the trademark sticky spirals—the snare's deadly vortex. All done in roughly 40 minutes, start to finish.

Q:Do kestrels really have ultraviolet vision that allows them to track the lemmings they hunt? —Julie Colhoun, San Francisco, California

A: "Birds would be lost to some degree without their ultraviolet vision," says Innes Cuthill, professor of biology at the University of Bristol in England, "in the way that a color-blind person is disadvantaged." That's because, ornithologists suspect, many birds rely on their ultraviolet vision, the ability to see short-wavelength radiation beyond the humanly visible spectrum, to detect food. According to a recent Finnish study, kestrel prey, like lemmings and voles, appear particularly cursed. Unlike the UV-dull berries and insects that many birds feed on, the urine of these small rodents reflects ultraviolet light, glowing brightly at the edge of Northern Hemisphere forests. The sheen is undetectable to us (thankfully), but to the falcons, a string of tinkle across an open field probably looks like runway landing lights. Circling 50 to 100 feet above, the keen-eyed kestrels connect the glowing dots to track the scuttling vermin easier than if they wore neon "Lunch" signs.

Q:I once thought I was awakened by the distant smell of smoke. Can olfactory stimuli wake one up the way bright light or noise can?—Nancy Rice, Sylvania, Ohio

A:If smoke had a pungent oniony odor, it probably could rouse you. That's because an irritant-detecting nerve in the nose, the trigeminal, becomes increasingly stimulated as scents become stronger and more bothersome. Smelling salts, fertilizers, and household cleaning products, for example, send the trigeminal nerve into a frenzy, thanks to their high concentrations of ammonia. The chemicals in smoke, on the other hand, generally aren't irritating enough to tickle the nerve to the point of disturbing us from sleep, or so experts theorize. It's possible that you have a delicate and hypersensitive trigeminal and it woke you after smelling smoke—but don't disconnect the fire alarm: Where there's smoke, there's carbon monoxide. The CO from a burning building is likely to put you into a coma before your nose alerts you. While this news doesn't bode well for readers currently asleep in flaming homes, most of the time snoozing through a faint whiff of smoke or other olfactory stimulus is an advantage, says Alan Hirsch, the neurological director of Chicago's Smell & Taste Treatment and Research Foundation: "If you were to wake up with every little smell, you'd never get to sleep."

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