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Outside magazine, June 1996

By Patrick Clinton

Why is it that an animal's eyes seem to glow when struck by light, while a human's don't?
Steven Rubin, Baltimore

Human eyes do reflect light, as all flash photographers know--they just don't do it very well. The distinctive "eyeshine" given off by cats and deer and crocodiles, among other species, comes from the tapetum lucidum, a mirrorlike layer of cells in or behind the retina. This structure is found mostly in nocturnal animals, for whom it serves as a kind of light amplifier: The retina captures some of the light that enters the eye, but some passes through. The tapetum lucidum bounces it back at the retina, giving the animal a second chance to "see" it. So why don't humans have this handy adaptation? Probably because we evolved in sunny climes where too much light, rather than too little, was our problem. Anyway, because the tapetum lucidum usually lies behind the retina, the light it reflects is slightly out of focus. Maybe you'd be willing to swap acuity for night vision; our species apparently wasn't.

It seems like the world's big continental "points"--like Cape Horn--all aim south. Is this significant?
Troy Kitch, Guam

You mean, did the continents long ago all melt and "drip" southward? Not at all. Geologists believe that Africa, South America, and India were all part of a megacontinent, which they call Gondwanaland, that fragmented millions of years ago. Africa and South America in particular have a similarly pointy aspect because they broke off Gondwanaland like two slices of the same pie. "If you drop a plate on the floor," says University of Texas geophysicist Lawrence Lawver, "you'll get some pointy pieces and some square pieces." That's what happened when Gondwanaland broke up. At the moment a lot of its pointy shards happen to be aimed south. In a billion years, they'll be pointing somewhere else. Geology is so fleeting.

Thanks for explaining why stars twinkle (January). But I'm curious: Why don't planets twinkle?
Ron Hasler, Highland Park, California

Stars are so far away that to us they're mere point sources of light; there's just one path for the light to follow from, say, Alpha Centauri to your eye, so the slightest bit of atmospheric distortion will make it appear to twinkle. The visible planets, even though they send us only reflected light, are much closer and occupy more space in the sky. Light from East Venus will follow a slightly different path to your eyeball than light from West Venus. There may be some twinkle on each separate path, but when you put them all together, your eye averages things out and you get a nice sober glow.

How do you extract the poison from a poison-dart frog?
Lynn Gayden, Shady Grove, Tennessee

In western Colombia, where the blowgun is still part of the hunter's kit, they do it by roasting one of two frog species (Phyllobates aurotaenia or Phyllobates bicolor) on a spit and collecting the toxic liquid exuded by the skin. Researchers there recently learned of another species, Phyllobates terribilis, which is so poisonous that natives only have to scrape a dart across its back. "I wouldn't touch that frog," notes John Daly, who studies frog juice for the National Institutes of Health. "It has enough toxin to kill lots of people, far more than it needs." If you're hatching some fiendish scheme, Lynn, bear this in mind: Of the 70 known species of poison-dart frogs, only the aforementioned three are poisonous enough to merit a place in the aboriginal arsenal. But since these frogs derive their poison from their forest diet, their toxicity fades once they leave the wild. So the pet-shop versions, I'm afraid, are basically just duds.

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