Most people keep papers in their file folders. Roger Knutson keeps animals. Dead animals, of course -- it's the only way they fit. Such as the "marvelously flat" Key Largo snake his daughter sent him, which was as smooth as a piece of construction paper.

"I don't go out and collect these things, but when I do come into possession of a particularly interesting one, I keep it," he says. "They're like a piece of leather. They genuinely become two-dimensional over a period of time."

Like Knutson's other prize specimens, the snake met its untimely demise under the wheels of a car. Exactly why and how this happens is a subject the professor of biology at Luther College in Decorah, Iowa, has become something of an expert on.

His preliminary conclusion -- that roads are animal habitats, much like marshes or forests -- was reinforced during field trips with students. Trying to keep them from snoozing, he says, "you point out these things, talk a little ... What shapes do animals have on the road after they've been there a couple of days? Well, that's predictable, depending on the species and a number of other variables."

Less predictable was that Knutson's book on the subject, Flattened Fauna, would be totally serious, even if it's written in a slightly deadpan way. (Hognosed snakes will play possum, "and, if harassed by a near miss, will curl up and give a most convincing demonstration of lifelessness -- a demonstration that almost inevitably turns into the real thing.")

Not everyone seems to be getting the message. Olsson's Books, for instance, stocks it in their humor section. Perhaps this is understandable considering the publisher is Ten Speed Press, which has previously issued such similar but distinctly unserious titles as The Original Roadkill Cookbook.

"I'd hate for people to think it was another 101 Uses for a Dead Cat," says the 54-year-old author. "It ought to be with the other field guides. Any humor is incidental, rather than the reason for the book's being. What I'm trying to do is communicate something -- the behavior of animals." And since the only wild animals seen by most people are lying on the road, that's his starting point.

However, most of the details in the book are about the fauna during life, not death. The subject that intrigues Knutson is the transition between the two, and why certain animals are more likely to make this passage on pavement.

"An obvious example is the skunk. Skunks are killed on the road often, and found in greater numbers than you'd predict from their numbers off the road," he says. The reason: "Skunks don't have a reason to run away from anything in the natural world. They're not smart, and so they don't treat a truck or car differently. In another part of the country, armadillos do the same thing."

A highway, in this biologist's view, resembles the supernatural element in a Stephen King movie. "Effectively, the road is a predator, and it functions like other predators do -- somewhat selectively. You don't find just anything there."

What you do find are snakes, toads (when flattened, "often a forelimb is extended as though the toad were waving goodbye"), sparrows, crows, pigeons, starlings ("you don't find many mammals in urban areas," he notes, "but a fair number of birds"), mice, chipmunks, cottontail rabbits, porcupines, and road runners ("often described as a striking bird, more often it is a struck bird").

Flattened Fauna doesn't want budding biologists to actually stop and look closely at the victims. For one thing, that's unsanitary; for another, it's a good way to get flattened yourself. The book is meant to be used at highway speeds, and focuses on those identification features that are most easily visible and most durable.

Knutson also emphasizes that he's "certainly not encouraging people to contribute to this assemblage" by purposely aiming for any live wild animals they see. (Since a dog or cat getting hit is a family tragedy, neither does he consider those.) "You study what's there -- not what you would like to have there, but what's there."

Somewhat morbid, yes, but the author doesn't find his new fame unpleasant. "Many people have come up to me and said, 'I never see a dead animal on the road without thinking of you.' Taken literally, of course, that doesn't convey exactly what I want, but I know what they mean."