The following is an excerpt from Michael Shermer's book How We Believe: The Search for God in an Age of Science. Shermer writes the "Skeptic" column for Scientific American.
Werewolves figure prominently in mythology. The peak of prosecutions for lycanthropy -- the "condition" of a human taking on wolflike characteristics -- was in the sixteenth and seventeenth centuries in France. The most famous case was that of Jean Grenier who, in 1603, boasted to three girls that he was a werewolf, telling them that a man "gave me a wolfskin cape; he wraps it around me, and every Monday, Friday, and Sunday, and for about an hour at dusk every other day, I am a wolf, a werewolf. I have killed dogs and drunk their blood; but little girls taste better, and their flesh is tender and sweet, their blood rich and warm." Looking at this tale with the distance of almost 400 years, it is likely nothing more than male boasting and posturing; but since several children had been murdered at the time Grenier was fingered and convicted. Why a wolf? The earliest myths are associated with a ceremony of a man putting on a wolf's skin for protection from the cold, or to act as concealment when hunting for food. This mutated into the theme that the wearing of the skin passed on to the man great magical powers of strength, speed, and stealth, not just for hunting, but for exacting vengeance or gaining power over others. From here it was but a small step to changing the man into a wolf through the common mythic motif of shapeshifting, where creatures or objects can change into other creatures or objects, either at will or under special conditions. An evolutionary argument could also be made that, as pack hunters, wolves were a principal competitor to early humans in northern latitudes. Dogs, as loyal friends and noncompetitors to humans, do not generate such myths as wolves. (It also should be noted that werewolves did not have a monopoly on the genre. There were werebears, weretigers, werehyenas, werecrocodiles, and werejackals. Vampires were a type of werebat. Shapeshifting is found in countless myths, including the Burma-Assam tiger men who can share a tiger's body, or the leopard men of certain regions of Africa.)