By day, Douglas Emlen studies insect weaponry, but at night, military history is his preferred reading. Eight years ago, these interests merged unexpectedly. As a professor of ecology and biology at the University of Montana in Missoula, he had spent 20 years examining the mating and territorial behavior of dung and rhinoceros beetles, well known for their very large horns, to understand why some insects spend so much energy building big weapons. Asked to write a review about weapons systems across the animal kingdom, he dug into the file cabinets full of papers on the subject that he had collected over the years. The more he read about tusks, horns, claws, antlers, and other weapons, the more enthralled he became. So, when the journal made him cut 10,000 words from the manuscript, he decided it was time to branch out and write a book for a general audience.