The horn of a Japanese rhinoceros beetle (Trypoxylus dichotomus) can grow to be two-thirds the length of the rest of its body. And size matters. The male beetles use their horns to battle over feeding sites, where they also get access to female beetles. The longer the horn, the more reproductive success. So what limits horn-size? And why do some beetles have big horns and others puny ones? Biologists Doug Emlen and Erin McCullough of The University of Montana are looking into it.