The male Japanese rhinoceros beetle, Trypoxylus dichotomus, which lives on Japan’s main island, has big horns that are used as weapons to fight other males for females. Scientists have sought the developmental mechanism that creates these horns, and to this end, a research team at the National Institute for Basic Biology in Japan has identified sex-determining genes for the rhinoceros beetle, and has succeeded in identifying the timing of sex differences that appear in horn primordia.