General mechanism for exaggerated sexuallyselected traits. Many animals wield sexually-selected exaggerated traits. 'Classic' examples include the peacock's train, and the antlers observed in male deer, as well as fiddler crabs with enlarged claws, and the enlarged head-horns of rhinoceros beetles (Trypoxylus dichotomus, cover). These traits are used for mate choice, or to deter rival males, because they act as unusually reliable signals of the condition of individual males: only the best-quality animals produce full-sized signal structures. But what keeps their expression honest? How can signal traits evolve that are resistant to invasion by cheaters who fake attractive signals? The answer may lie in the ancient insulin-like signalling pathway, which is found in all animals and directly links individual condition to growth in a dose dependent manner. Warren et al. discuss recent evidence suggesting that exaggerated sexually-selected signal traits arise when specific structures become extra-sensitive to physiological signals like insulins or insulin-like growth factors (pages 889-899).