The horns of creatures as different as elk and rhinoceros beetles - along with other decorative, mate-attracting structures -  are sensitive to changes in nutrition. They aren’t diabetic, but they are insulin-dependent; if they want to grow big horns and therefore attract mates.

Michigan State University zoologist
Ian Dworkin was part of a team that found that the key ingredient in both elk horns and peacock feathers, and therefore mating success, was insulin. Sexual selection has roots back to Darwin’s research. Subsequent research revealed the so-called 'handicap principle' which labeled males as burdened for toting such unwieldy baggage. The researchers believe that when insulin-dependence is part of the picture, the showy males are not handicapped at all. Instead, the insulin-dependence of these big horns provides a way for the males to show how great they are.

Lowering insulin levels dramatically reduces size of beetle horns, elk antlers and peacock tail feathers. Credit: MSU.

“Clearly elk antlers, peacock tail feathers and beetle horns are very different, but it appears that they do share similar mechanisms to make these structures so big,” he said. “And lowering insulin levels dramatically reduced the size of their ornate structures. It’s a sign that these males are thriving, made of some pretty sturdy stuff and certainly mate-worthy,” said Dworkin.

They determined that each time such exaggerated traits evolve, they repeatedly but independently seem to use insulin-dependence. This suggests that the traits are more likely to have evolved as honest indicators of quality rather than handicaps.

“While more work needs to be done, our results provide and important way of linking genetic mechanism with the ultimate evolutionary reason for the trait exaggeration,” Dworkin said.

Citation: Douglas J. Emlen, Ian A. Warren,Annika Johns,Ian Dworkin,Laura Corley Lavine, 'A mechanism of extreme growth and reliable signaling in sexually selected ornaments and weapons', Sciencexpress, July 26, 2012