Why size matters for horned beasts
Horny beetles and stags share a common mechanism that helps them attract mates with over-the-top ornaments, scientists believe.
Insulin is the key to growing enormous horns, antlers and claws across a wide range of species, research suggests.
The appendages are acutely sensitive to the hormone's signals, providing a neat way to advertise fitness.
"If you have a lot of food, you have a lot of insulin," said Dr Laura Lavine, a member of the US team from Washington State University.
"You respond to that by making a really giant, exaggerated horn. Then the female can tell she wants to mate with you because you are truthfully advertising your condition."
The research focused on Japanese rhinoceros beetles, big insects whose males grow horns two thirds the length of their bodies.
When the insulin-signalling pathway was blocked, the horns were far less likely to grow and ended up stunted. Other parts of the beetle's body, including the wings, were much less affected.
Professor Doug Emlen, from the University of Montana, who led the study published in the journal Science, said: "Our research explains how these enormous traits get to be so enormous. People have known for 100 years that the best males produce the biggest structures, but nobody has really understood how."
The findings in rhinoceros beetles are likely to apply to many other species, he said. Previous studies have linked the same signalling pathway to the growth of red deer antlers and crab pincer claws.
Prof Emlen added: "Horns and antlers matter. Animals pay attention to them when they size each other up for battle. And females pay attention to horns or are attracted to males with really big tails. Why? Because only the best of the best can have really big horns or tails."