This month, the Japanese rhinoceros beetle (Allomyrina dichotoma) is making news because of its headgear (2nd image), which can grow to as much as 2/3 of its body length. Scientists have discovered that not only is the size of the forked horn useful for flipping rival males off a branch (see video here*), it is an unfailing indicator of his health and prowess. The horn is 8 times more sensitive to insulin and related molecules called insulin-like growth factors (IGFs) than any other body part, so well-fed beetles may have larger wings and bodies than poorly-fed ones, but they have much larger horns. A study by entomologist Doug Emlen of the University of Montana confirmed that the loss of insulin signals - triggered artificially in his experiments, but naturally by lack of nutrition - resulted in their horns growing 16% smaller. Dr. Emlen has shown that the reason the beetles can't grow flashy horns is not that they would need their strength to bear the heavy burden, but that the size of those ornaments is tied to their nutrition-dependent insulin levels and that can't be cheated. "They can’t fake their way to showiness."
*While researching this post, I learned that insect fighting is a popular spectator sport in Japan, China, and other Asian countries!