For Horned Beetles, Size Equals Sexy
July 27, 2012

Horned Beetles With Bigger Horns Are Healthier, Sexier

Brett Smith for - Your Universe Online

Males of a species are always strutting their stuff for the sake of reproduction; peacocks have their plumage, rhinoceros beetles have their horns, and elk have their antlers.

These often unwieldy displays were deemed a burden to well-endowed males, but a team of scientists led by Douglas Emlen from the University of Montana has just uncovered a biological mechanism that suggests these ornamentations are the physical embodiment of healthy diet.

According to a report recently published in the journal Science, when the researchers disturbed the insulin-signaling pathway in male Japanese rhinoceros beetles, their horn growth was significantly stunted while the rest of the insect´s body grew at a normal rate.

"If you have a lot of food, you have a lot of insulin," said co-author Laura Corley Lavine, a Washington State University entomologist who took the male beetle´s point-of-view. "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."

Insulin is more commonly known as a hormone that some diabetics take, but it and the insulin growth factors it reacts with play a key role in animal development. Their levels control how fast different tissues can grow, depending on nutrition and overall health.

To test the impact of insulin on various types of tissue, researchers injected RNA that was engineered to shut down a specific insulin gene into the beetle larvae. Although standard insulin signaling had resumed after 72 hours, the injected beetles´ horn growth was stunted. Researchers noted that the affected insects´ genitalia grew normally despite the hormonal disruption. Additionally, their wings were slightly affected, developing 2 percent smaller than unaffected beetles.

"We're the first ones to make the link by explicitly tying the insulin pathway to the evolution of these kinds of male weapons," Lavine said in a prepared statement. "The discovery of the actual mechanism might now open new avenues of study for how exaggerated traits evolved, their genetic basis and the evolution of animal signals."

The new study repudiates the so-called “handicap principle”, which states that animals with outsized sexually-selective features are burdened with these displays. In fact, the more endowed animals are simply broadcasting their history of a well-nourished development.

Emlen explained that the insulin-growth mechanism has an immediate impact on an animal´s chances for reproduction and would be selected for accordingly.

“Historically, when bigger versions of traits cropped up, they would have been conspicuous and receivers, choosy females or rival males, that noticed them and responded to them would have fared well, since the trait is an unfakeable signal of male quality," he told the BBC.

"This would have favored receivers who paid attention to these traits, and individuals with bigger and bigger traits. The bigger they got, the more reliable they would become, enhancing this process and helping drive the evolution of bigger and bigger size."

The study´s authors said that the insulin signaling pathway extends throughout the animal kingdom because it is inherently successful at being passed from generation to generation.

"Horns and antlers matter," Emlen said in a statement. "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."