Seductive Rhino Beetles Boast Bigger Horns

By Chelsea Whyte on July 27, 2012 7:55 PM EDT

Rhino beetle
Rhino beetles don't just sport their horns for fun - they use them to attract the ladies. And because they're a direct reflection of their health, the horns act as truthful advertising for potential mates. (Photo: Creative Commons: e3000)

In the animal kingdom, healthy equals sexy. And beetles show off their superior health by growing exaggerated horns which advertise their robust bodies to potential female mates.

It all sounds a little tawdry, doesn't it?

Researchers found that these come-hither appendages, which vary greatly from small nubbins on tiny beetles to enormous prongs on larger beetles, are dependent on the amount of insulin the beetle's body takes in.

"If you have a lot of food, you have a lot of insulin," said co-principal investigator Laura Corley Lavine, a Washington State University entomologist. "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."

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The researchers disturbed the insulin-signaling pathway in Japanese rhinoceros beetles - large insects that can grow horns two-thirds the length of their body - by injecting a cocktail of double-stranded RNA into their larvae, shutting down the desired gene.

Within 72 hours, normal insulin signaling had resumed, but horn growth was significantly stunted. The appendages were more sensitive to a lack of the hormone than the wings, which continued to grow eight times as much as the horns. Genitalia grew normally and body growth was only slightly affected.

"Our surprise finding that the same change in mechanism that appears to make an ornament or weapon especially big also makes them hyper-variable and super-sensitive to nutrition, means that these structures are likely to be intrinsically reliable as signals," co-principal investigator Doug Emlen told BBC Nature.

In essence, that means that beetles don't engage in false advertising. Healthy looking horns mean a truly healthy insect.

"Our research explains how these enormous traits get to be so enormous," Emlen said. "People have known for 100 years that the best males produce the biggest structures, but nobody has really understood how. Our work looks under the hood to explain why so many sexually selected structures get so massive."

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