If the rhinoceros beetle were the size of an actual rhinoceros, its horn could be 16 feet long. Male beetles grow this gargantuan face-fork so they can win mates (why else?). And even though evolutionary science would predict that the beetle pays a price for this appendage, it seems to come absolutely free.
Males of many animal species wear showy accessories: antlers on deer, long tails on birds. Growing one of these accessories often comes at a cost. For example, energy spent growing one large body part may leave another body part smaller, as seems to be the case with the dung beetle's horns. Or the showy feature may make the animal more vulnerable, as in the Bahamas mosquitofish, which grows a large sperm-delivery organ to impress females but then can't swim away as quickly when chased by predators. Females benefit from being choosy, because males that can afford to spend resources on a fancy headpiece or tail demonstrate that they're hardy or have good genes.
Erin McCullough, a PhD student at the University of Montana, Missoula, and her advisor, Douglas Emlen, have been putting rhinoceros beetles through the wringer to try and find the cost they pay for their giant horns. Individual males grow horns of widely varying sizes. In the Japanese rhinoceros beetle, Trypoxylus dichotomus, horns range from a stubby 7 millimeters to a towering 32. In other species, the largest horns are 10 times the length of the smallest ones.
In a previous paper, the researchers showed that larger horns—somehow—don't hurt the rhinoceros beetle's ability to fly. Now, they measured the horns of T. dichotomus beetles and compared their size to the insects' legs, wings, eyes, and genitalia. They also tested the strength of the beetles' immune systems. And by marking beetles with paint, releasing them outdoors, and recapturing them later from the same area, the researchers assessed whether larger horns make a beetle more likely to die.
The result was a big goose egg. Nothing. If you're a rhinoceros beetle, there is apparently no trade-off to growing the biggest horn you can.
So why aren't all horns huge? Males with larger bodies are able to grow disproportionately longer horns than smaller beetles; Emlen found in an earlier study that this is tied to the beetles' insulin levels. "Males that have poor nutrition and therefore have low levels of circulating insulin simply can’t produce big horns," McCullough explains.
Still, if big horns are so great, evolution might favor males who can use their good nutrition to grow ever-larger appendages. Why is there any limit on the size of the horn? "I think the primary reason...is because they are weapons that are continuously tested in combat," McCullough says. Male rhinoceros beetles use their horns to fight each other for the best territory on tree trunks and branches. Grappling over sap-rich sites, they wield their horns like pitchforks to pry rivals loose. "So it doesn’t benefit a male at all to have a horn that’s so large that he can’t use it properly," she says.
McCullough is currently testing that idea by measuring the force needed to pry a male beetle from a tree and comparing it to the force needed to snap the beetle's horn. She thinks longer horns are at more risk of breaking, and that this may be what limits their size.
The reason rhinoceros beetles escape paying for their horns might be that they're functional, and not merely a decoration. When birds pay a price for a showy tail, it ensures that only the genetically strongest birds can give the best display to females. If unhealthy birds could cheat and grow fancy tails at no cost, females would no longer benefit from favoring good tails—so they'd stop paying attention at all, and males would stop bothering. But because there's a cost, the system works. In the case of the rhinoceros beetle, McCullough and Emlen believe cheaters are weeded out because they can't fight with their oversize horns. This means the flashy gear comes for free—as long as the beetle knows how to use it.
Erin L. McCullough, & Douglas J. Emlen (2013). Evaluating the costs of a sexually selected weapon: big horns at a small price. Animal Behaviour DOI: 10.1016/j.anbehav.2013.08.017
Image: McCullough & Emlen.