The huge horns on male rhinoceros beetles come surprisingly cheap.

Credit: DOUG EMLEN

Biologists have long thought that the extreme body parts that males use to gain mates — such as showy plumage or aggressive antlers — are costly because they make the animals vulnerable in other ways. By that logic, the horns of a male rhinoceros beetle should exact a toll.

But when Erin McCullough and Douglas Emlen at the University of Montana in Missoula set about quantifying probable costs in dozens of wild and laboratory-raised beetles (Trypoxylus dichotomus; pictured), they found none. Horn size had little or no effect on flying ability, survival, immune response or the growth of other body structures.

The researchers suggest that weapons may evolve along with traits that let males make best use of them, offsetting survival costs. The low cost could help to explain how the beetles' horns became so elaborate and diverse, the authors say.

Anim. Behav. http://doi.org/n34 (2013)