Abstract
Sexually selected ornaments and weapons are among nature's most extravagant morphologies. Both ornaments and weapons improve a male's reproductive success; yet, unlike ornaments that need only attract females, weapons must be robust and functional structures because they are frequently tested during male-male combat. Consequently, weapons are expected to be particularly costly to bear. Here, we tested the aerodynamic costs of horns in the giant rhinoceros beetle, Trypoxylus dichotomus. We predicted that the long, forked head horn would have three main effects on flight performance: increased body mass, an anterior shift in the centre of mass and increased body drag. We found that the horns were surprisingly lightweight, and therefore had a trivial effect on the male beetles' total mass and mass distribution. Furthermore, because beetles typically fly at slow speeds and high body angles, horns had little effect on total body drag. Together, the weight and the drag of horns increased the overall force required to fly by less than 3 per cent, even in the largest males. Because low-cost structures are expected to be highly evolutionarily labile, the fact that horns incur very minor flight costs may have permitted both the elaboration and diversification of rhinoceros beetle horns.