Elaborate horns in a giant rhinoceros beetle incur negligible aerodynamic costs

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.

Cost of flight and the evolution of stag beetle weaponry

“…The mandibles of the male C. metallifer individual that was used to build the model weighed 8.7% of its body mass is relatively slow compared with similarly sized rhinoceros beetles [3,4]. We implemented these characteristics in a CFD model, to calculate the aerodynamic forces and moments on a steadily and horizontally flying male stag beetle body (without the wings).…”

“…This is, for example, the case for flying with rhinoceros horns (1.5% of body weight), hummingbird tails (0.4% of body weight) and peacock trains (6.9% of body weight) [3,5,24]. Species for which substantial costs of ornaments were observed often compensate for these costs, for example, by increasing power generation with a larger wingspan (e.g.…”

“…Even in species with very impressive ornaments and armaments, the costs are often relatively small. For example, peacock trains (6.9% of body mass) do not decrease take-off performance, and rhinoceros beetle horns (1.5% of body mass) do not influence flight distance, travel speed or aerodynamic costs [3][4][5]. The detection of such costs is even harder when animals have also evolved compensatory traits [2].…”

“…rhinoceros horns and avian tails), it was argued that the low cost of these armaments/ornaments causes their remarkable diversification owing to more labile evolution [3,24]. However, the origin of the low armament/ ornament cost differs: the selective pressure on stag beetle mandible morphology is minimal because of the dominant effect of weight on the net flight cost, whereas for rhinoceros horns and avian tails this is the case because of a relative large wake from the body compared with the armament/ ornament [3,24].…”

Finite-element modelling reveals force modulation of jaw adductors in stag beetles

“…Male stag beetle mandibles should be lightweight, to enable efficient walking and flying [7,11]; their shape may have to be aerodynamically favourable [7,12]; and for the fights themselves, the mandibles should not only provide grip, transfer pinching forces and reach towards opponents, but they must also be strong enough to withstand the forces acting upon them. That these conditions pose conflicting demands on the weapon morphology has also been observed in other taxa.…”