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Weapons important for stag beetle evolution.

By JW Dowey - 16 Apr 2015 8:56:28 GMT
Weapons important for stag beetle evolution.

Stag beetles fighting image; Credit: © Jana Goyens

When you fly, the mass you carry is a payload, for which you must gain some advantage. A flying animal can be expected to gain from any structure that is not contributing to perfect streamlining or an aerofoil shape. With stag beetles like this Cyclommatus metallifer the males’ large mandibles or horns would seem to be counter-productive to any flight efficiency, despite their usefulness in breeding behaviour.

If we were to examine the peacock train, the flight of an eagle with its enormous talons and beak, or a bird like the shoebill with an enormous beak the mass of the creature should be related to an ideal flight weight. Because we are dealing with beetles, the running of the animal has been noted to be unstable because the heavy offensive weapons take up half the length of the body. The stag finds running 40% more costly to its energy expenditure when it has the horns. Flying must therefore be undertaken so that females or nesting sites can be reached aerially rather than by terrestrial means.

Using fluid dynamics simulation, Antwerp and Ghent Universities (Belgium) researchers, Jana Goyens, Sam Van Wassenbergh, Joris Dirckx and Peter Aerts investigated the energetics of flight required 26% more mechanical work when the horns were present. These painless simulations come as completely novel experiments, given that their real-world equivalents have always been impossible! The size and the shape was also varied in the steady state models, so that the effects on flight could be gauged with various alignments. Only a negligible influence was detected, at around the order of 0.1% of the flight performance.

Conclusions about the weight of these extraordinary horns are related to any excess mass to be carried, not the head designs, so that stags have been able to evolve a diverse range of weapons from their basic mandible shape/size. As in the infamous Irish elk, sexual selection has produced these horns. They signal to females and males that the individual is able and fit to breed, with little in the way of energy costs - or as little as possible. With walking so costly, the bite force afforded by these mandible must be providing the insect with strong advantages in battle. Rhinoceros beetles have very light horn, with little musculature, so they fly light, and faster. The stag beetle copes with ¬his heavy mandibles by flying at an angle of 58o at a top speed of 0.57 metres/sec.

The stag beetles stand out therefore as incurring a great cost in maintaining their mandibles as unstable heavy weapons. The massive head muscles and great horns form 28% of body mass in Lucanus, for example. The result is small wings and low flight speed, leading to another conclusion that the heavy mandibles are constrained in mass by natural selection for flight costs. Sexual selection seems to lose out, until you consider the massive size that the stag’s armour has already attained. The theory is that the selection of the large weapons in so many stag beetle species has swamped any aerodynamic effect and size and shape of the .horns. The mandible evolution of shape and size has blossomed worldwide, pleasing Asian schoolboys, in this case with an Indonesian (Sulawesi) rainforest species, and entomologists everywhere.

The Journal of the Royal Society Interface publish this paper as Cost of flight and the evolution of stag beetle weaponry.