Stag beetles' unwieldy jaws are surprisingly slick in flight

Stag beetles wield fearsome mandibles almost as big as their bodies, but you'd think they're a bit of a drag when it comes to flying. Now it seems the oversized jaws aren't an aerodynamic burden after all, allowing them to take on a variety of shapes and sizes.

To exert dominance and win the attention of females, male stag beetles use their jaws to fight each other. Their weaponised heads comprise as much as 18 percent of their body mass, meaning a running male beetle uses up 40 per cent more energy than a female. The top-heavy mandibles give the males an unstable gait that threatens to send them tumbling. To find females and nesting sites, males have to fly – with their rather awkward baggage.

"You see that these jaws are so large and they're not streamlined at all," says Jana Goyens of the University of Antwerp in Belgium. "You might think there is a very large aerodynamic cost."

But when Goyens ran computer simulations of a beetle inside a wind tunnel, she found that flowing air exerts negligible aerodynamic forces on the insect. The beetle is so heavy that the force of gravity on it is much larger than any drag forces, so it can fly just as well no matter how its jaws are shaped. Drag also doesn't matter because beetles fly so slowly, at roughly one-twentieth the speed of small birds, which limits the amount of air resistance they feel.

Goyens' results are consistent with recent studies on rhinoceros beetles, which are also plodding flyers. "This is another piece of evidence that for animals that fly slowly, subtleties of drag are not that important," says Bret Tobalske of the University of Montana in Missoula.

In the case of stag beetles, the lack of drag means there is no evolutionary pressure for the mandibles to become aerodynamic, Goyens says. Instead, the beetles have evolved mandibles in a range of killer styles, from stubby and claw-like to long, spear-like protrusions.

Goyens now wants to explore the mechanics of the jaws and how they stand up to the males' fierce battles.