Why Do Stag Beetles Have Big Jaws? Antlers for Fighting

Antlers for wrestling, not for eating

In the European stag beetle, only the males have the famous “antlers”. They are enlarged mandibles, the same pair of jaws that other beetles use to bite and chew food, stretched to a length that can be nearly a third of the beetle’s total length.

Males meet at the places where females are found: sap oozing from damaged trees, fallen fruit, and old stumps and logs where females come to lay eggs. When two males meet there, they face each other with jaws open, then lock together. Each tries to grip the other around the body, lift him off his feet and throw him off the branch or log. The beetle that keeps his footing usually stays with the female.

This kind of competition between males, called sexual selection, favors bigger weapons generation after generation, in the same way as deer antlers or the horns of rhinoceros beetles. Charles Darwin discussed stag beetles and their exaggerated jaws when he first described sexual selection in 1871.

Why do jaw sizes vary so much?

Not all males carry huge antlers. Jaw size depends mainly on how well the beetle fed during its years as a larva underground. A larva that grew large on plenty of rotting wood becomes a big male with very long jaws, while a poorly fed one becomes a small male with short jaws. Adult beetles do not grow, so this size is fixed for life.

The relationship is not simply proportional. In stag beetles, larger males have relatively longer jaws than small males, so a big male’s antlers look disproportionately huge. Small males with short jaws are common in the wild, and they sometimes avoid fights and try to mate with females while bigger males are busy.

How long the larvae take to grow, and why dead wood matters, is described in how long do stag beetles live.

Do females bite harder than males?

To a person handling them, females often do. A female’s jaws are short, curved and sharp, so the force of her jaw muscles acts close to the head and is concentrated at the tips. She can give a painful nip that may draw a little blood. A male’s long jaws act as long levers, so the force at the tips is weak, and a male usually manages no more than a mild pinch.

That does not mean male jaws are weak. Research on a tropical stag beetle found that males have greatly enlarged jaw muscles and bite much harder than females when clamping something near the base of the jaws, as they do when squeezing a rival. Their jaws are specialized for gripping and lifting another beetle, not for biting a finger. Neither sex is venomous or dangerous.

The cost of carrying antlers

Big jaws are heavy and awkward. Male stag beetles fly with their bodies held almost upright, a posture that helps balance the weight of the antlers, and they are clumsy, noisy fliers. The jaws also make males easier for birds such as magpies and crows to spot and catch.

Males have only a few weeks as adults to find a mate, and they cannot eat solid food during that time. They live mainly on fat reserves built up as larvae and top up with sap and fruit juice using a soft, hairy tongue-like mouthpart. The jaws play no part in feeding.

What do females use their jaws for?

Female stag beetles use their jaws in more everyday ways. Their strong, compact mandibles are thought to help them dig down into soil and rotten wood when they search for a place to lay eggs. They also give females a means of defense.

During mating, the male uses his jaws to hold the female and to keep rival males away, which is another reason large jaws pay off.

Even bigger jaws around the world

The European stag beetle is only one of more than 1,000 stag beetle species. In some tropical species the jaws are even more extreme. The giraffe stag beetle of southern and southeastern Asia has exceptionally long, slender jaws and is one of the longest stag beetles in the world, and the Chilean stag beetle (Chiasognathus grantii), which Darwin collected and wrote about, has long, curved, toothed jaws. Other beetles use horns instead of jaws, such as the Hercules beetle, whose males fight with a huge pincer formed by a horn on the head and another on the thorax.

Across all these species the rule is the same: the most extreme weapons are found in males that compete directly for females.