Stag males give nasty nips despite massive jaws 26 March 2014

Armed with a ferocious pair of mandibles, male transducer whenever it was held before them. And stag appear well prepared to take on the when she compared the strength of the male's and world. 'Their jaws are not just for ornamentation, female's bites, she was impressed to see that the they really use them to fight', says Jana Goyens male's jaws gripped with an impressive 7N force from the University of Antwerp, Belgium, adding (which is like having a 700g weight pressing on a that males grapple over the choicest patches of pin): six times stronger than the female's bite. Even rotten wood for their mates to lay their eggs in. when she scaled up the female's bite to take Describing a battle, Goyens explains account of the female's smaller stature, the male's that one beetle grabs the other one around its bites were still three times stronger. So, even body and then rears up in an attempt to hurl his though the male's long mandibles should reduce opponent over his head and onto its back. 'It is the strength of their nasty nips, they were still able clear which one is the loser', says Goyens. But to clamp on much harder than the females. They something puzzled her: 'It seemed unlikely that must have somehow compensated for their stag beetles could bite forcefully because they oversized mandibles; Goyens decided to take a have these long jaws', she says. Instead of closer look at the beetles' heads. amplifying forces, the beetles' long mandibles would reduce the forces exerted at the sharp teeth Comparing the males and females, Goyens halfway along the mandibles when gripped realised that the males' heads were much wider together. Intrigued, Goyens and her supervisors, than the females', suggesting the males had Joris Dirckx and Peter Aerts, decided to test how increased the size of some of the internal structures powerful stag beetle bites are. that move the mandibles to compensate for their size. Goyens then teamed up with Luc Van They report their discovery that male stag beetles Hoorebeke and Manuel Dierick to take CT scans of have enlarged their heads and muscles to insure the ' heads to find out what was going on that their bite force is not weakened in The Journal inside. She could clearly see that the muscle that of Experimental Biology. pulls on the mandible is almost four times larger in male stag beetles than in females, and the lever on Although the beetles are native to Belgium, they the side of the mandible that is pulled by the are endangered, so Goyens turned to Asia, where muscle is more than three times longer than the betting on fights is popular, to obtain the lever on female mandibles. And when Goyens . However, when 10 pairs of the animals checked the direction that the muscle pulls in, she arrived from just before Christmas, the saw that both the male's and females' muscles pressure was on. 'I had been told that they only were perfectly oriented to generate the hardest bite. live for a couple of months but I didn't know how So, the males have successfully compensated for old they were and it was the Friday before the their ungainly mandibles by increasing the size of Christmas holidays so I worked the entire holiday the internal structures that wield their mighty to get as much data as possible while they were weapons. still alive', says Goyens, who laughs when she recalls that the beetles went on to live for another More information: Goyens, J., Dirckx, J., Dierick, 12 months. M., Van Hoorebeke, L. and Aerts, P. (2014). Biomechanical determinants of bite force Testing the beetles' bite force, Goyens found that dimorphism in metallifer stag beetles. the aggressive animals were extremely J. Exp. Biol. 217, 1065-1071. cooperative, biting enthusiastically on the force jeb.biologists.org/content/217/7/1065.abstract

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