Biomechanical determinants of bite force dimorphism in Cyclommatus metallifer stag beetles

In the stag beetle family (Lucanidae), males have diverged from females by sexual selection. The males fight each other for mating opportunities with their enlarged mandibles. It is known that owners of larger fighting apparatuses are favoured to win the male-male fights, but it was unclear whether male stag beetles also need to produce high bite forces while grabbing and lifting opponents in fights. We show that male Cyclommatus metallifer stag beetles bite three times as forcefully as females. This is not entirely unexpected given the spectacular nature of the fights, but all the more impressive given the difficulty of achieving this with their long mandibles (long levers). Our results suggest no increase in male intrinsic muscle strength to accomplish this. However, morphological analyses show that the long mandibular output levers in males are compensated by elongated input levers (and thus a wider anterior side of the head). The surplus of male bite force capability is realized by enlargement of the closer muscles of the mandibles, while overall muscle force direction remained optimal. To enable the forceful bites required to ensure male reproductive success, male head size and shape are adapted for long input levers and large muscles. Therefore, the entire head should be regarded as an integral part of male armature.