Genetic correlations between weapons, body shape and fighting behaviour in the horned beetle Gnatocerus cornutus

Male fighting frequently results in the evolution of traits used as weapons. Additionally, males with well-developed weapons typically differ behaviourally and morphologically from weaponless males. However, the genetic basis to these phenotypic correlations has not been investigated. The broad-horned flour beetle, Gnatocerus cornutus, is a species in which males engage in fights using their enlarged mandibles. We conducted bidirectional selection on beetle mandible length to investigate the correlated responses in male behaviour and body shape. Mandible size diverged significantly after 10 generations of selection. We also found that the microevolutionary enlargement of mandibles affected male morphology and behaviour. Compensatory or supportive traits of the mandibles (head, prothorax, genae and forelegs) also became enlarged, but eye area, antenna, head horn and elytra length were all reduced. These correlated responses in morphology may be the result of developmental integration of these traits and mandibles, but the reduction in size of some traits could also be caused by trade-offs generated by resource competition between characters. In any case, the enlargement of the weapon (mandibles) altered male body shape into a suitable form for fighting. Fighting endurance also evolved as a correlated response to selection on mandible size, with lines selected for larger mandibles able to fight for longer. It therefore appears that morphology and fighting are genetically correlated with each other.