Sexually selected traits and life history traits of larger and smaller males of the horned flour beetle Gnatocerus cornutus

1. Expression of sexual weapons is strongly affected by environmental factors in armed insects, resulting in a remarkable morphological difference between larger and smaller males. 2. Morphological differences in weapon between larger and smaller males can promote differences in non‐sexual traits. This is because weapon sizes are often associated with developments of non‐sexual traits such as somatic traits and life history traits. The resulting different subsets of multiple traits may contribute to the reproductive success of larger and smaller males, respectively. 3. There were relatively few studies that couple differences in life history and reproductive traits between larger and smaller males. Here we investigated differences in morphology, behaviours, and life history in Gnatocerus cornutus. 4. Larger males have relatively larger mandibles and the advantage in male fighting to access females. Also, the developmental period was significantly shorter in the larger males than in the smaller males. 5. Smaller males with rudimentary weapons have higher locomotion. This suggests higher performance in the dispersal to new territories. Larger and smaller males showed different suites of multiple traits, and the combinations of multiple traits are probably related to expression of weapons. In armed beetles, expression of weapons exhibits remarkable phenotypic plasticity, resulting in positive allometry. In Gnatocerus cornutus, larger males have relatively larger mandibles and show faster larval growth. By contrast, the smaller males with rudimentary weapons have higher locomotion. Larger and smaller males showed different suites of multiple fitness‐related traits, and the combinations of multiple traits may contribute to the reproductive success of larger and smaller males.