Fighting, dispersing, and sneaking: body-size dependent mating tactics by male Librodor japonicus beetles

Abstract 1. Scaling relations between weapons and body size depart from linearity in many male beetles. In many previous studies, these males have been divided into major and minor males with a switch point, that is male dimorphism. Major and minor males adopt strikingly different reproductive tactics.

2. We found three size-dependent behaviours, i.e. fighting, dispersing, and sneaking, however, among Librodor japonicus males with dimorphic mandibles. We statistically classified males into large, medium, and small (L-, M-, and S-males) sizes and then compared the dispersal of males from a foraging site, behaviours to gain access to females, and sizes of mandibles, wings, and testes.

3. M-males dispersed earlier than L- and S-males from a territory in a field, but no difference in the frequency of dispersal was observed between L- and S-males. Observations of male–male interactions in the laboratory showed that L-males frequently fought with other males in a fighting arena, while S-males often showed sneaking behaviour without fighting.

4. On the basis of the morphological analysis, we concluded that S-males invested their available resources more in sperm (= testes), M-males more in wings, and L-males more in mandibles in L. japonicus.

5. Even though a morphological male dimorphism was detected, it might be possible to classify the males of the armed beetles into more than two behavioural tactics if we examine their behaviours.